Joaquin is very sensitive; or why meteorologists are freaking out about this storm

We have a hurricane in the Atlantic at the moment---Hurricane Joaquin, a category 1 storm.  It's currently milling around in the Bahamas, slowly strengthening as it churns over the warm Gulf Stream waters.

Obviously any hurricane near the US east coast would raise concerns, but in this case meteorologists don't know what to do with the forecast.  Why?  Because even with our many state-of-the-art weather models, it's still not clear where this storm is going to go.

Here are some forecast tracks for the center of the storm from the Weather Underground site.  They show many of our major models' predictions for where the storm center will go.

Each colored line represents a different model.  You can see that most of these models take the storm into the Carolinas by this weekend.  One model (the NAM) seems to go a bit off to the east.  This actually looks like fairly good consensus between the models---almost all are agreeing on where the storm will go.  But there's one model missing from this map---the ECMWF weather model, arguably the best global weather model we have today.  Below we have forecasts from the US GFS model on the left and the ECMWF model on the right, both valid at 12Z Sunday morning.

Forecasting nightmare: GFS and EC w/ wildly different solutions Sunday morning. #Joaquin (c/o @TropicalTidbits) pic.twitter.com/i7rfrr02xN

— Jen DeHart (@jcdehart) September 30, 2015

The GFS shows a strong hurricane impacting the Carolinas, like we see in the track map above.  But the ECMWF shows a much weaker storm that actually tracks out toward Bermuda...well away from the US.

And this is what's giving meteorologists the fits.

Just about everyone is agreeing on a land-falling storm this weekend, except for the best model we have.  We can break this uncertainty out even further by looking at ensemble forecasts---several different forecasts produced by the same model, but starting with slightly different initial conditions (after all, we're not 100% certain about the exact status of the atmosphere when we start the model).

Below is a plot (again from Weather Underground) showing the GFS ensemble forecasts from this morning.  These are 21 forecasts using the GFS model.  Most of them take the storm into the coast, though admittedly there are a few that keep it off shore.

The ECMWF also has an ensemble of 50 forecasts.  Here's an animation from Ryan Maue (@RyanMaue) showing the minimum surface pressure centers from all the ensemble members (i.e., where each ensemble member is putting the center of the storm over time) from this morning.  You can see that the different forecasts have the storm centers all over the place...some move it over land, but others keep it out to sea.

What are ensembles? Potential forecast scenarios based on slightly diff initial conditions. Minimum across 51-members pic.twitter.com/6KgkmlEvYJ

— Ryan Maue (@RyanMaue) September 30, 2015

So there is a lot of uncertainty.  Adding to the uncertainty is how stubborn these model forecasts have been.  The single operational ECMWF forecast (not the ensemble)---again, our most accurate and trusted model---has consistently taken the storm out to sea over its last four model runs.  Even as the ECMWF ensemble has slowly moved the storm westward on average, that single deterministic ECMWF forecast has stubbornly kept it to the east and away from land.  However, the other models (like the GFS) have increasingly brought the storm further west and towards landfall in the same time period.

So what do you do when a home run would win the forecasting game, but your best slugger comes out and shows a bunt?  Trust their instincts or hope they come around and get in line?

One thing the meteorological community is trying to do to reduce the uncertainty is to get more observations.  As I said above, we're never 100% sure of what the atmosphere is doing at a given time, so our models always start out with a bit of uncertainty.  We can try to reduce that uncertainty by taking more observations.  And that's exactly what we're doing.  You may have seen the "Hurricane Hunter" airplanes which fly into tropical cyclones to get more detailed observations about what the storm is doing.  These planes also release "dropsondes"--think weather balloons without the balloons.  These packages of weather instruments fall through the atmosphere and collect valuable information about the structure of the atmosphere on the way down.  These observations are then "assimilated" into the next cycle of weather model forecasts to reduce the uncertainty.  Below, Alex Lamers (@AlexJLamers) shares the plan for the Hurricane Hunter flight this afternoon.  Each "dot" is a planned location where a dropsonde will be released.

Flight plan for aftn NOAA G-IV mission. Lots of dropsondes, including in the Gulf. Sampling features around #Joaquin. pic.twitter.com/BzG5SXoXbe

— Alex Lamers (@AlexJLamers) September 30, 2015

You'll notice that many of the dropsondes aren't even around the storm...they're off to the west in the Gulf of Mexico.  Why are we getting information for areas not around the storm?

As I've blogged about before, hurricanes, despite how powerful they are, are still steered by the much larger ridges and troughs in the atmosphere.  Knowing how these troughs and ridges will evolve is key to figuring out where the hurricane is going to be steered.  One experimental tool we now have is something called "sensitivity analysis" or "ensemble sensitivity analysis".  We can basically use our ensemble model forecasts to look at different times of the forecast and see how changing the atmosphere in one region affects the forecast downstream in another region.  Here's an example ensemble sensitivity plot from Dr. Brian Colle at the University of Albany using last night's ECMWF ensemble.

Here they set the target to be the upper-level pattern over the eastern US 3 days from now.  The plots go back in time through the forecast from Friday evening in the upper left down through Thursday evening in the lower left and then over to this (Wednesday) evening in the middle right.  You may have to click on the image to make it bigger to see the details.  Notice that in the forecast for Friday evening (upper left), there's a lot of dark colors over the eastern US...those are highlighting regions where the pattern is extremely sensitive and uncertain.  We've seen this in the hurricane tracks above.

But let's trace those areas of uncertainty back through the forecast...where do they come from?  You'll note by the time we get back to this evening (+1 day, middle right), the sensitive areas are in these shortwave troughs over western Ontario and the Great Lakes area and into eastern Ontario and Quebec.  That means that it's actually uncertainty in the upper-air pattern over the Great Lakes and eastern Canada tonight that's turning into uncertainty in the hurricane track on Friday!  To improve our forecasts, we actually need more information about what exactly is going on over most of eastern North America...not just around the storm.

The National Weather Service may start doing special weather balloon launches over the next few days to try and reduce that uncertainty.  Each model run that comes out will be able to take advantage of any additional observations to start their forecasts off with greater accuracy.

I haven't even talked about the intensity of the storm, which also varies quite a bit between the models.  Some develop Joaquin into a major hurricane (Category 3) over the next 72 hours, others keep it as a weak hurricane (Cat 1).  Those extra dropsondes released by the Hurricane Hunters in the storm may help refine that aspect of the forecast.

For now, I'll leave you with the latest forecast from the National Hurricane Center (as of 22Z Wednesday) and encourage you to read their discussions over the next few days to see how our forecasts are progressing!

