Tuesday, September 27, 2011

A stacked low

We have an aesthetically beautiful storm system (at least, from a satellite perspective) over the midwest that has been lingering around for the last few days.
GOES-E IR satellite image from 1815Z, Sept. 27 2011
A nice spiral shape is visible over the Chicago area, indicative of what we call a stacked low--a situation where the center of low pressure (or the lowest heights of all pressure levels aloft) is vertically contiguous over the entire atmosphere.  That is to say, the troughs of lower pressure or geopotential height are all lined up over each other throughout the atmosphere.

We can see this on this morning's model analyses.  Here's the 12Z GFS 500mb analysis:

The height of the 500mb pressure surface off the ground is contoured in black.  You can see that the lowest heights are found right in the center of that bulls-eye over northeastern Illinois.  This is a closed low seen at 500mb.  Why is this a "closed" low?  Notice that the contours surrounding that very low height center are circular and completely closed off.  This is not an open wave in the mean flow--air is moving in full circles around this depression.  Thus this is a closed low in contrast with open waves that we often see moving through (like the broad open wave off the Pacific Northwest coast.

If we drop down to 850mb, we see the low in exactly the same place:

Once again, the lowest heights of the 850mb surface are centered over northeastern Illinois.  Finally, if we look at the surface pressure, we see the same thing:
Surface pressure is contoured in black on the above image.  You can see that there is an area of low pressure once again centered in northeastern Illinois.  Since all of these areas of depressed height or pressure are stacked right on top of each other, this is why we call this a stacked low.

One interesting feature of stacked lows is that they often indicate a weakening storm.  Look at the surface temperatures in the surface map above surrounding the center of low pressure.  See how the temperature is just about the same (in the greens and blues) all the way around the low pressure center?  The warmer air (the yellows) is well off to the east and south--far displaced from the center of the low.  A strengthening low pressure center draws its strength from these thermal contrasts--these fronts.  If the fronts move away from the center of low pressure, it gets more difficult for the surface low to strengthen.

Why is this?  Remember that the strong upper-air winds (and, consequently, the associated jet streak divergence that low-pressure centers need to form) are driven by temperature gradients below.  If the strong temperature gradients (usually fronts) move away from the center of the low, the strongest upper-level winds will move away with them.  This makes it less likely that the low will grow.

Another interesting feature of closed, stacked lows is that they don't usually move out quickly.  Since the temperature gradients and therefore the strong upper-level winds move away from the center of the trough, it's difficult to get enough strong motion to move the trough along.  Furthermore, when the low is closed instead of being an open wave, the flow around the upper-air low is more circular and less east-west.  This also doesn't help move the low off to the east.

So, in short, this means that the midwest has been and will continue to experience more rain and cloudiness for the next day or two as that low lingers around.  Most models have it transitioning to an open wave again by Friday, and with that it should begin to move out.

Wednesday, September 21, 2011

A new radar is born!

Today, the newest radar in the NEXRAD system began publicly transmitting its data.  This radar is known as the Langley Hill radar (KLGX) and it's located in Grays Harbor County in western Washington.  Here's a sample reflectivity image from this evening:
As you can see, the radar's position on the coast affords us the new ability to watch storm systems and rainfall before they reach the coast--giving us advance warning of the structure and strength of precipitation.  This is going to be amazingly helpful both to forecasters and anyone curious when the rain in Seattle is going to start.

The KLGX radar is also equipped as a dual-polarimetric radar--one of the first in the country to receive this new technology.  I'll do some future blog posts talking about what dual-polarimetric radar means, but for now, those who already know will be happy to see that the variables seem to be coming in nicely.  Here's an example cross-pol correlation coefficient plot from the same time.
You can view live data yourself from the radar online currently through the University of Washington's radar loops at:

Also, for those people who use the Gibson Ridge Level II or Level II analyst program, here's the modifications I needed to make to add this radar:

1)  In the directory where Windows installed GR (usually something like C:\Program Files\GRLevelX\GR2Analyst), you can edit either of two files (depending if they exist or not)--radars.gis or curstomradars.gis (It may not let you save changes to the file since it's in the Program Files directory.  I find I have to copy the file to my desktop, edit it, save it, then copy it back into the GR2Analyst directory for it to work).

