Wednesday, November 18, 2015

Very Un-El-Nino-Like cold next week for the NW

The GFS ensemble (and the ECMWF ensemble as well) are showing a large pattern shift to highly-amplified flow coming by the end of this weekend and into next week over the western US.  This would represent a pretty stark change from the weather we've been experiencing here (which has been wet, but not unseasonably so).  Let's look at what this change means.

I apologize for the somewhat ugly graphics, but here's a "spaghetti plot" of two 500mb height contours from the GFS ensemble.  There are 21 ensemble forecasts here of 500mb height and each line represents a different forecast's idea of where that particular contour will be.  The cyan color is the 5520m line and the red color is the 5820m line.  Let's focus on the cyan lines for now.  This is a 96 hour forecast for 00Z Sun (so, Saturday evening in local time) showing a very zonal (east-west) pattern over then north Pacific.  This is more or less what we've been seeing for the last week or two.  There have been some wiggles along this line as shortwave troughs have come through (bringing us all that rain), but the mean pattern has been rather flat like this.
NCEP Ensemble forecast product

Fast forward three days to 00Z Wed (Tuesday evening).  We can see over the northwest that the pattern is very different.  Though there is some (a lot) of uncertainty as to the exact structure here, most of the ensemble members agree that a large ridge will amplify into the Gulf of Alaska with a deep trough digging down into the western US.
NCEP Ensemble forecast product

This signal has gotten rather robust in the last few runs.  What does this mean for our weather?  Cold.
The Climate Prediction Center (CPC) has caught on to this pattern change and their longer-term temperature forecasts show it.  Below we see the CPC forecasts with high probabilities of below-normal temperatures for the 6-10 day and the 8-14 day timeframes throughout the northwestern quarter of the continental US.  There's also a high probability of above-normal temperatures in Alaska associated with that high-amplitude ridge.

And, yet, we are in an El Nino year...a very strong El Nino year at that.  One of the things that seasonal forecasters have tried to bank on over the years in the more "robust" patterns in temperature and precipitation that seem to accompany El Nino events.  You'll note that the three-month outlook from the CPC shows what is the more canonical El Nino-type pattern, that being warmer temperatures in the Northwest and across the northern tier of the country.
So this is coming as a surprise to a lot of people who had been expecting a warmer-than-average winter.  Does this mean everything we understood about what El Nino does is wrong?  Not at all...this troughing pattern will probably end up being transient as most weather signals are.  We may have a week or so of colder temperatures, but at this point it seems unlikely that this deep trough will be the story of the winter.  Remember that when we get toward monthly and seasonal scales we're getting into the realm of climate, where the El Nino patterns are more applicable.  Just because on average we expect warmer temperatures doesn't mean we can't get super-cold for a few days.

This could be quite the cold snap, though.  If we look at a single model (the University of Washington extended WRF run) we can get an idea of one possibility for how cold the temperatures may be.  Here's the extended forecast for 850mb temperatures (which are a few thousand feet above sea level) for next Wednesday at 12Z.  The light blue line is the 0 Celsius line (32 Fahrenheit), which you can see dipping all the way into southern California.  Remember this is at 850mb, so this is a few thousand feet up.  Still impressive.
Actual temperatures at the surface aren't as bad, but still showing below freezing (in the 20s) on Wednesday morning in western Washington.

Temperatures like that would be cold enough for lowland snow in western Washington, which only tends to happen about once a year (at best).  That assumes we get moisture, and there is a fair chance of that.  This forecast will definitely be something to watch develop this weekend.

Tuesday, November 10, 2015

The OLYMPEX field campaign

After being told enough that I should write a blog post sometime soon, I figured I could write up a blog post about why I've not been writing blog posts recently...

I'm a part of the OLYMPEX field campaign to observe precipitation processes in complex terrain.  "Why do we need to do that?" you may ask.  "Haven't we observed enough precipitation in mountains? Why do it again?"

The US (NASA) and Japan (JAXA) have cooperated to launch a new weather satellite called the "Global Precipitation Mission" (GPM).  This satellite has a whole suite of sophisticated instruments for observing precipitation from orbit, including both active and passive microwave sensors.  Think of it as a weather radar floating around in space.  We previously had another satellite carrying a radar called the "Tropical Rainfall Measuring Mission" (TRMM) which was in orbit since 1997 before finally coming to the end of its life earlier this year.  As the name implies, TRMM was mostly limited to the tropics for measuring precipitation (because of its orbit).  GPM will be able to sample precipitation in the mid-latitudes (e.g., the continental United States).  It's still a low-earth-orbiting satellite, so it only passes overhead maybe once or twice per day.  But it will be able to give us much better estimates of precipitation, particularly in areas where we don't have good radar coverage, like over the ocean mountainous regions.