Cyclone Pam, Seattle rain records and the MJO

Last week the small island nation of Vanuatu was devastated by a major tropical cyclone: the Category-5-equivalent Cyclone Pam.  There has been terrible damage to the infrastructure on the islands, with estimates of up to 90% of the buildings in the capital of Port Vila.  Power was lost early on in the storm and is just now being restored in some places.  We can actually see this looking at the weather observations from Port Vila, here from Weather Underground:

You can see that before they lost power late on the 13th, the pressure was plummeting (down to 965 hPa) with wind speeds rapidly increasing to 45 mph.

The CIMSS Satellite blog had a post showing several satellite animations of the cyclone as it hit; I encourage you to take a look at it here.

The cyclone continued south and is giving a glancing blow to New Zealand today, where it has caused some power outages and a lot of rainfall.  Remember that in the southern hemisphere low pressure centers rotate clockwise.  Here's a satellite image from the New Zealand Met Service this morning, showing that the cyclone has basically been sheared apart, but there is still a lot of moisture with strong winds.

As The Weather Channel pointed out in an earlier blog post, Pam isn't the only cyclone we had in the western Pacific over the past week.  There were actually four active at once: Pam, which hit Vanuatu; Olwy, which is affecting the west coast of Australia; Nathan, which is hitting the York Peninsula of Austrlia, and Bavi, which is headed west towards the Philippines, but is expected to weaken before it arrives there.

Is there some uniting factor behind this burst of tropical cyclone formation?  It would appear so---the Madden-Julian Oscillation (MJO) is currently, by many measures, the strongest we have seen it in a very long time.

What is the Madden-Julian Oscillation?  The exact nature of it is still a somewhat poorly-understood topic in meteorology, despite intense research activity in recent years.  Basically, it's an area of enhanced convection (thunderstorm activity) that propagates around the equator every 30-90 days.  (See the Wikipedia article for a decent overview).  We can see this looking at a global satellite loop over the past week.  There's been a lot of cloud cover and convection moving out from Papua New Guinea and northern Australia and into the central Pacific.  You can see that as it moves east, it looks like there are tropical cyclones (Bavi and Pam) that are being "shed" from the convection.

It turns out that the basic structure of the MJO has an equatorial "heat source" being trailed by flanking "gyres" on either side of the equator.  If strong enough, these "gyres" can break away and form tropical cyclones, which is exactly what we saw last week.

How strong is the MJO right now?  Here's a diagram that tracks the strength and position of the MJO as it goes around the globe, called the Wheeler-Hendon diagram.  

You can see that there are different "zones" on the diagram, labeled with different geographical regions (the "Maritime Continent" (Indonesia, New Guinea, etc.) , "Western Pacific", "Western Hemisphere and Africa" and the "Indian Ocean".  So as the MJO moves around the globe, it's supposed to make a big circle around the diagram.  The magnitude of the MJO (how strong the convection associated with it is) is given by how far from the center you are.  The line on the plot shows where the MJO has been for the past 40 days.  You can see that for much of February (the purple part of the line, the MJO was very weak (close to the middle of the diagram) and not clearly being tracked around the globe.  But in March (the red part of the line), the MJO exploded in strength over the Maritime Continent and is now moving out over the western Pacific, getting even stronger.  This is WAY stronger than the MJO has been for a long time!  No wonder we are seeing powerful cyclones.

It turns out that an "active" MJO in the western Pacific can also contribute to heavy rain ("pineapple express") events on the west coast.  Here's a diagram from the MJO wikipedia article describing this connection:

So as the active area of convection on the equator associated with the MJO moves out over the Pacific, the moisture associated with that gets drawn northward to the west coast of North America, causing heavy precipitation.

Well, over the past two days we've had just that here in Seattle.  Here's an animated map of the total column water vapor over the past two days.  You can see the plume of moisture drawn northward from the tropics that brought heavy rain to Seattle.

So much rain fell (2.2") that Sunday was actually the second wettest March day on record in Seattle, and the single wettest day we've had since 2009 (courtesy of Scott Sistek).  It's amazing how a single weather anomaly like the MJO can contribute to both tremendous rain in Seattle and an extreme Cyclone in Vanuatu.

Snow, severe weather and a tropical storm...all in the next few days

It's an active weather period for the continental United States.  Our relatively tranquil summer weather pattern has been shattered in the past month as the flow has become far more amplified. Autumn is here.  Take a look at this morning's 500mb analysis from the University of Wyoming:

A fairly deep trough is digging through the intermountain west.  As this trough begins to cross the Rockies today, strong pressure falls are forecast in the lee of the mountains in the central high plains.  Take a look at this morning's 12Z NAM surface analysis:

And then the forecast for tonight at 00Z:

The low pressure center in the central US is forecast to deepen and pressure gradients are on the increase.  This means stronger winds.  In addition, you can see that there is a sharper contrast between the warmer air to the southeast of the low and the cooler air to the northwest.  This is showing us frontogenesis---the strengthening of horizontal temperature gradients into sharp fronts.  This points to a continually deepening cyclone, as mid-latitude, extratropical storms like these derive their energy from strong temperature gradients.

What does this mean for weather?  The colder air and northeasterly (upslope) winds on the northwest side of the low point to snow for Colorado, Wyoming and the northern Plains.  In Colorado, that's not very good for a place still recovering from devastating floods less than a month ago.  As the storm moves east over the next 48 hours, that low is really forecast to deepen.  Here's the NAM forecast of 500mb heights and 1000-500mb thickness for Saturday morning:

You can see really tight height gradients around that upper-level low, indicating strong winds aloft.  These winds are going to drive severe weather chances on Friday into Saturday.  Furthermore, strong winds on the back side of the low combined with ongoing snow should deliver blizzard conditions to the northern plains.  However, I showed the thickness map to point out that most forecasts still keep the lower atmosphere too warm for significant snowfall as this storm moves away from the Rockies.  That solid blue line off on the northern fringes of the map is the 5400m thickness line, usually a good indicator of the rain-snow divide.  That's well to the north, though there are so colder pockets near the low.  I'm not expecting major snow with this in the midwest.

However, severe weather is definitely on the ticket.  The SPC has slight risks for severe weather out for parts of the central plains and into the midwest for today and tomorrow (with a "see text" on Saturday) and even a moderate risk for Iowa tomorrow:

Lots of thunderstorms are expected, and with a height gradient like you see in the thickness map above, there should definitely be enough wind shear to support severe weather.