2) In either the radars.gis or customradars.gis file, add the following line
"klgx, ksew, 47.1169, -124.1066, 73, 1, WA, Langley Hill"
(Those may not be the official values for the location, but it's the closest I could find.)
This line gives the following information--the radar ID, the WFO that runs the radar (KSEW--Seattle, in this case), the latitude, longitude, elevation, some random flag, the radar is in the state of Washington, and its official name is "Langley Hill".
Save the file and, if needed, copy it back into the GR2Analyst directory.

3) Open up GR2(Analyst) and go to File, Configure Polling. Highlight the address for your level II radar source in the text box, then click the "Refresh" button.  This makes Gibson Ridge confirm that your online data source actually includes the KLGX radar.  Then, click Ok.

4) Navigate to the radar site and start polling for radar data.  It should start downloading the level II data and displaying images.  At least, I use the Iowa State level II source and they have the data flowing.

That's it!  You should be able to see data from the new radar, now, too.  Enjoy watching our rain tonight and tomorrow...

Monday, September 19, 2011

New blog format--back to weather basics

This will be a short post, describing a few changes in the content of this blog that we'll hopefully see over the next three months.

I know that the audience of this blog is a mix of people with greatly different backgrounds.  Each new blog post I have generates, on average, about 150 views after it is posted.  Of those people, I know many are fellow friends and colleagues who are also meteorology professionals.  And, as such, many of my posts are geared toward that audience.

However there is also a large component of my audience containing people who aren't meteorologists by trade, but have a strong interest in weather nonetheless.  I frequently get emails from these readers, asking for clarification about some concept or sharing with me their own stories and images from unique weather events.  I really enjoy these interactions, and it has really helped me get to know my audience.

For this fall quarter, I will be a teaching assistant for the class ATMS 101 -- Weather -- at my home institution of the University of Washington.  This is a class designed to cover the fundamentals of our understanding of meteorology, geared toward students who are not meteorology or atmospheric sciences majors.  It's basically designed as a science elective class for undergraduate students who have an interest in learning about the weather.

I'd like to use this blog as a supplementary teaching tool to the time I'll be spending with the students in their classes.  By offering my own perspective on the topics covered in class in a written format like this that is easily accessible and retrievable, I hope to be able to give the students another resource as they are studying the material.   Furthermore, this would give me a platform to respond to questions that many students may have and perhaps lead to some interesting discussions of some of the finer subtleties of meteorology.

However, I don't see this as just a resource for the students--as I said, I have gotten a pretty good idea of who my audience is on this blog.  And I know that each and every time I casually talk about a shortwave trough at 500mb, there are a lot of people who really don't have a good idea what exactly I mean by that.  I want to thank those particular readers for sticking with the blog and hopefully learning a lot in the process.  By following this class and starting from the basics, I hope that in my blog posts over the next three months you'll be able to maybe pick up a little bit more background information about what I'm talking about here and learn a lot about the weather in the process.  I hope you'll find this enjoyable.

With that said, I still plan to do several posts about ongoing weather events across the country as I have been doing.  This should hopefully help to satisfy those readers who have a little bit more background and are looking for a different perspective on the current weather.  In an ideal world, I would hope to be able to tie in the current weather to some of the more basic concepts from this class.  We'll see how that goes.

So, just to let you know, my blog posts starting next week will get back to the basics.  I hope that you'll enjoy this review of the science of meteorology and how we as scientists treat the weather and make forecasts.  As always, feel free to email me or comment with any of your thoughts, concerns or suggestions about the blog.

Thanks again for being such loyal and thought-provoking readers...

Friday, September 16, 2011

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.

Wednesday, September 14, 2011

First rainy weekend in Seattle in a while?

As anyone who has been in Seattle over the past few days knows that we've returned to more "normal" conditions for fall.  Which includes a sky that perpetually looks like this:
Webcam image looking across Hood Canal to the west on Sept. 14, 2011.  From Dr. Dale Ireland's page.
The marine layer is in constantly and so far we haven't had enough heating to erode it away.  Upper-level troughing is helping keep an onshore flow with cooler temperatures aloft, both good conditions for maintaining this cloud cover.

However, we haven't seen rain with this onslaught of cloudiness.  In fact, Seattle hasn't had any rain for a while.  We've been enjoying the usual summer "drought".
Climatological norms, records, and observed values for temperature and rainfall year-to-date at KSEA.
You can see in the rainfall chart that while Seattle is wetter than usual so far this year, there hasn't been much of a change in rainfall for quite some time.  In fact, aside from a .12 inch event on August 22nd and a few scattered "trace" readings, Seattle hasn't seen any real rain since the end of July.

