Tuesday, May 8, 2012

The "simplest" kind of weather model

When we talk about our weather models these days we're talking about complex systems of equations coded into computer programs that are expensive and time-consuming to run.  There's so much detail in what goes on in the atmosphere that it takes so much memory storage and processing power to compute forecasts for the time period we're interested in and at the detail we want.  Models that I show on this blog like the GFS or the WRF model contain hundreds of thousands of lines of code and can take supercomputers to run them efficiently (particularly in the case of the GFS).  But do we really need all of this power?  If we're willing to settle for a bit less detail in our models and trust our meteorological instincts some more, just how well can we make a forecast?

Do we really need big supercomputers and fancy, complex codes to make decent weather predictions on a global scale?

By doing a lot of simplifying, we can actually get a fairly good idea of the general flow in the atmosphere from some very basic models.  Today I'm going to talk about what is probably the "simplest" model that can realistically model atmospheric flow around the globe--the barotropic model.

The barotropic model was the first kind of numerical weather model ever successfully implemented--it was based on work by Charney in the 1940s.  You can see their original paper describing the work at this link.  So what goes into a barotropic model?

First, what does barotropic even mean?  Meteorologists use that word to describe an environment where pressure is only a function of temperature or density.  This means that if we were to look at, say, the 500mb surface in a barotropic environment, we'd be assuming that the temperature (or density) on that 500mb surface is everywhere exactly the same.  In other words, there are no temperature gradients on constant pressure surfaces.  Because of that, we don't have to worry about the affects of cooling or heating or temperature advection.  Immediately you'll notice that we run into problems--the entire atmospheric circulation is driven by heating by the sun, and yet we're ignoring heating?  How are we going to make things happen in this model?

Well, in some ways, we don't make things happen in this model--there are no features that add energy to the atmosphere in this model, or in other words there are no forcings.  In the barotropic model, since there are no external forcings, all we're really doing is taking the current state of the winds in the atmosphere and letting them blow until they blow themselves out.  The only thing we keep around is the fact that the earth is turning, so admittedly there is a Coriolis "force" present.  But there are no mountains causing lift, no ocean/land differences, and no solar heating.

"Well this is silly," you might think.  "How can we make any kind of decent weather forecast without having any impact from the sun or the land or anything?"  It turns out that, on the large scale, atmospheric motions in the relatively short term (the first few days or so) are pretty well dominated by this barotropic motion--the simple continuation and evolution of the flow without external forcing.  This has been known since the work of Carl Rossby in the 1930s.  I'm going to show examples of how well this actually works here today.

Based on a description of a modern-day implementation of the barotropic model from Issac Held and collaborators at Princeton, I coded up a "simple" global barotropic model in Python.  You can see animations of the output from the model on my webpage:


Though I warn you it's not updated that regularly, as my initialization data comes in several days late.  Still, it's there...and kind of fun to watch.

All this model does is predict the future wind, vorticity and height anomalies of the 500mb pressure surface.  That's it.  Here's how the barotropic model basically works:

  • We start with a global field of vorticity--that is, a measure of how much rotation there is in the atmosphere.  We're talking rotation on large scales--not small scale tornado-style rotation.  Remember in our barotropic assumptions above that we assumed that density or temperature was the exact same everywhere on a constant pressure surface.  If density is everywhere the same, we can't have any place where the winds are pushing air together to increase the density.  This means that we make an assumption of incompressibility which leads to an assumption of non-divergence--the winds everywhere can never be divergent or convergent.  If we make this assumption and we know the degree of rotation in the air (the vorticity), we can back out a wind field.  Here's an example of the global vorticity and non-divergent wind field I use to initialize my model:

Focus on the northern hemisphere in the image above.  Blue areas are areas with positive absolute vorticity--wind around them blows counterclockwise--think troughs or low centers.  Red areas are areas with negative absolute vorticity--wind around them blows clockwise--think ridges or high centers.  By going through and looking at how strong the vorticity gradients are, we can figure out how strong the winds should be, and we know what directions they're going based on the shape and orientation of these vorticity areas.  It's actually pretty simple to figure out if you know what you're doing.
  • Now that we have our wind field and vorticity field at the current time, we can use the wind field to advect the vorticity field.  That is, we know what the winds are right now, so we use the winds right now to push the vorticity values around for a certain amount of time.  In my model, I have a 15 minute time step, so I assume the winds are constant and use them to push along those blobs of vorticity for 15 minutes.
  • After doing this, I have a new vorticity field that has been advected around some.  However, I still have the old wind field.  Here, the model stops, takes that new vorticity field and computes the new wind field that corresponds with that new vorticity field, just like we did in the very first step. Now I have a new wind field that matches the new vorticity field.
  • From here, the model just repeats those same steps over--it takes these new winds and uses them to push the new vorticity around again for another 15 minutes.  Then we stop, recompute the wind field from the next vorticity field, then go on.  We can keep going for as long as we want...
So you see, all the barotropic model does is use the winds to basically push vorticity around, which gives new winds that push the vorticity around some more, and so on.  Once again, to reiterate, this model has:
  • No temperature or density gradients
  • No fronts
  • No mountains or terrain
  • No oceans
  • No heat sources from the below
  • No heat from the sun
  • No divergence or convergence
  • No rising or sinking motion
So how well does it do?

Well, here are some comparisons.  One thing we can also back out from the wind and vorticity fields is a field called the "streamfunction", which, for all we need to talk about here, is like a height anomaly map.  It highlights where there would be troughs and ridges on the 500mb map, were I actually forecasting the 500mb height.

On the left below is the 18 hour forecast of 500mb winds and that streamfunction/height anomaly field (the black solid and dashed contours).  On the right is the actual 500mb map from that time.
As always, you can click on the image to make it pop up bigger.  I've highlighted the major trough axes on both of the images just for reference.  You can see that, at least in terms of the placement of ridges and troughs, it's actually doing very well in the 18 hour forecast.  It has a deep trough over the western Gulf of Alaska, and the tightly-packed height contours on the actual 500mb map do imply that there are strong winds through the base of that trough, just like the barotropic model is predicting.  The model also does a good job forecasting the elongated trough over the western US and the compact upper-level low over northern Quebec.  It's a surprisingly good forecast for a model that has no real external forcing.

Let's go a bit further out into the forecast.  Here's a comparison of the 44 hour forecasts:
In many ways the forecast is still very good.  The Gulf of Alaska trough is in about the right place with strong winds right where they should be.  That little upper-level low over northern Quebec is also still being handled rather well.  However, we can start to see differences creeping in.  Notice that the elongated trough over the western US doesn't look quite right in the model.  The barotropic model has kept the trough much more compact and has it centered over the western plains whereas, in reality, the trough still extended quite far back off to the southwest.  Remember that in real life the Rocky Mountains stretch across the western US, but the barotropic model has no concept of mountains or topography.  So, any impact the high terrain would have on the flow is totally beyond anything the barotropic model would predict...

Finally, the 66 hour forecast comparison:

Now at almost three days out, things are beginning to fall apart.  The barotropic model still has that northern Quebec low in about the right position with a wind maximum in the right place, so at least that's good.  But now that Gulf of Alaska trough isn't in the right spot--the barotropic model has moved the trough to fast to the east, placing the trough axis over the coast whereas in reality the trough stayed cut off and sort of hung back in the central Gulf of Alaska.   That western US trough is almost totally absent from the barotropic model--the barotropic model kept its more compact, lower amplitude trough and moved it slightly east, centering it over the Mississippi valley.  In reality, the trough stayed elongated to the west and even formed a cutoff low off the coast of California--something not at all present in the barotropic model.