Finally, not to be outdone in this lackluster tropical year, we have a tropical storm (Karen) that has developed in the Gulf of Mexico.  Unlike pretty much every single tropical storm that has formed this year, the upper-level conditions are marginally favorable for development of this storm as it drifts north, though there still is great uncertainty as to where the storm will make landfall and how powerful it will be.  It's looking to hit either as a strong tropical storm or a weak hurricane at this point.  Here's the HPC forecast track:

Though there remains disagreement even among our best hurricane models.  The Hurricane-WRF model run from this morning has the storm making landfall in the Florida Panhandle as a strong tropical storm:

However the GFDL hurricane model has the storm making landfall over in Louisiana, again as a strong tropical storm.

We'll have to watch this as it approaches over the next day or so.  It will also be interesting to see what will happen after this storm makes landfall and starts interacting with the trailing cold front from the low-pressure center that's going to bring the snow and severe weather to the central US.  Longer-range forecasts from the GFS hint that by next Monday, the combination of tropical moisture from Karen's remnants and lift provided by that trailing cold front could bring heavy rain to the mid-Atlantic states.  Just what Washington needs...

Five crazy weather events you could be looking at right now

1) The extremely deep low in the Bering Sea
An unusually deep low pressure center moved through the Bering Sea last night and is now moving northward through the Chukchi Sea in western Alaska.  At its peak intensity, the low was down to 943 mb (or so we assume).  It was still at 946 mb, at least according to this morning's 12Z surface analysis from the NAM-WRF model over Alaska:

Hurricane-force winds are being reported with this storm, which is unusually far north to be of such a deep magnitude.  Here's the Alaska surface observations from 06Z early this morning.  Look at the wind observations in the Bering Sea.  Anywhere you see a triangle-shaped flag on the wind barb, that means winds over 50 knots were reported.  Amazing.

NOAA runs an ocean wave modeling system called WAVEWATCH III that forecasts wave heights over areas in time.  Here's their 12Z analysis of wave heights across the Alaska region.

Within that orange area in the center of the Bering Sea, they're showing waves as high as 9-10 meters--that's 30 feet or more.  In fact, some reports have come in from buoys and ships reporting 40-foot waves with a wave period of only 14 seconds.  That means that if you were on a boat, every 14 seconds you would fall 40 feet, then rise 40 feet on the waves.  Spectacular.  You can also see on this analysis map the grayed-out area to the north.  This represents areas that are already completely iced over for the winter.  That area is still pretty far north, so villages on the western Alaska Coast that would normally be protected from these waves by sea ice don't have that protection yet.  Major coastal flooding and storm surge is expected.

2) The anomalously deep cut-off trough forecast over the eastern Pacific
Anyone who was planning on heading down to southern California this weekend hoping to escape the winter weather may be in for a bit of a shock.  The same highly-amplified upper-air pattern that helped get that Alaska storm going is going to help spin off a deep, cut-off, upper-level low that looks like it will hover off the coast of California for the next few days.  Here's the 36-hour forecast for 500mb heights and temperature on Thursday evening from the UW-WRF 36km model:

You can see that this low is paired with a very strong ridge of higher heights over the central Pacific and that the main jet stream (where the iso-height lines are packed closely together) is much further north.  This means that the low center is going to hang around for a while.  Notice that with counterclockwise flow around that low height center it's directing onshore flow right into California in addition to bringing in colder air aloft.  This combination is going to help bring cloudiness and rain showers to that area over the next few days.

We can compare this trough and ridge to the climatological normal values for 500mb heights to see just how unusual of a pattern this is. Here's the CPC's 3-day forecast for 500mb height anomalies over this area:

Anywhere surrounded by dashed red lines is anomalously high and anywhere surrounded by dashed blue lines is anomalously low.  You can see that, particularly with respect to that ridge in the central Pacific, this pattern in somewhat unusual.  It will definitely make for some interesting weather.

3) First snowfall across parts of the plains and the upper midwest
With the jet stream shifted so far north due to this huge ridge over the northern Pacific, it's getting to be kind of hard to make other shortwaves move along and propagate away.   The same surface low that helped create severe thunderstorms in Oklahoma the other day has been slowly moving northeastward and getting better organized.  It's currently located over western Michigan:

Note the very tight isobar packing on the western side of the low.  This implies strong northwesterly winds in that area, helping to advect in some very cold air.  Behind this low, temperatures look cold enough to support snow.  Here's the RUC analysis of 1000-500mb thickness (an indication of how cold the lowest part of the atmosphere happens to be) for 18Z today.  Anything north of the solid blue line should be cold enough to support snow:

As the low moves eastward and the cold front continues to swing south, areas in eastern Illinois and Wisconsin and into Michigan should see temperatures start to fall over the next few hours, eventually to a point where any precipitation that falls could be snow.  Combined with the high winds behind the low pressure center, we can see why winter weather advisories have been posted for parts of Wisconsin.

4) Sub-tropical and now Tropical Storm Sean

Take a look again at the surface analysis that I showed two images ago.  Notice the rather deep low analyzed off the coast of the southeastern US?  That's tropical storm Sean (formerly sub-tropical storm Sean), a late-season tropical storm that should stay well-away from land.  On visible satellite, we can see that it has a good spiral shape, but it's not very symmetrical--there are bands within the storm with little cloud content and no well-defined eye.  These are all evidence that Sean isn't particularly strong--but still a tropical storm.

The National Hurricane Center official forecast has Sean staying a tropical storm for a while, feeding off the warm waters in the Gulf Stream.  There is some evidence in the dynamical models and satellite imagery to suggest that Sean may strengthen to a weak category one hurricane, though, so I wouldn't be surprised to see it upgraded sometime soon. However, they don't project it to hit mainland North America at all.  It may clip Bermuda, but otherwise it's staying well out to sea.

5) The "tropical storm" that formed in the Mediterranean Sea

If you really want an unusual cyclone, for the last several days a warm-core low developed and lingered over the western Mediterranean Sea.  The CIMSS satellite blog had several loops of satellite images as the cyclone reached its peak intensity.  Based on the warm-core structure and that the peak winds reached tropical storm strength, this was considered to be a tropical cyclone.  In the Mediterranean.  Strange...

As you can see, there's been a lot going on in the weather recently.  Plenty to keep any meteorologist, from amateur to professional, occupied for quite some time...