That may be about to change, though.  Here's the UW-WRF 12-km extended forecast image showing 24-hour accumulated precipitation as of next Sunday evening:
108-hour UW-WRF 12-km forecast of previous 24-hour accumulated precipitation, valid 00Z, Monday, Sept. 19, 2011.
The units on the color bar an in hundredths of an inch.  This map shows around half an inch of rain in the south sound area, with maybe a tenth to a quarter of an inch further north in the Seattle area.  Higher amounts are forecast further south.  You can also see the area of lower precipitation forecast directly to the east of the Olympic Mountains--a rain shadow effect.

The trend in most of the major models has been to slowly increase our precipitation amounts this weekend.  Some factors seem to support this.  For instance, water vapor imagery shows a nice plume of upper-level tropical moisture headed our way over the next few days.
GOES-W water vapor imagery at 18Z, Sept. 14, 2011.
And there does appear to be a bit of a 300mb shortwave trough with associated jet streaks popping into the upper-left corner of the model domain by Sunday.
108-hour UW-WRF 12 km forecast of 300mb winds and heights, valid 00Z, Monday, Sept. 19, 2011.
Such a setup should generate some strong onshore flow.  This will definitely keep the cloud cover in place, and added lift as the air moves over the Cascades (or the Olympics) could help to squeeze some showers out of the moist layer aloft.  A vorticity maximum is also forecast to move over this weekend in advance of the upper-level trough, as seen on this 500mb vorticity forecast map:
87-hour UW-WRF 12 km forecast of 500mb absolute vorticity and heights, valid 03Z, Sunday, Sept. 18, 2011.
Positive vorticity advection (yes, if we're being technical, differential advection, but generally we have lower absolute vorticity at the surface...) generally supports stronger vertical motions, so this would also aid in getting enough lift for their to be some rain.

And the forecast rainfall amounts in the models just keep growing...

Monday, September 12, 2011

Autumn starts crashing in

It's that time of year--time to start looking ahead to the cooler temperatures that are inevitably on their way.  A taste of more fall-like temperatures is due this week for much of the central part of the country.

Right now, we still have the same general upper-air pattern that I talked about a week ago--a ridge over the west with troughing in the east:
GFS analysis of 500mb heights and winds, 12Z, Sept. 12, 2011.
However, by mid-week, a relatively strong upper-level trough is forecast to dig down across western Ontario and into the upper Great Lakes.
GFS 54-hour forecast of 500mb heights and winds valid 18Z, Wed, Sept. 14, 2011.
Notice that for the first time in a while we have a really significant jet streak visible (the strong winds indicated by the bright colors) over the upper midwest.  This jet streak represents a particularly strong portion of the larger jet stream which circles the globe over the boundary between cold polar air to the north and warmer, subtropical-like air to the south. During the autumn in the US as the northern hemisphere begins to be pointed more away from the sun, cold polar air creeps slowly southward.  We see this manifested in the upper-air pattern as the polar jet stream also creeping slowly southward.  When we start seeing jet streaks (remember--these are embedded in the larger jet stream) over the northern US again, it's evidence that the jet stream is on its way south and cold polar air is soon to follow.

Sure enough, the GFS is forecasting a large high-pressure center to move down from Canada and settle over the middle of the country.  The air mass accompanying this high pressure center is continental and polar in origin, bringing with it cold air and clear skies.  Here are the forecast low temperatures on Thursday morning:
GFS 72-hour forecast of 2-meter temperature (colors), mean sea-level pressure (contours) and winds (barbs) valid 12Z, Thursday, Sept. 15, 2011.
This forecast shows low temperatures in the low 20s in parts of Minnesota--with lows in the 30s and low 40s also predicted for much of the midwest and northern plains.  You might also notice how the leading edge of the cold air has a rather distinct boundary--there is a pronounced wind shift along the leading edge as well as a strong temperature gradient.  These are the classic marks of a cold front, and we really haven't had a decently strong cold front move through the country in quite some time.  As this front moves south and encounters rich moisture, storms and rain along the front will probably be possible on Wednesday and Thursday.  The model forecast for Wednesday night would seem to confirm this:
GFS 60-hour forecast of 6-hour accumulated surface precipitation valid 00Z, Thursday, Sept. 15, 2011.
There's a fair amount of precipitation forecast right along that frontal boundary.