So we can see that, at least for the first day or two, the barotropic model with no real forcing actually does a fairly good job at predicting the major upper air pattern--not that bad for a very simplistic model. However, since there is no vertical motion, no divergence, no heating, etc., the barotropic model really cannot capture the formation and development of new troughs very well.  That cut-off low that formed by 66 hours is a prime example--the forcings that led to that cut-off low being formed were just not present in the barotropic model, so it missed it.

So we'll stick with our far more complicated global models that actually include all those things we ignored for now.  They may be a lot more expensive to run, but they're a bit more reliable beyond 48 hours.  As an interesting comparison, looking back at the history of those first barotropic numerical weather models, the first 24-hour global barotropic model forecasts on the ENIAC computer took 24 hours to run in the 1950s--not much of a forecast if you don't get it until right when it's happening!  By comparison, my barotropic model integrates out to 120 hours in about 20 seconds on my desktop.  Amazing how far computers have come...

Tuesday, May 1, 2012

Using Doppler, dual-pol radar to interrogate storms

Last night there were some pretty impressive-looking storms that moved across northern Oklahoma, dropping a few tornadoes along the way.  I pulled a few screenshots of radar images from these storms to look at a lot of the helpful and hazardous things that our radar system gives us.

First, one of the primary advantages of having a Doppler radar--that is, a radar that uses the Doppler effect to measure wind velocity--is that it lets us more accurately see areas of rotation within thunderstorms.  Before we had Doppler radars (and we just had regular radars), all we had was reflectivity to look at.  Sometimes we got lucky and tornadic storms would produce a well-defined "hook echo" that let us know where a tornado was likely to be.  Here's an image (borrowed from Cliff Mass's blog, where he borrowed it from someone else) of an old-style, 1960s era radar image:

This works out great if you have a storm that has a well-defined hook echo structure.  But we don't always see clear hook echoes in tornadic storms.  Take a reflectivity image from last night, for example.

There are tornado warnings out for these storms and storm spotters reported a tornado on the ground with at least one of them at this time.  But where is the tornado?  There are a few candidate locations that look a little like hook echoes, but nothing too clear.

This is where being able to see Doppler wind velocities is really helpful.  Here's the base velocity image from this time:
I've circled the areas where there are nice velocity couplets that clearly show locations of strong circulation where there could potentially be tornadoes.  If you want to know a bit more about how to look for rotation in these radar velocity images, I wrote a blog post some time back about it that can be found here.

So getting Doppler radars was a huge boon to our ability to forecast and warn for tornadoes.  In fact, a lot of research suggests that adding Doppler capabilities to our radars is the single most important advance we made to increase our warning lead times and probability of detection of tornadoes.

But now we've added even more capabilities to our radars--this whole dual-polarization business.  What can dual-pol show us about these thunderstorms?

If you go through the training material for dual-pol, there are a lot of nice, relatively clean examples that are given that show you how to differentiate between things like rain, hail and even tornadic debris using dual-pol products.  However, in my experience, real-world interpretation of these products is often not nearly so clear-cut.  There are some things that dual-pol nails every time and other things that it doesn't.  Let's start looking at the dual-pol images from the time I showed above.

We'll start with differential reflectivity.  In dual-pol parlance, this is simply the difference between the strength of the horizontally-oriented radar return and the vertically-oriented radar return (eventually I'll write a more detailed blog post about all of this).  This means that the bigger and fatter objects (like, big raindrops which tend to flatten out as they fall) will have higher ZDR (differential reflectivity) values.  Nearly spherical objects or objects that are tumbling as they fall (like hail) will tend to have much lower, near zero ZDR values.  If you have a lot of different objects in an area (like, for instance, if there is tornadic debris being thrown around) we'd expect to see very noisy ZDR values.  Here's my annotations on the ZDR radar image for the time above:

There's a lot going on in that image.  Let's start with my top comment.  If we compare this to the reflectivity image above, you'll notice that in an area where there's a lot of high reflectivity to the north of the potential tornado, we see low, zero, or nonexistent ZDR.  This can happen if the radar beam is traveling through so much material (so much rain or hail) that too much of the beam gets absorbed to make reasonable inferences about what we're actually getting back from the radar beam.  There's just too little left to work with.  This is a phenomenon called attenuation--the absorption of the radar beam by some material that decreases the radar beam's power and resulting sensitivity.  I'll get back to that again later.  For now, it's somewhat disappointing that this northern part of the storm seems to be missing a lot of signal.