A Later-Season Hurricane

The "official" hurricane season lasts from the beginning of June through the end of November, so we're beginning to near the end of it.  In fact, we're getting to a time of the year when there are fewer hurricanes forming in the Atlantic.  Here's a chart from the National Hurricane Center showing the annual distribution of Atlantic hurricane frequency:

You can see that the number of tropical cyclones typically peaks in mid-September and then trails off.  However, interestingly enough, notice that there does appear to be a secondary maximum (though not nearly as prominent) in mid- to late-October.  Is there some physical reason for this?  I'm not sure.  But, the fact remains that here we are in late October and we have another hurricane to track:  Hurricane Rina.  (Oddly enough, the name "Rina" was selected to replace the previously-retired name "Rita" from this cycle of hurricane names.  I think that sounds a bit too similar, but...it's the way it is...).  Here's a visible satellite image of Rina from early this afternoon:

You can see that Rina is a fairly well-organized storm with a clear eye and that it isn't experiencing much wind shear.  How can you tell that there's not much wind shear going on from this image?  Look at the high-level cirrus clouds coming off of the storm--the wispy, fibrous like clouds on the outer edges of the circulation.  Notice that you can see those clouds pretty much all the way around the storm (maybe not so much on the western side, but they're still mostly surrounding the storm).  If we had strong wind speed shear (that is, winds strongly increasing with height in a direction that's NOT circling the storm), we'd see all of that cirrus "outflow" on one particular side of the storm, since the upper level winds would blow off all the clouds in one direction.  Because there's cirrus pretty much everywhere, we can guess that there's not a lot of vertical wind shear.  That's good for hurricanes, as it allows them to stay organized.

Strength-wise, Rina is a category 2 hurricane, forecast to become a stronger category 3 storm in the next day or so.  Warm sea-surface temperatures in the western Caribbean are helping fuel this storm.  Also, that lack of strong wind shear is allowing the storm to stay together and not get blown apart.

In contrast to the usual paradigm of hurricane forecasting, while we're pretty sure that the hurricane is going to strengthen, we aren't so sure about where the hurricane is going to go.  In a previous blog post, I talked about using model ensembles to forecast tropical cyclone paths. Remember that ensembles involve running multiple models or multiple perturbed versions of the same model to get many different possible forecasts of the future.  We can see how well these forecasts agree to make decisions about where the storm may go.

Well, here's an example of an ensemble forecast of Rina's path from this morning's ECMWF model ensemble.  Each yellow line represents a different ensemble member's forecast of Rina's path:

The black line shows the previous path of the center of the storm and where it is now.  But look at how many different paths the ensemble members are forecasting for this storm!  Some are sending it south into Nicaragua.  Some are bringing it northwest, then abruptly southwest into Belize.  Many are bringing it into the Yucatan Peninsula.  And a few even have it crossing the Yucatan and then curving back northward to hit Florida.  That's a lot of uncertainty in the path of this storm.  The National Hurricane Center's forecast favors the Yucatan path (and you can see that a fair number of the ensemble members above do too).  Here's the NHC's forecast:

The cone of white surrounding the storm indicates their uncertainty in where the hurricane center will be.  The forecasters at the Hurricane CEnter use their expertise and experience with past hurricanes to try to rule out some of the more unlikely forecasts.  As such, you can see here that they really don't believe the models that are taking the hurricane into Nicaragua or Belize.  But there's still a lot of uncertainty in their forecast track.  Furthermore, they haven't ruled out a landfall in Florida yet.  But Rina is a slow-moving storm, and we will have until the end of the week to see what Rina does and where it goes before determining if it will hit the US.

Extratropical Transition

Hurricane Maria is currently up near Newfoundland and is undergoing a period of its life known as the "extratropical transition".

GOES-E wide view infrared satellite image from 1615Z Sept. 16, 2011.  Maria is in the top right corner.

What does this mean?  In the classification of large-scale cyclones, we usually think of two broad classes: tropical and extratropical (that is, "outside the tropics").  Alternatively, you could consider these warm-core cyclones and cold-core cyclones.  If we go way back to my fourth blog post ever, I talked about hurricanes as warm-core cyclones.  Right now, Maria is transitioning from a tropical, warm-core cyclone to an extratropical, cold-core cyclone.

How can we see this?  Well, for one thing, the structure of the cyclone on satellite has changed dramatically.  Notice how Maria (in the upper right corner of the image above) no longer looks like a symmetrical storm with an eye in the center.  It looks far more comma-shaped, with a long, trailing band of convection stretching down to the south.  That band marks the position of a front, which is our first bit of evidence.

Extratropical cyclones have fronts.  Tropical cyclones do not have fronts.

As the hurricane moved north, it moved away from the tropics--where the temperature is basically warm everywhere. Instead, it moved up north and began encountering the polar front -- the divide between the cold polar air to the north and the warm subtropical air to the south.  As the swirling winds around the cyclone encountered this temperature gradient, they started wrapping cold air around the western side of the storm.  And, with that, a front was born.

Here's the GFS analysis of 850mb virtual temperature (just think low-level temperatures) over the region.

GFS analysis of 850mb virtual temperature valid 12Z, Sept. 16, 2011.

Keep in mind Newfoundland off the east coast of Canada as the location of Maria.  You can see how this tropical cyclone has brought up a lot of warm air from the tropics as it moved northward (it also helps that the Gulf Stream current is right there along the east coast, too).  But the storm is now running into much colder air to the north and west.  In fact, the formation of the strong cold front in the Atlantic that we saw on satellite imagery isn't exclusively due to the hurricane--that front actually started out as the leading edge of the very cold air that has sunk down over the central part of the US this week.  As the cold front at the leading edge of the cold air moved eastward out over the Atlantic, it was picked up by the circulation surrounding Hurricane Maria, helping to convert that cyclone into a cold-core, extratropical type storm.

We can also see that Maria is in an intermediate phase by looking at upper-air soundings.  Here's this morning's sounding from Stephenville, Newfoundland:

12Z sounding from CYJT on Sept. 16, 2011.

This sounding has some tropical cyclone characteristics and some extratropical cyclone characteristics.  Notice how high the tropopause height is--that's the height where the temperature stops cooling with height and abruptly starts warming.  On this sounding it's at just above 200mb--unusually high for some place that far north (for comparison, this morning's sounding in central Quebec had a tropopause at about 250mb).  Furthermore, in extratropical cyclones, the tropopause tends to drop closer to the ground as the trough moves overhead--not remain high. 

Also, notice how the temperature profile is nearly moist adiabatic all the way up to the tropopause--the air is nearly saturated all the way through the troposphere.  Once again, this is far more typical of tropical cyclones than extratropical cyclones, where usually only the near-surface layer(s) tend to be saturated with moisture.

However, the wind structure tells a different story.  In a mature or maturing tropical cyclone, the lower part of the storm rotates cyclonically while the outflow in the upper part of the storm rotates anticyclonically.  It's a high pressure area on top of a low pressure area, basically.  As I explained in that blog post I liked to above, this is a characteristic of a warm-core cyclone.  However, if this were happening, then the winds in the top half of the storm would be blowing in the opposite direction as the winds in the bottom half of the storm.