GPM was just launched earlier this year and we're still trying to calibrate it. Basically, we want to be sure that what the GPM sensors are telling us is actually what we're seeing in these clouds and on the ground.  That's why we need this field campaign--to observe lots of details about the precipitation over mountains and see if that matches what the satellite sensors are telling us.

To do this, we've chosen to focus on the Olympic Mountains of western Washington.  This is one of the wettest places in the country (particularly this time of year), so for the next two months we'll be doing an extensive observational study of every precipitation event that crosses the mountains.  There are many components to this observational study including:

Lots of ground-based instruments, including rain gauges, disdrometers (to measure raindrop size), vertically-pointing radars, and standard weather instruments.  We've installed these observations all over the Olympic Peninsula.  This was a bit of a challenge since most of the peninsula is wilderness or the national park.  A brave team of graduate students has spent the last few months hiking around installing these instruments (even having to use pack mules to carry in some of the equipment...).  We'll also have several supplemental radiosonde launching groups to give us observations of the atmospheric structure aloft.
Rainfall map

We've got several supplemental radars set up around the peninsula, including the NASA NPOL radar (below, image from NASA), a "Doppler on Wheels", and an Environment Canada X-Band radar up on the southern end of Vancouver Island.  These radars will provide a continuous, 3D picture of what's going on during rain events.


And last, but not least, we'll have three research aircraft: a Citation jet, a DC8, and a NASA ER2, which is basically a U2-type spy plane converted for research purposes (seen below, image from NASA).  The DC8 and Citation will mostly fly in and around precipitating clouds to sample them directly.  The ER2 is equipped with the same type of radar instruments that are mounted on the GPM satellite.  As I mentioned before, the GPM satellite has an orbit that only brings it overhead twice a day at best.  That's not often enough for us to get a whole lot of samples in our two-month window.  However, the ER2 plane will fly at over 60,000 feet (the pilot actually wears a special "space" suit to fly at such high altitudes) and "simulate" what the satellite would see if it were flying overhead.  This will give us a lot of samples of what the GPM satellite instruments would see so we can make more rigorous evaluations.
ER-2 In Flight

This week we'll finally have the DC8 and the Citation jet around (with the ER2 arriving next week) so we'll finally start our sampling.  But how do we know when and where to fly these planes and point these radars?  That's where my team comes in--the forecasters.

I'm working as one of the lead forecasters to try and project when will be the best times to sample precipitation over the next 3-5 days.  We're interested in sampling a lot of different phenomena, from the warm, wet atmospheric rivers to isolated, post-cold-frontal showers that are common to this area.  For instance, the next four days look to be a gold mine of opportunity for the project, as we're looking to get a strong "atmospheric river" event pointing right at us, giving non-stop waves of precipitation.  Here's a 12-hour precipitation forecast for this Thursday from our latest local WRF run:

The higher elevations of the Olympics are forecast to receive between 20-50mm (0.75 to 2.0 inches) of rain in just that 12 hour period.  Here's the next 12 hours:
Another 75-125+mm (3-5 inches) in just that time alone!  Let's look at the next 12 hours!
Another 75-100mm (3-4 inches).  We're looking at over 10 inches of precipitation in the Olympics through the end of this week.  

You can see in the above maps how complicated this precipitation might be to observe.  There are steep gradients in the precipitation, with large amounts over the high elevations, but only light rain expected just north of Seattle.  We really want to see how well the satellite instruments might be able to catch these kinds of variations.

Another concern is snow.  How much of this precipitation will fall as snow as opposed to rain?  Where is the melting level actually going to be?  This model suggests we'll see some snow at the high elevations, upwards of 100mm (4 inches) at higher parts of the Olympics:
But how well can this model actually do at predicting precipitation type?  What kinds of small-scale structures lead to one mountain getting a lot of snow, while others getting barely any?  These are the kinds of questions we can try and answer over the next few months.

So that's why I've been busy...getting ready to support this campaign.  I've written a new website to view the output from our local WRF model (with more modern graphics) that's located at:

We're also heavily using ensemble-based products, including the SPC SREF, UW mesoscale ensemble, and the GEFS for longer-range forecasting.  It's quite the experience to take all of your collected meteorological observations and musings over the years and be forced to put it into practice every single day...