Is this unusually cold weather for this time of year?  Indeed it is.  Here's a plot of the climatological records and normals so far this year for Chicago:
2011 observed, normal, and record values so far as of Sept. 11, 2011.  From the NWS WFO Chicago.
In the top panel of the graph, the bottoms of the pale blue bars indicate the record low temperatures.  For mid-September, the record lows are around 40 degrees, and based on the forecast above it looks like we may get close to that.  So, an unusually chilly few nights are coming up this week.

Thursday, September 8, 2011

A wildfire in...western Washington

I have decided to stay local today (at least local for me) and talk about a wildfire that's ongoing in the Olympic Mountains of western Washington.  With the huge wildfires in Texas, it seemed like a good topic to briefly talk about. I know...there are three tropical cyclones in the Atlantic that would also be worthy subjects. However, I feel like I've somewhat neglected my many blog readers who live in the Seattle area.  Besides--I can see the smoke from this fire from my window, and that makes this a compelling subject for me.

As noted in the news article I linked above, this fire (known now as the 'Big Hump Fire') started last Thursday, but has really grown this week.  They also note in the article that we haven't had any lightning recently so...they assume the fire was manmade somehow.

We can see the smoke from the fire on Tuesday's high-resolution MODIS imagery:
MODIS image from the Aqua satellite on Tuesday, Sept. 6, 2011.
My crudely-drawn arrow points at the approximate location of the fire.  You can see smoke plumes trying to spread out from it, though they don't seem to be sure of where to go.

So, where is the smoke from this fire going to go?  We can once again use NOAA HYSPLIT trajectory model to look at smoke plume trajectories over the next 24 hours.  This model is freely available and you can run your own simulations using it online at http://ready.arl.noaa.gov/HYSPLIT_traj.php.
NOAA HYSPLIT model forecast trajectories for 24 hours starting at 12Z, Sept. 8, 2011.
In the image above, I started a new trajectory every three hours over a 24 hour period.  You can see in the map view that all of the plumes seem to head generally to the south or southwest.  This roughly agrees with the 850mb winds forecast by models for this evening over the region:
UW 4km WRF 12-hour forecast of 850mb winds (barbs), temperature (colors) and heights (contours) valid 00Z, Sept. 9, 2011.
 Overall we do have flow out of the north-northwest along the coast and into the mountains (though the mountains are interrupting the flow somewhat).  It's pretty weak flow, though, so the smoke won't get evacuated very quickly.

But let's return to the trajectory model above. The lower panel of the graphic above shows a time-height series for the plumes.  Time increases as you go to the right, and each star represents a time when the model started another plume trajectory calculation--every three hours.  You can see how high above the ground each of the plumes gets by following the lines that come out of those stars.

Notice that for plumes starting or already traveling during the mid to late afternoon (from about 18Z through 00Z), the plumes get pretty high--up to 1000 meters above the ground.  But then, once we get to the evening (after 00Z), the plumes stop getting that high--they get trapped very low near the ground.  This continues until the following day in the afternoon, when once again the plumes are able to rise higher.

This shows the effect of the nocturnal inversion extremely well (at least, in this model).  Remember from my discussions about diurnal cycles and about the nocturnal boundary layer wind maximum that at night the land surface cools off very rapidly.  This, in turn, causes the air close to the surface to cool more than the air above it.  The result is a layer at the surface where temperature increases with height--a stable "inversion".  Stable layers inhibit vertical motion.  Thus, at night, we can see that the stable layer that has formed near the surface is keeping the smoke plume low to the ground--it can't rise up.

In contrast, during the afternoon when the sun is beating down, the ground surface warms up very quickly.  Similarly, the air near the surface warms faster than the air above.  This leads to a layer near the surface where the temperature decreases with height--a more unstable layer.  The more unstable the layer, the more easily you can have vertical motion.  We see that in these smoke plume trajectories--the smoke rises much higher during the afternoon when the near-surface layer becomes less stable (and we get more convective mixing).

As I noted in my last blog post, the Pacific Northwest will be stuck under a ridge for the next several days, with warm temperatures and low humidities persisting.
UW 4km WRF 72-hour forecast of surface dewpoint temperature valid 12Z, Sun., Sept. 11, 2011.
The forecast graphic above shows dewpoint temperatures on Sunday morning throughout the Pacific Northwest.  Note the purple blob over the Olympic Mountains--those are dewpoints down in the 30s and 40s.  With high temperatures forecast to be in the upper 80s on that day (the joys of living under a ridge...), that represents very low relative humidity.  This fire could be difficult to put out.