But we don't see that above!  Instead, we see that the winds are all blowing in pretty much exactly the same direction.  This is more typical of a dying extratropical cyclone than a tropical cyclone.  So, the wind field is starting to show evidence of this extratropical transition as well.

The exact mechanisms involved in extratropical transition are still not well-understood--how does a storm flip its structure like that while not tearing itself apart? It's an active area of research, and hopefully we'll understand it better some day soon.

Ensembles in hurricane track forecasts

Let's look at the tropics today.  As most people are aware by now, Hurricane Katia has formed in the eastern tropical Atlantic and is chugging along westward, forecast to become a major hurricane.  Katia is a Cape Verde type hurricane, which means that it formed just off the coast of Africa (near the Cape Verde Islands).  Because of this, it will go through a long, slow trek across the tropical Atlantic before approaching the Caribbean or the US.

Where is Katia headed?  We can use the NCEP's experimental GFS ensemble to get an idea of possible tracks.  Remember than a model ensemble involves running the same model dozens of times with slightly different initial conditions each time.  We then get several possible outcomes.  If all the outcomes are similar, we have pretty good confidence in the forecast.  However, if they are vastly different, there's low confidence in the forecast.  Here's several tracks from the 20-member GFS ensemble:

10-day track forecasts for Katia from GFS 20-member ensemble.  http://ruc.noaa.gov/tracks

The current position of the storm is about where the dot that says "OBS" is located--it's a little to the left of the name "Katia".  From that point, every white dot further westward represents the mean location of the storm over all the ensemble members every 48 hours.  So, each subsequent dot to the west is 2-days later.  We can see that the tracks all agree fairly well out to about 6 days, so we have high confidence in our track forecast out to that time.  We start to see some spread after that, but only one of the ensemble members actually brings Katia into the US coast in Maine.  It seems far more likely that Katia will stay out to sea, perhaps impacting Bermuda, but staying away from the US coast.That's still several days away, however.

Another way of graphically looking at ensemble track forecasts is to look at the spread of the ensemble members about the mean.  For instance, say that at 48 hours out you calculated the mean position of the center of the storm from all of your ensemble members.  You could then go to each individual ensemble member model and calculate how far the storm in that particular model was from the mean position.  You can then find the mean error (a rough standard deviation) of the ensemble members from the mean location.

What does this tell us?  If the ensemble members are all doing different things, then the average difference between any one ensemble member and the mean position will be rather large.  But, if all the ensemble members have the storm in about the same place, then the average difference between any one ensemble member and the mean position will be rather small.

We can plot this on maps.  First, we can plot the mean position of the storm out of all the ensemble members at various forecast times.  Then, around each of those points, we can draw a circle whose radius represents the average distance between the position of the storm in each ensemble member and the mean position of the storm.  As uncertainty in the position grows, the circles will get larger.  As circles start overlapping each other, you get the charactaristic "cone" shape we so often see in hurricane forecasts.  Here's an example for the first 168 hours of the Katia track:

GFS Ensemble 168 hour track forecasts and uncertainties.  http://www.esrl.noaa.gov/psd/forecasts/gfsenkf/.

In the figure above, each white circle with a number in it represents the mean position of the storm in the GFS ensemble. The number tells you what hour of the forecast it is, starting at 6Z this morning.  As time goes on, the colors transition up the rainbow from purple to blue to green to yellow to red.  The colored areas represent the circles drawn around each mean position to show the uncertainty.

You can see that for the first 24 hours of the track forecast, the colored swath formed by those overlapping circles is very small--it's hardly visible.  Remember that small circles means that there is low uncertainty--the ensemble has a good idea that this is going to be the track of the storm.

However, after that, the circles start widening considerably.  By the time we get to the end at 168 hours, the uncertainty has grown quite a bit.  In theory, the ensemble suggests that by 168 hours (that's seven days from now), the center of the storm could be anywhere in the big red circular area at the end of the track.  Even with that large area of uncertainty, you can still clearly see a track emerging in the colored swath.  The ensemble is pretty confident that the storm will continue moving northwestward over the next week or so.

Let's look at another example.  There's currently a disturbance (identified as "93L" by the folks at the Hurricane Center) in the central Gulf of Mexico.

GOES visible image of invest 93L at 1715Z, Sept. 1, 2011.

It may not look like much now, but most of our models are developing that junky-looking area of clouds and storms into a tropical depression and tropical storm in the next few days.  Since this storm is forming in the Gulf, it probably poses a much greater threat to the US than it's looking like Katia will.  What do ensemble track forecasts say about this one?

GFS Ensemble 168 hour track forecasts and uncertainties.  http://www.esrl.noaa.gov/psd/forecasts/gfsenkf/.

This plot looks like a big mess.  Note that there doesn't seem to be a clear "track" developing like we saw in the Katia case.  The average position of the storm over the next week jumps all over the place in the north-central Gulf.  By seven days out, the average position really doesn't seem to have changed much, and the red "circle of uncertainty" covers pretty much the entire Gulf Coast (except for west Florida).

So what can we take from this graphic?  It gives us a couple of possibilities to watch out for:

1. There's clearly a lot of uncertainty in the forecast track, if the models suggest that the storm could be anywhere in the northern Gulf over the next several days.  This makes forecasting the track very difficult, and as such it's possible that a lot of people on the Gulf Coast might be put under tropical storm or hurricane watches and yet never really see significant impacts of the storm.  This could be a very difficult storm to predict, and everyone will have to be cautious.
2. Since, after seven days, the center of the storm really hasn't moved much (and the storm may never even have made landfall in that time, despite being very close to land), this storm may be a very slow mover and stick around for a while.  What does that mean?  Prolonged periods of heavy rain and windy conditions on the Gulf Coast.  The storm winds don't have to be too powerful--it doesn't even have to be a hurricane--for the storm to cause lots of flooding damage if it sticks around for a while and keeps bringing wave after wave of rain to the coast.

As of early this afternoon, the NHC still has this as an invest area in the Gulf--it's not a depression or a tropical storm yet.  However, a hurricane hunter aircraft currently investigating the area has found winds both measured and estimated to be at tropical storm strength.  So, should a closed circulation center be found, this could become tropical storm Lee in the next 24 hours.  We'll have to wait and see.

Fallout from the storm...and a rebuttal

It's been a week and a half since my last post, as I've been on vacation in northern California during that time.  But what a week to miss--a strong hurricane moving up the east coast with lots of media hype surrounding this storm.  I wanted to just briefly cover some highlights of the fallout from this storm by looking at what other people are saying.