For more information on OLYMPEX, you can visit the main project website at:

Of particular interest may be the "Science Summaries" tab, where every day some of our project scientists describe the kinds of weather that we observed and what we saw.

You can also follow updates on the project's Facebook page, or on Twitter by following @UW_OLYMPEX or #OLYMPEX.

We also made the front page of The Seattle Times today...

Wednesday, September 30, 2015

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.

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.

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.

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!

Friday, September 11, 2015

Waviness in the tropics

Despite this being one of the most significant El Nino years we've had in a while (with the potential to become one of the strongest on record), we've still managed to have some tropical storm activity in the Atlantic.  So far there have been eight named storms, only two of which have actually reached hurricane strength.  Depressed tropical cyclone activity in the Atlantic is expected in strong El Nino years, so the small number of hurricanes so far isn't too surprising. (There's still a ways to go in the hurricane season yet, though...)

However, every few days it seems we get a new "feature" in the far eastern Atlantic that the National Hurricane Center  identifies and keeps an eye on as it moves to the west across the ocean.  These features are called "tropical waves" and they happen all the time.  The NHC forecast discussions have an entire section devoted to describing the current "tropical waves".  So just what is a "tropical wave"?

When I was much younger, I would hear them talking about tropical waves on the Weather Channel and picture something like this:

And, admittedly for a very long time, I thought that hurricanes ultimately came from these giant waves on the surface of the ocean that somehow triggered hurricanes above them.  Let's just be clear here...while a warm ocean surface plays a tremendous role in supporting hurricane development, ocean waves are not what we mean by "tropical waves".  Let's look a little higher up in the atmosphere.

To understand what makes a tropical wave, we have to understand the air pressure patterns on the earth's surface.  Some of you may be familiar with what's called the "Hadley Circulation": the large scale motions of the atmosphere that dominate the tropics.
In short, near the equator it is very warm and there is a lot of convective (thunderstorm) activity.  All this warm air rising in the thunderstorms migrates away from the equator at higher altitudes before sinking again in the "subtropics"--about 30 degrees north and south of the equator.  When all that air rises near the equator, it creates lower pressure near the surface (think of all that air lifting up and away from the surface...the pressure you feel pushing down on you is lower).  Oppositely, the sinking air in the subtropics tends to create higher pressure in those regions.  Pressure again decreases as you get towards the northern mid-latitudes.  The result is a series of pressure "belts" that circle the earth in a mean sense:
These belts are an "average" condition of the atmosphere.  When we're looking at weather (particularly stormy weather), we're interested in the deviations from this average.  We have "troughs" where the pressure is lower than normal and "ridges" where the pressure is higher than normal.  This are where the idea of "waves" in the atmosphere come from---they are perturbations along these average, static belts.

Since storms are typically associated with lower pressure, we tend to look for "troughs" in the flow when we care about storms.  In the mid-latitudes (some 30-60 degrees N or S; the latitude band of the continental US), we live in a zone where there is typically higher pressure to the south and lower pressure to the north.  Therefore, our troughs tend to show up as perturbations extending down from north.  They "perturb" the average pressure pattern in a way that looks (and behaves) like a wave.  Here's an old Weather Channel map showing what this looks like in an idealized sense:
Note the "waviness" of the flow shown by the jet stream as it navigates around these areas of higher and lower pressure.  These are the "waves" we are talking about when we're talking about the atmosphere.  Not ocean waves, but pressure waves.

As I mentioned above, when we look at maps like this in the northern hemisphere mid-latitudes, our terminology makes sense.  A high pressure "ridge" looks like a "ridge" on this map, reaching up toward the north.  And a low pressure "trough", looks like a deep "trough" being dug down into the flow.  That's where this terminology comes from.

But near the tropics, remember that the pressure pattern is different.  There is lower pressure near the equator on average with higher pressure to the north.  We still look for troughs of lower pressure to cause storms---the same mechanics are at work there.  But the way troughs look is different: with low pressure to the south and high pressure to the north, a low pressure "trough" would actually be digging UP from the low pressure near the equator into the higher pressure to the north.  Here's a Wikipedia image that shows what these sorts of troughs look like:

The troughs are "upside down", but we still call them troughs.  Sometimes you'll hear them referred to as "inverted troughs".  This is one of the hardest things to explain to our Weather 101 students---a trough of low pressure doesn't always look like a "trough" on the map.  In the tropics they are upside down.