Just south of that, closer to the core of the storm, we have a patch of higher ZDR--on the order of 6-7 dB.  We know that higher ZDR values ten to correspond to big, flat objects like big raindrops. So we could probably infer that there was a very heavy downpour going on in the middle of the storm.

But what about the area right near the potential tornado?  The ZDR values are somewhat weaker there, though still slightly positive.  We know that near-zero ZDR values are typical of hail, so maybe there is hail there?  They also look a little noisy, and the base velocity image suggests there could be a tornado there.  Perhaps this is a debris signature?  It's really difficult to tell.

Let's try looking at another dual-pol image for help--the correlation coefficient.  Correlation coefficient basically tells us whether or not everything the radar beam is bouncing off of in that area of space is the same thing or not.  If we have high correlation coefficient, we're probably looking at all rain or all hail.  Lower correlation coefficients mean more of a mix of different precipitation types.  Really low correlation coefficients mean that what the radar beam is bouncing off of is probably not normal precipitation and is instead something like debris, bugs, dust...

Here's the correlation coefficient image from the same time.

Let's start with that area to the north of the core of the storm again, where we saw the low to nonexistent ZDR before.  We see here that that area is full of very noisy, lower correlation coefficients. This could either mean that we're seeing a lot of different sized and shaped objects floating around in that area, but another way to get low, noisy correlation coefficient is for there to be a very weak radar signal.  This is consistent with my early statement that the radar beam is probably somewhat being attenuated in that area.

Looking back down toward the area with the potential tornado in it, we see that the correlation coefficient is still quite high in that area.  This isn't very consistent with debris, as tornadic debris is all different shapes and sizes and should have a lower correlation coefficient.  So, high reflectivity with lower ZDR and high correlation coefficient makes me lean more toward hail being seen in this area.  But, it's very difficult to tell.  Even though we get so much more information from these dual-pol products, it still can be very difficult to interpret.

Let's look at a slightly later time when the storms had merged more together.  Our old friend reflectivity can show us a lot about the storm structure:

One of the striking features of this reflectivity image is the clear outflow boundary seen on reflectivity.  This is a narrow band of slightly higher returns that's making an arc shape out in front of the main storms.  An outflow boundary is the leading edge of the cool, downdraft air that accompanied the rain falling out of the thunderstorms.  As this cool air falls to the ground with the rain, it spreads out and races along the ground out away from the storm.  Those higher returns along the leading edge of the boundary are not actually caused by rain--they're caused by dust, dirt, insects and birds that are caught along the leading edge of the advancing air.  Want proof of this?  We only have to turn to the correlation coefficient image from the same time:
Remember that hydrometeors like rain and hail tend to be rather highly correlated--they're the same everywhere.  Even in mixed precipitation the correlation only drops a small amount.  But other things that are not so evenly shaped--things like dust, birds, insects--those show up as very low correlations.  You can see int he correlation coefficient image above that the outflow boundary is completely missing--the correlations of those returns are so low that they're actually off the color chart.  In fact, a lot of the clutter off to the west of the storms also basically disappears in correlation coefficient.  This highlights one of the best uses of correlation coefficient--to differentiate between what's actually precipitation and what's not.

Here's a look at the velocity image from this time.  Note the strong velocity couplets where there are potential tornadoes.  I've drawn in arrow to help show where the radar says the air is moving--remember green is toward the radar and red is away from the radar.  You can also see how the air behind the outflow boundary is moving to the south, out away from the storms.  There's also strong convergence right along the outflow boundary.