1) Irene caused/is causing major flooding across much of the northeast.
As seen here:

 Much of the northeast remains under flash flood watches or warnings.  It's not completely safe to be out there just yet.  In the coming days we'll see the true extent of the damage caused by the storm.  The highest rainfall amount (so far) as reported by the NWS was in Bunyan, NC with 15.66 inches of rain.  However, greater than 10 inches of storm total precipitation was also reported in Virginia, Maryland, Delaware, New Jersey and New York.

2) Irene brought near-record storm surge to the Chesapeake Bay region.
Surge heights of 7.5 or 7.63 feet were reported near Norfolk at their peak.  This is only a few inches shy of the 7.89 foot record set by Hurricane Isabel in 2003.  Since storm surge height is often used as one proxy for the "strength" of a storm, this indicates just how significant of a hurricane this was for the east coast.

3) Radar images along the coast showed excellent detail of the storm as it passed over.
This is just a subject that I happen pay attention to.  Some of the newly-updated dual-pol NEXRAD radars got glimpses of this storm.  Since I've really only seen dual-pol products applied to severe thunderstorms and snowstorms, it will be interesting to see what dual-pol can do for hurricanes.  If you have liked my previous radar interpretation blog posts, then I recommend this post by Patrick Marsh (everybody's favorite graduate student) about interpreting wind fields in the near-hurricane environment from doppler radar.

4) The hype surrounding this storm is generating a lot of press...about the press.
I'm told that the network news coverage of this event was pretty spectacular and omnipresent.  There was no escape from it.  Being on the road most of the time, I didn't get to witness it.  But, several news outlets are wondering if the media went too far--or did just fine.  I suppose it depends on your viewpoint.  I have a feeling that had the storm been worse than predictions that we'd then get complaints that there was not enough warning, we were caught completely off guard, and so on.  Actually, based on the numbers above, I don't think that, at least for the meteorological community's part, this was over-hyped.  By the media, perhaps.  But I think the meteorological community did well.

5) Budget cuts to the weather service?
Of course, in the midst of all this, a few articles did come out both supporting and criticizing the NWS.  Some articles, like this Huffington Post article, highlight the impeding budget cuts to the National Weather Service and NOAA satellite programs.  They argue that our aging weather satellite fleet would be in danger of falling apart over the next few years if the program lost money.  We depend on weather satellites for so much of our observations in meteorology today that it's almost unthinkable that we'd let such a valuable tool disappear for budget reasons.  As the article points out, we have almost no observers taking weather readings out on the open ocean.  But that's exactly where hurricanes form and develop, and exactly where we need the observations the most.  Satellites are our greatest tool to fill that immense data void and without their observations, our hurricane models (and even our everyday global weather models) would be reduced to shots in the dark.  The GOES-R series, due to be launched in the next few years, has significant technological upgrades and much higher resolution than our current satellites.  With this GOES-R series, we have replacements for our aging satellite fleet in the works.  It would be one of the most short-sighted and foolish things I can think of to cut the program off now.

6) Fox News's "Do we need a national weather service?" editorial
This article has been receiving a lot of buzz (at least among my friends and colleagues in the meteorological community) over the past few days.The article criticises the NWS as a "relic of America's past that has outlived [its] usefulness."  The editorial writer argues that private companies provide better forecasts than the NWS and that cutting the NWS's budget by $126 million to levels of almost a decade ago wouldn't have any serious effects.

I believe this position represents an extremely limited vision of what the National Weather Service does, limiting it to just "forecasts".  It's true that private companies can often provide better forecasts on a point-by-point basis than the National Weather Service (NWS).  There is a thriving private sector in meteorology that makes serious money off of specialized and localized forecasts.  These companies can run high-resolution models and conduct internal research to improve their forecasts.  All of this is definitely doable by the private sector and I'll even admit--the forecasting aspect of the NWS is probably its most expendable feature with respect to the private sector's abilities.

But let's consider some of the other roles of the NWS.  Let's take those high resolution models that private companies run.  Let's step back to the very first step of the forecasting process.  To do any sort of model, you need to have initial conditions.  We have to know what the atmosphere is like now (and how it has been recently) to be able to look into the future.  We know this by looking at observations.  How many types of observation platforms are managed by the NWS and its parent organization NOAA?

• Weather satellites (GOES, POES, MODIS, and a suite of other specialized satellites)
• The NEXRAD doppler radar network (jointly with the FAA and the DoD)
• All upper-air sounding stations (weather balloons)
• River level gauges
• METAR observation stations at airports (jointly with the FAA)

And that's just a sample.  Almost our entire supply of upper-air observations and most of our surface observations come from the NWS or NOAA in some form or another.  Without these observations, no forecaster would be able to do predictions anywhere near as accurately as we do now.  No model would be able to get anything close to thunderstorm initiation or rain-snow dividing lines correct without this data.  We'd just maybe get the overall pattern correct over the next few days--sometimes.  All of our current weather prediction--be it public or private sector--depends on data from the NWS and NOAA.

One might argue that all of these data information services could be privatized.  We do have a couple examples that I know of where this has happened. The National Lightning Detection Network (NLDN) is administrated by the Vaisala Corporation.  Ever wonder why we don't see more lightning maps around, particularly in real-time?  Vaisala charges customers for access to lightning data in near-real-time, and those that buy it are prohibited from publicly releasing it until after a certain latency time (unless they pay more).  Another pay-for-data model has been adapted by the European Centre for Medium-Range Weather Forecasts.  I usually don't show many ECMWF model graphics and you won't find many detailed ECMWF model output images online either.  Once again--the ECMWF community charges people in non-member nations for their model output.  As such, even though they have one of the most advanced modeling systems around, we don't see much of their output online.  That is, unless you happen to belong to an organization that pays for this data.

My point is, we do have instances where private or near-private entities are controlling weather observations and model output.  In these cases, there is invariably a fee involved in accessing the data--that's how the private sector makes its money.

This is actually one of the things that worries me the most about eliminating the public sector from weather forecasting.  From the perspective of a researcher in meteorology, I enjoy and heavily rely on the plethora of free data that the NWS and NOAA provide.  You can go online and get both archived and real-time radar data whenever you need them.  You can look at output from all major weather models run by NOAA as soon as they are complete. You have access to weather observations at tens of thousands of locations through this free network.  I can see satellite images of anywhere in the country updated as frequently as every minute at times.  All of this for free.  Not only is the data free, but the mechanisms for producing, decoding or displaying the data from the NWS and NOAA are almost all open source--even the Radar Product Generator (the program that controls the doppler radars) and major weather models (like WRF) have all of their source code online, free to download and run. 