However, regardless of how they are oriented, it still makes the flow "wavy".  And that's where we get our "tropical waves".  They're still troughs of lower pressure, but they're just oriented in the opposite sense of what we are used to.  They also move from east to west, giving them another name: "easterly waves".   These areas of lower pressure extending north into the subtropics provide the seeds for eventual development of tropical storms and hurricanes, and they can also bring stormy squalls just on their own.  For weather forecasting in the tropics and subtropics, monitoring these tropical waves is essential.

On a side note, it actually is also possible for us to get "inverted" pressure troughs here in the mid-latitudes.  In fact, it's a common occurrence on the west coast in the summer, where you may have heard of the "thermal trough".  But that's an entirely different blog post...

Tuesday, August 4, 2015

El Nino and Precipitation--Depends on Who You Ask

I came across an interesting divergence in opinion on what the impacts of El Nino on wintertime weather here in the Pacific Northwest might be.  First, a reminder that El Nino conditions are present in the Pacific Ocean.  Here are the latest weekly sea-surface temperatures, with anomalies on the bottom.  The next several images are from the Climate Prediction Center's ENSO page.
It's a classic El Nino signature with a tongue of warmer-than-average sea-surface temperatures stretching westward along the equator from western South America.  One way we often track the strength of El Nino over time is to look at something called the Nino 3.4 index, which is basically the average sea-surface temperature anomaly in the central Pacific.  Here's what that index has looked like over the past year.  The Nino 3.4 index is the second panel from the top.
Positive Nino 3.4 values indicate warmer than normal sea-surface temperatures.  Since about mid-March of this year, the Nino 3.4 index has been climbing steadily, now up to near 1.7.  Anything above 1.0 is considered an El Nino event, so this is definitely a legitimate, full-blown El Nino.

We have dynamical and statistical models that try to forecast what the Nino 3.4 index will be like for the coming months.  Here are those model projections, though they are almost a month old:
These forecasts are for three month periods, given by the letter abbreviations at the bottom on the x-axis.  You can see that by the late fall into winter (around the OND, NDJ, and DJF time) the Nino 3.4 index is predicted to peak.  There's a lot of spread, but the average maximum is between 2.0 and 2.5.  Keep in mind that the 1997-1998 El Nino event---the largest we have on record---peaked at a Nino 3.4 value of 2.3.  It's definitely possible for us to meet or beat that strength...

One of the major reasons we care so much about El Nino is that it is one of the few phenomena that can give us information about what the large-scale weather patterns will be like months in advance.  We've gotten rather good at synoptic-scale weather prediction, with forecasts good out to 10 days or so.  We also feel we have a reliable handle on large-scale climate impacts on the scale of years to decades.  But it's that in-between area---regional variability in large-scale weather patterns on the orders of weeks to months---where we really don't have much predictive skill.  Thus, anything that can give us a signal as to what may happen in that time frame is highly valued, and El Nino is the best tool we have.

We can look at what happened in past El Nino winters to get a good idea of what might happen this year.  For instance, here is a map showing the average precipitation anomalies in November through March of  El Nino years from NOAA ESRL:

We see that during El Nino years, California has usually been much wetter than normal, as has the Gulf Coast.  Interestingly enough, El Nino tends to suppress Atlantic hurricanes, so the extra rain on the Gulf Coast is NOT due to more tropical cyclones---just a rainier winter pattern.  Where does El Nino mean drier conditions?  Here in the Pacific Northwest, it seems, and also to some extent in the Midwest.

But's another map from the Climate Prediction Center that is supposed to show *almost* the same thing.  The upper left panel of this figure claims to show precipitation anomalies for December through February of major El Nino years:

Some features are very similar---we still see the increased precipitation in California, along the Gulf Coast and up the eastern seaboard.  But note the Pacific Northwest---here we see the opposite connection: significantly wetter than normal during El Nino years!  How can these two maps give such different results?

There are a couple of things that are different.  One is that the first map gives anomalies for Nov. through March whereas the second gives anomalies for Dec. through Feb.  But, there is a significant degree of overlap between the two (and actually looking at other maps from the CPC that include Nov. and March show the same "wetter" pattern).