One slightly positive effect of this wildfire is that its smoke can contribute to some absolutely amazing sunsets.  Below is a screen capture from Dr. Dale Ireland's high-definition webcam that was pointing in the direction of the fire as the sun went down on Tuesday evening.
Image from Dr. Dale Ireland's page.
Incredible.  On his webpage, Dr. Ireland actually has a time lapse movie of the entire day on Tuesday.  Outside of watching it to enjoy the beautiful weather and excellent scenery across Hood Canal and in the Olympics, at around 4 PM during the day you can see the fire really start to explode--the smoke plume shoots up into the sky almost like a volcanic eruption.  The smoke then spreads out as it hits the capping inversion aloft (yet another example of how a stable layer stops vertical motion).  Then, the sun setting through the smoke-filled sky fills the environment with brilliant and exotic hues of red.  This video is definitely worth a watch.

Tuesday, September 6, 2011

A steadfast pattern for the week ahead

I thought I'd do a quick look at what the GFS model is saying for the weather over the next week.  Let's start with this morning's analysis.
GFS 500mb analysis from 12Z, Sept. 6, 2011.  Annotations by LM.
This is this morning's 500mb analysis from the GFS model.  We can see the remnants of Tropical Storm Lee as marked by the deep trough with a small cutoff area in the south.  Lee's remnants are interacting with a frontal boundary draped through the east coast states.  From the thermal wind relation, we know that surface temperature gradients (like fronts) imply strong winds aloft parallel to the front axis.  Therefore, we can infer that there is a front across eastern New England and the mid-Atlantic states from this jet streak aloft.  Lee has brought with it very deep tropical moisture, which is helping to fuel some incredible rainfall amounts up and down the east coast.  Here's this morning's precipitable water analysis from the GFS:
GFS precipitable water analysis at 12Z, Sept. 6, 2011.
That swath of dark greens running up the east coast indicates precipitable water values in the 2-inches-plus range--large amounts of water in the atmosphere.  No wonder we're seeing such flooding rains.

Further to the southeast and just coming onto the map is Hurricane Katia.  As far as Katia goes, it may be a powerful category three storm right now, but ensemble track forecasts don't show it making landfall.
ECMWF ensemble track forecast (50 members) for Katia as of 12Z, September 5, 2011.
However, dangerous rip currents and some high swells will definitely be possible due to Katia's approach, even if it remains well off-shore.

Get familiar with this overall synoptic pattern, though--it's going to stick with us.  Particularly with respect to the idea of a large ridge over the western US and a trough over the Mississippi and Ohio River valleys.  Here's the GFS 500mb forecast for Thursday morning:
GFS 54 hour forecast of 500mb heights and winds, valid 12Z, Thursday, Sept. 8, 2011.
We can see Hurricane Katia spinning off of Cape Hatteras, staying well off shore.  However, I'm kind of suspicious of this upper-air pattern from the GFS as we don't often see big, circular cutoff lows hovering around the Ohio River valley and the upper midwest.  But, the ECMWF is showing something somewhat similar (though the trough is a little more compact).  Regardless, low heights aloft mean cooler air aloft, which in turn means a more unstable atmosphere. A more unstable atmosphere is susceptible to vertical motion and, consequently, clouds and rain.  As long as that trough lingers, there will be daily threats of rain for that area.  Now, the moisture available to work with won't be like what we are seeing now on the east coast.  Here's the GFS precipitable water forecast for the same time:
GFS 48 hour forecast of precipitable water valid 12Z, Sept. 8, 2011.
We can see the copious amounts of tropical moisture that Katia is bringing with it off the coast.  However, in the Indiana-Ohio area, while there is still elevated precipitable water values, nothing is nearly that deep.  As such, there will be moisture to work with, but not enough to get really intense rainfall.

Meanwhile, out west, things look pretty dry.  Being under a ridge, general subsidence will prevail everywhere and we should see lots of clear skies, warm temperatures, and little chance of rain.

Let's fast forward to Sunday.  Here's the 500mb forecast from the GFS again.
GFS 120-hour forecast of 500mb heights and winds valid 12Z Sunday, Sept. 11, 2011.
We still are forecast to have the same overall pattern--trough in the east with big ridge in the west.  But note that the character of the trough has changed. It's no longer a cutoff trough--the contours around it are not closed circles.  Instead, the trough has become an open-wave trough again.  Open-wave troughs tend to be more progressive in moving along than cutoff troughs, which tend to stick around for a while.  So, this gives us some hope that the trough might move on by next week, hopefully bringing an end to the perpetual chances of showers throughout the midwest.

Thursday, September 1, 2011

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.