As I mentioned before, an observational network controlled by the private sector would virtually end all the free data and open source decoding.  Universities that are already cash-strapped would have to pay more money to get information that used to be free.  Cities and towns whose emergency managers relied on real-time radar and model updates to get emergency planning going would have to pay for access to this data.  Or, for a few dollars more, pay the company to have one of their expert meteorologists interpret the data for you.  Maybe we'd save some money by cutting spending for the NWS, NOAA and their programs.  But we'd probably end up paying it all right back to private companies in the end.

Which brings me to my last point--the accountability of the private sector.  While, as with any government organization, the politics will somehow find a way to infiltrate the system, I feel that the NWS really hasn't succombed to that as much.  Having worked in an NWS forecast office and for the NWS NEXRAD program, I have only seen the fringes of politics enter the business they do and never directly impacting the forecasting or warning process.  I feel that the NWS still provides an objective forecast and warning system.  The forecasters I know there are driven by their desire to get the forecast right--not by some other political or administrative agenda.

I also know several people who work for private sector meteorological companies.  They, too, possess a keen interest in getting the forecast right and that same thrill as NWS forecasters feel when things play out the way they thought they would.  I strongly support these forecasters and really respect the skill and passion they bring to their jobs.  It's not them that concerns me.

It's more the administration of these corporations that has me worried.  As any private-sector corporation, they are driven by profit.  With the way Wall Street has gone recently, we've seen that there are some darker sides of this system, even as robustly proven as it has been for our nation. We'd hope that higher profits are obtained by delivering a better product than your competitor.  But when your products can mobilize people and impact transportation or resource allocation, things get dicey.  Whole markets have developed that trade based on weather forecasts--people buy agricultural futures based on how they think the weather will impact crops.  A change in the forecast can cause the cancellation of hundreds of flights or the rerouting of dozens of container ships.  A change in the forecast can mean a change of millions of dollars.  So much of what we do depends on the weather.  Having a (hopefully) objectively-based, public-sector weather outlet that provides forecasts (and particularly warnings) without worrying about profit seems to make the most sense to me.  I'm not saying that the free-market economy would fail here--if a company makes bad forecasts, no one will trust it and it will fail.  But that strong link between the weather and our economy is something to think about when considering the role of private-sector weather companies in a country without a National Weather Service.

I usually try not to be opinionated (or voice my opinions) on these sorts of subjects, but I felt I should add my thoughts here.  After all, that's what a blog is good for, right?   If anyone else has any comments or thoughts, I'd love to hear them.

I also will do a write-up soon on my thoughts about the growing or shrinking weather-literacy of the American people.

The storm that won't form

Looking out into the tropics today.  Last week, Tropical Storm Don failed to live up to expectations in south Texas.  The rainfall associated with the storm dissipated rather quickly, bringing little relief to that drought-stricken area.

However, we have a new disturbance in the tropical Atlantic, labeled by the National Hurricane Center (NHC) as Invest 91L.  Here's how it looked early this afternoon on visible satellite.

GOES-E visible satellite image of invest 91L, 1815Z, Aug 1, 2011.

It's definitely a somewhat-organized area of convection and the NHC says that tropical storm force winds are already being reported in the vicinity as it approaches the Lesser Antilles.

However, this storm has been on the watch list for days--since Friday, the NHC has maintained on its website that this disturbance has an 80-100% chance of developing into a tropical depression or storm in the next 24-48 hours.  Three days later, they still haven't officially upgraded it yet.  Why not?  Hurricane hunter airplanes flying through the storm failed to find a closed circulation about a clear low-pressure center.  Without that level of organization, the NHC doesn't assign storms a unique identity as a depression or as a storm, even if the winds are of tropical storm strength.  So, we continue to wait...and watch.

The tropical cyclone modeling community is ever-expanding, and they're trying to solve what is becoming one of the classic dilemmas of current numerical weather prediction.  As our computer models have become more advanced, our ability to forecast cyclone tracks has increased, while our ability to forecast cyclone strength really hasn't changed much at all.  Many, many scientists are working on that problem, and the result is many fancy model configurations that try to get it right.

Here are three examples of advanced hurricane weather models and what they are predicting will happen with this storm.  I'm going out to the end of their model run at 126 hours--that's over 5 days.  Already we're stretching the limits of predictability even in relatively quiet weather conditions.  But, I wanted to illustrate the differences in these models in both track and intensity over this time frame.

I got these images from Bob Hart's Tropical Cyclone Genesis Potential Fields page--my absolute favorite page to visit to get a nice summary of what all the models are saying with respect to possible tropical cyclones.

First, here's the Geophysical Fluid Dynamics Laboratory (GFDL) model forecast for 126 hours out from 12Z this morning:

GFDL 126 hour forecast for incest 91L at 18Z, August 6, 2011.

The left panel shows a wider view of the tropical Atlantic and eastern US.  You can see that by August 6th (Saturday), the GFDL model forecasts the storm to be well off the coast of Florida and rather small in size.  The right-hand panel shows a moving nested model grid inside the large model.  This shows the output of a second model over a small area centered about the storm at much higher resolution.  In theory, the overall synoptic pattern in the larger-scale model will help determine where the storm will go and the higher-resolution, moving nested grid will help determine the storm strength.

One thing I really like about the graphics on this particular website is that they've color-coded their wind contours to match the different levels of tropical cyclone strength.  For instance, any areas in cyan are where winds are of tropical depression strength.  Dark blue is tropical storm strength.  Green corresponds to category one hurricane strength, yellow for category two, and so on from there up.  This lets you see at a glance what the structure of the wind field is forecast to be by this model, and also what the maximum intensity would be.

In the GFDL case above, there's a significant area of green, signifying category one strength, with a few small pockets of yellow (category two) strength.  So this forecast offers the possibility of a category 1-2 hurricane well off the southeast US coast by Saturday. The minimum pressure is given as 978.9mb.

Now here's the Hurricane-WRF model (the HWRF) forecast for the same time:

HWRF 126 hour forecast for incest 91L at 18Z, August 6, 2011.

There are significant differences between this and the GFDL model.  First, the storm position is different.  The HWRF is predicting it to be much closer to the Florida coast on Saturday.  Also, in this model the storm is predicted to be much larger in area, with tropical-depression- and tropical-storm-strength winds over a much larger radius from the center.  However, in terms of strengh, I don't see any signficantly visible areas of category two strength winds in this output.  The minimum pressure is forecast to be lower--971.4 in this model as opposed to 978.9 in the GFDL model.  So lower pressure, but also weaker winds.  Still probably looking at a high-end category one hurricane in this model.