Another is the number of cases used.  Both plots list the years that they used to make these composite averages.  The first uses what looks like only nine winters, while the second uses 22.  Looking at the strength of the El Nino events in these years, the second plot uses several weaker El Nino events not included in the first one.  So that could affect things.

We also have to pay attention to the different baselines used for making these plots.  Remember these plots are showing average departure from normal.  But what is "normal"?  In the top plot, they say that they are comparing to the 1971-2000 average.  In the second plot (if you find it on the CPC website), they are using a 1981-2010 average.  So both maps have a different view of what is "normal".  I pulled the NCEP reanalysis data for precipitation and plotted up the average Dec. -  Feb precipitation for both 1971-2000 and 1981-2010.  Here's a map of the percent change between the two:

In this plot, blue means the average precipitation was higher in 1981-2010 than in 1971-2000 while red means it was lower.  Over the Pacific Northwest there's not much of a difference.  The later time period may be slightly drier, which could maybe a explain a little bit of why the El Nino anomalies from the second plot showed it more likely to be wetter in El Nino years.  But it doesn't look like a big enough change to account for the differences we saw in the above plots. 

So the differences probably come down to the second point---using different years for the composite.  We probably don't really have enough samples to reliably draw a connection between El Nino years and what happens for Pacific Northwest precipitation during the winter.  It seems that in very strong El Nino years (as this one looks like it might be), we may tend to be drier than normal, as seen in the first plot.  But, as we include weaker El Nino years (the second plot), the signal becomes more muddled and can actually show the opposite.

One thing both of the above sources agree on, though, is warmer than normal temperatures during El Nino winters in the Pacific Northwest.  Probably not so good if you're looking to ski...

Though these anomalies are strongest during the winter months, we're already starting to see early signs that our precipitation patterns are beginning to look El Nino-like (even though we're still definitely in the summer).  Here are the most recent 90-day precipitation anomalies:

Wet in southern California and also into the southern Plains.  Not so much of a signal on the Gulf Coast, but, then again, it still is summer...

Monday, June 15, 2015

Even the tropics are aiming the faucet for the central US

After the eastern Pacific got off to an early, rapid start with its tropical storm season (which isn't unexpected in an El Nino year), we're looking at our first tropical storm developing in the western Gulf of Mexico in the next 12 hours.  On satellite, this storm has been getting its act together:
And the National Hurricane Center thinks we'll likely see Tropical Storm Bill by tonight.  This storm is forecast to run out of ocean really fast, though, and hit the Gulf coast of Texas tonight and tomorrow.   From Weather Underground, here are the recent 18Z GFS ensemble forecasts for the track of this cyclone over the next several days.
You can see that there's pretty high agreement that the storm will cross central Texas before curving north through Oklahoma then back northeastward across Missouri and Illinois and into the midwest and mid-Atlantic states.  Of course, almost as soon as this storm hits land in Texas it will stop being a tropical cyclone and turn into an extratropical cyclone.  Though this means weaker winds, the storm will still bring a lot of moisture into the heart of the country.   Here's an animation of the satellite-derived total precipitable water in the atmosphere over the last five days.  You can see towards the end of the animation that the developing storm in the western Gulf of Mexico (with its characteristic spiraling motion) is bringing in lots of moisture towards the Texas coast.
That moisture will move inland over the next several days.  Here's the 18Z NAM forecast precipitable water for tomorrow afternoon (from HOOT):
 Lots of moisture over Texas and Oklahoma, with residual moisture from the current frontal zone  across the Ohio River Valley.  Looking ahead to Wednesday afternoon there is still a lot of moisture forecast over eastern Texas and Oklahoma.
And even by Thursday afternoon, east Texas is still looking at a ton of moisture, though we're also seeing a lot of liquid move north into Arkansas and Missouri.
Not unsurprisingly, the Weather Prediction Center (NOAA/WPC) is forecasting some intense rainfall amounts over the next three days, with a swath over 6 inches from Texas into Oklahoma and western Arkansas, and over 3 inches from Missouri into central and southern Illinois.
It's interesting to note that, despite this storm being a tropical cyclone, its ability to influence the larger-scale synoptic pattern is somewhat limited.  Notice how the storm path follows a nice, sweeping curve around the southeastern United States.  Why is that?  Because we have a relatively modest ridge of high pressure/high heights sitting over the southeastern US, as seen in this 24-hour 500mb height forecast from the NAM:
Though that ridge looks so weak (it only has a single height contour!), there's actually still more potential energy sitting in that ridge than kinetic energy in a tropical cyclone.  So, even though it seems to be a strong storm to us, our tropical storm is still a slave to the synoptic pattern, and that "weak" ridge over the southeast will "shield" that area from most of the effects of this storm.