One final thing to look at here.  Sometimes when a radar gets hit with a particularly heavy downpour, the dome around the radar gets coated with a thin layer of water.  As the radar beam tries to pass through the radar dome, some of it gets absorbed by that layer of water, creating more attenuation.  Since the radar beam suddenly loses power as soon as it crosses through the layer of rain, it causes the beam's sensitivity to drop--we have less power going out, so there's even less power that can be reflected back.  This can make radar echoes suddenly appear weaker as a heavy downpour moves over the radar.  This happened last night.  Here's a reflectivity image from right before a downpour hit the radar:

Now watch what happens to the radar returns in the circled area in the next radar scan when there is a downpour right over the radar:

The strength of the return suddenly drops off!  This happens actually quite frequently, and we often call this the "wet radome effect".  Something to watch for if you suddenly see the radar echoes all weaken dramatically.

Wednesday, April 25, 2012

Connecting height changes to weather changes

After a two-month hiatus, I'm back to writing blog posts.  While still a graduate student at the University of Washington, I'm now on temporary assignment at the National Center for Atmospheric Research in Boulder, Colorado.  And, now that I'm settled in, I can get back to blog writing...

Often in forecast discussions we talk about "height rises" and "height falls" as a way of quickly summing up what is going on with the overall weather pattern.  It turns out that the changes in the height of pressure surfaces aloft are related to changes in the wind and temperature, which in turn have ramifications for the potential for rain and storms.

First, let's remind ourselves about what these "heights" are measuring.  Remember that the air pressure decreases as you go up in height.  In meteorology it's often convenient to talk about the pressure structure aloft, so we talk about the atmosphere in terms of how far up you have to go (above sea level) until the pressure has decreased to a certain amount.  We refer to these values as the "heights" of various pressure levels.  500mb is a common pressure level for us to talk about, as it's about "halfway" up the troposphere.

"Heights" of various pressure levels are simply how high you have to go above the ground until the pressure falls to that particular value.

Every time I show an upper-level chart (like the 500mb chart), we're looking at a map of how high 500mb is above sea level across the country.  When we talk about troughs and ridges, we're talking about areas of lower and higher heights, respectively, of the 500mb surface.
Sample 500mb map showing where 500mb is relatively lower (troughs) and where 500mb is relatively higher (ridges).  The "heights" are a description of how high or low the 500mb surface is above the ground.
So, the easiest way to think about "height rises" and "height falls" is that these just describe whether a trough or a ridge is approaching.  If a trough is approaching, the heights will be falling.  If a ridge is approaching, the heights will be rising.  Since we associate troughs with unsettled weather (or, even just the transition from a ridge to a trough), the simplest way to think about interpreting height rises and falls is that height falls foretell  an approaching storm, whereas height rises often mean clearing, calmer conditions on the way.
500mb map from the SPC mesoanalysis at 17Z, Apr 25, 2011.  The 500mb heights are contoured with black lines.  Areas where there have been height rises over the previous 12 hours are shaded and contoured in red.  Areas where there have been height falls over the previous 12 hours are shaded and contoured in blue.  Notice a trough (low heights) over New England with a big ridge (high heights) over the Rocky Mountain corridor in the west.  As that trough moves out and the ridge (with its higher heights) moves in, we see a large area of hight rises being shown over much of the eastern US.

But we can look a little bit deeper into connections between these heights and temperature.  It turns out that heights and temperature are intimately connected.  It's pretty common knowledge (or, I think it should be...) that as you warm air it expands and as you cool air it contracts.  If you throw sealed containers onto a fire, they can explode because heating the contents causes them to expand.  It's basic chemistry.

The same thing happens on a larger scale in the atmosphere.  When we have warming in some level of the atmosphere, it causes the air to expand outward from the center of that warming.  As the air expands, it changes the pressure structure.  For instance, let's imagine our atmosphere like I showed in the first figure above, but this time add warming in the lower atmosphere.