And now, another version of the HWRF, but this one with modifications from NOAA's Hurricane Forecast Improvement Plan (HFIP) initiative.  This group has modified the HWRF model somewhat and also runs their smaller nested grid at very high (3 kilometer!) horizontal resolution.  Here's their forecast for the same time:

HWRF-HFIP 126 hour forecast for incest 91L at 12Z, August 6, 2011.

This forecast is actually from 6 hours earlier, but it paints an even more sinister picture.  In this model, the storm is very close to the Florida coast by Saturday morning.  The storm also is forecast to be much stronger--there are significant areas of orange in the wind field, indicating category three strength.  That would make this a major hurricane.  The forecast minimum pressure is also much lower than the other two models at about 940mb.  If this were to happen, then south Florida would really need to watch out.

So, you can see that we still have a long way to go in our hurricane models.  Granted, these are forecasting for five days out, and even in non-hurricane situations our weather models do poorly at that range.  These only represent possible futures for this storm.  If my experience in watching these models is any indication, the end result will be something that avoids the extremes.  In fact, usually these storms turn out weaker than forecast.  But not always...  So, just be aware of this storm as it develops and moves eastward.

Heavy rain in the midwest, tropical storm down south

Repeated rounds of storms around the periphery of the still-persisting ridge in the southern US have contributed to rainfall totals that are well above climatology for the last few weeks in some locations.  In Chicago this is the wettest July on record with 9.75 inches so far this month.  Furthermore, this month is now the 9th wettest month of all months in Chicago since records began--pretty impressive.

There's a lot of water vapor to work with in the atmosphere over that region at the moment.  Here's the satellite water vapor image today:

GOES-E water vapor image from 1615Z, July 28, 2011.

The upper-level ridge across the south causes air to advect clockwise around its periphery.  As such, you can see a big plume of moisture stretching from Arizona and New Mexico through the central plains, into the midwest and out through the mid-Atlantic states.  This plume of moisture has been in place over the same area for several days now, and any storms that form in that area follow the same path.  Repeated rounds of storm after storm (MCC after MCC...) following the same path have contributed to the high rainfall totals.

The more moisture there is in the atmosphere, the greater the potential for heavy rainfall.  This seems like a very obvious statement.  To that end, one parameter that meteorologists use to evaluate the potential for heavy rainfall is called the precipitable water value (often abbreviated as PWAT or PWTR).  It's a pretty simple index to derive--basically, all precipitable water does is look at the profile of moisture content over a given location.  It calculates how deep the water on the ground would be if all of the water vapor over a given location were to immediately condense out and fall as rain.  It's saying, if all the water in the air over your head immediately condensed into rain and fell out, how much rain would fall.  Clearly this is never fully realized, as the atmosphere never dumps out all of its water vapor content at once.  But, in general, the higher the precipitable water values, the higher the likelihood for heavy rain.

Most soundings that you see will compute the precipitable water value for that sounding location.  The HOOT website soundings are no different.  Here's this morning's 12Z sounding from Davenport, Iowa, which is in the middle of that moisture plume.

KDVN sounding from 12Z, July 28, 2011.

First, let's look at this sounding for a moment.  Notice how the dewpoint trace (the green line) is rather close to the temperature trace (the red line) from the surface up to somewhere around 430mb.  That's a very deep layer of very moist air.  We'd therefore expect the precipitable water values to be rather high.  You can see the calculated preciptable water value under PWTR above the upper right corner of the sounding.  The black value is the value for the current sounding, and it shows a PWTR value of 2.29 inches.  So that's saying that, based on this sounding, if all the water vapor in the atmosphere over Davenport were to immediately condense out and fall as rain, 2.29 inches of rain would fall.  That's a lot.

But we've already established that this is a record-breaking month for rainfall in this region.  Just how anomalously moist is the atmosphere within this plume of moisture?  The Rapid City, SD forecast office of the National Weather Service has put together a really nice webpage with climatological values of precipitable water for each sounding site.  Here's their yearly graph of climatological precipitable water values for Davenport:

Monthly climatological precipitable water values for KDVN.

There are several curves on this graph, which are explained by a legend in the upper left corner.  Each curve represents a different percentile of the normal precipitable water values based on the averages from 1995 to 2010.  The mean value (the 50th percentile) is given by the red curve.  You can see that for the month of July, Davenport's mean PWTR value is about 1.2 inches.  This morning's value of 2.29 inches is well above that.  In fact, if you look at the range of values for July, you'll see that 2.29 inches isn't quite up to the maximum curve (the light green curve) but it is above the 99th percentile curve (the dark green curve).  That means this precipitable water is in the 99th percentile of all PWTR values seen during July over the past 15 years or so.  That's very high.

Furthermore, the dashed green line shows a value that is two standard deviations above normal.  This value is often used by forecasters to try and separate extreme values from more climatologically normal (but still high) values.  Any time you're more than two standard deviations above normal, it's considered to be a more "extreme" case with a high potential for heavy rainfall.  2.29 inches is definitely above the two-standard-deviations line.  All of this points to the potential for very heavy rain.

Unisys publishes an interpolated precipitable water map for the US after every sounding release time.  So, here's their map of precipitable water across the US this morning at 12Z:

Interpolated precipitable water at 12Z, July 28, 2011.

There's a swath of elevated precipitable water values from eastern Nebraska east through southern Michigan.  This is the corridor where rounds of storms are able to produce very heavy rainfall and bring us these extreme events.

Also notable is the large area of very high precipitable water values right on the Gulf Coast.  They cite 2.4 inches at a point near Lake Charles, Louisiana.  There, too, precipitable water values are in the 99th percentile and thunderstorms with heavy rain are forecast for the next few days.

Of course, a very potent rainmaker looks to be bearing down on the Texas gulf coast in the next day or two.  Tropical Storm Don has formed in the Gulf of Mexico and is moving northwestward.

NHC predictied 5-day path cone of TS Don as of 10AM CDT, July 28, 2011.

It's not forecast to strengthen beyond tropical storm strength, which is good news for the gulf coast in terms of lessening the potential for damaging winds.  What's even better news is the amount of rain this storm could bring to central Texas, which is currently suffering from "exceptional" drought, as seen on the map below.  In contrast, with the extreme rainfall over Chicago recently, I'm pretty sure the little "A" for "abnormally dry" over northern Illinois is going to be disappearing soon...

US Drought Monitor from the Climate Prediction Center/UNL, as of July 19, 2011.