Unfortunately, it's that same ridge that is directing the storm over areas that really don't need any more rain right now.  Here's a map of the percentage of normal precipitation during this May.  Much of Oklahoma and Texas received 200-500% of their normal precipitation for May.
Soils in that area are also saturated.  Here's a map of soil moisture percentiles as of the end of May.  Much of eastern Texas and southern Oklahoma is in the 99th percentile for soil moisture...that is, soils likely contain nearly record amounts of moisture for this time of year.
Add on top of this the fact that several major rivers throughout that corridor (and also into Missouri and Illinois) are currently at flood stage from all the rain we've already gotten recently (the orange and red dots on this map)...
And you have a recipe for a flooding disaster across a large swath of the central US.  Flood watches are already up for most of eastern Texas and eastern Oklahoma, with hydrologic outlooks suggesting at future flood watches going up throughout Arkansas, Missouri, Illinois and Indiana as well.  If you live in these areas, you need to be paying attention to the weather this week and be alert for possible flooding.  This is going to be the weather story of the week.

Tuesday, May 12, 2015

The high desert gets soaked, but not as bad as Oklahoma

After days and days of high-precipitation thunderstorms with embedded tornadoes and large hail, Oklahoma is finally getting a bit of a break.  Just how wet was it there?

The Oklahoma Mesonet recently published rainfall totals for the past two weeks.  Here's what it looked like:

Huge rainfall totals across the state.  Over 12 inches in some locations in only 2 weeks.  The annual percentage of normal rainfall has spiked through the roof.  So far through the roof, in fact, that it's maxing out the colorbar on the Oklahoma Climate Survey plot, with most places greater than 180% of their total rainfall.
Needless to say, the latest monthly outlook from the Climate Prediction Center has drought conditions in Oklahoma and Texas "improving" over the next month...
But drought persists elsewhere---California is seeking no help and the persistent troughing has kept most of the heavy rain-makers to the south of the upper midwest, causing drought conditions to develop in Minnesota and the Dakotas.

Eastern Washington and Oregon have also been running a bit low on their precipitation and are in "drought" conditions as well.  A warmer-than-average winter has allowed the western side of the Cascades to see about normal precipitation, but without much snowfall accumulating in the mountains.  This is bad news for the eastern sides of the states, where they don't see as much precipitation, but rely on meltout from the snowpack to supply them with water for agriculture.

Here's a plot showing the snow water equivalent normals and what we've had this year at Stampede Pass, which is in the Cascades southeast of Seattle.
The light blue line is the normal Snow Water Equivalent, or the mass of water in the snowpack.  The dark blue is this year.  You can see that by this time of year we should be just dropping down from our peak snow in mid April, but this year we're already at zero snow.  Looking at a map of percentage of average snowpack as of last week, we can see pretty much everyone is basically melted out (or at least below 25% of where they should be).
But there is some hope for the eastern side of the mountains!  An upper-level low is churning its way through Oregon at this time.  It's moving slowly and a bit further south than usual, and this is allowing a fair amount of moisture to get to the eastern sides of the Cascades.  Here's the latest water vapor satellite image.
We can see the swirl of the low with its attendant moisture streaming up through Nevada and into Idaho.  That low is forecast to slowly lift north-northeastward over the next couple of days.   With cold air underneath that trough, we're expecting some destabilization and a few rounds of thunderstorms to develop.  All of this in an area that does not get much rain to begin with.  Eastern Washington and Oregon are basically deserts; here's a map (from the PRISM group) of annual average precipitation across Oregon.  Much of the high desert east of the Cascades sees 10-15 inches or less of rainfall per year.
Here's a similar map for Washington, with the same story away from the higher terrain:
Our middle- to long-range forecasts are calling for several inches of rain by the end of the week east of the Cascades.  This is an unusual setup.  Here's the total rainfall accumulation by Friday evening from this morning's local WRF model run.
Large swaths of area getting 1-3 inches of rainfall.  Great for farmers!  It may seem like these numbers are small in comparison to Oklahoma, but keep in mind that these are places that are lucky to get a tenth of an inch of rain with a good convective shower.  There are usually a few thunderstorms that occur throughout the year that will give a more thorough soaking; this will be one of those periods.