As the air below heats up, it expands outward, increasing the pressure above it.  This causes the heights of the pressure surfaces aloft to increase.  You can think of the expansion due to warming "pushing" up  the heights of the pressure surfaces above it.  Therefore, height rises are usually associated with warming below.

The opposite happens when there is cooling.  As an area of air cools, it contracts and the pressure decreases on the air above it.  This contraction pulls the heights of the pressure surfaces above down, leading to height falls.  So, height falls are usually associated with cooling below.

Things get more complicated if the warming or cooling is in the middle of the atmosphere.  For instance, if there is warming in the middle of the atmosphere, the air expands--both up AND down.  this has the affect of increasing the heights above the center of warming, but  decreasing the heights below.  So it can get rather complicated.

It gets even more complicated when we consider heating right at the surface.  If the surface heats up, the air expands in the only direction it can expand--upward.  This has the effect of decreasing the pressure at the surface. Then we also have to worry about how the heating is oriented in the atmosphere--it turns out that these warmer and cooler columns tend to tilt westward with height, so heights are increasing in some places and decreasing in others.  There's really a lot going on...

But, let's go back to our basic picture.  Remember that height falls tend to precede troughs which can be associated with storms. In particular, lets think about situations where we might be expecting thunderstorms. To get thunderstorms, we have to have an atmosphere that is (conditionally) unstable.  To destabalize the atmosphere, we have to increase the lapse rate (that is, make the temperature cool off rapidly with height).  Two basic ways to do this are to either increase the heat (and/or moisture) near the surface, or to cool off the atmosphere above the surface.

So let's consider a forecast discussion that says, for example, "mid-level height falls are occurring over a region with very warm surface temperatures and high dewpoints".  If we know that there are mid-level height falls, then from our discussion above we know there should be cooling going on below those mid-levels.   This is a great case for instability to increase, because we have a warm moist surface, but the height falls imply that the temperatures are cooling off just above the surface.  That will increase the lapse rates and make it easier for thunderstorms to form in the more unstable environment.

What about the opposite statement -- "height rises are occurring over an area with warm surface temperatures and high dewpoints".  Initially you might think that the warm, moist surface would be good for thunderstorms.  However, if height rises are occurring, that implies that there is warming going on just above the surface.  This is not good for increasing the instability--it's doing nothing to steepen the lapse rates in the right direction, and could even create an inversion (where temperature increases with height).

So that's a simple look at what height rises and height falls mean.  You can look at them as just signals of approaching ridges and troughs.  But, they also have importing implications for how the temperature is changing aloft which, in turn, can affect chances for thunderstorms.

Thursday, February 23, 2012

A "typical" winter storm

As most people in the central US are probably well aware, a powerful winter storm will be moving across the area over the next few days.  In fact, it's already pretty well developed.  Here's the surface analysis at 17Z this morning (11 AM CST):
We see a very well-developed low over western Missouri, with strong pressure gradients to the northwest and southeast of the low.  Strong pressure gradients mean strong winds, and already we've had damaging winds reported across eastern Colorado and western Kansas.  The tightening pressure gradient on the southeastern side of the low is going to help increase the winds out of the south, helping to advect very moist air out of the Gulf of Mexico and into the southeast.  Right now, the cold front doesn't look too impressive behind the low--in fact, as the low is analyzed now, it looks to be in the middle of the warm sector with only a weak frontal wave evident in the temperature field.  As more cool air pours down the high plains, though, this low should only become better organized.

Here's this morning's 500mb GFS forecast at 18Z this morning (12 PM CST).  Notice that the surface low over western Missouri is in a region between two jet streaks, though the streaks themselves are not too impressive.
However, as cool air continues to move down the high plains to the west of the low, it's going to encounter warm air in the 60s and 70s over Oklahoma and north Texas.  This is going to greatly strengthen the temperature gradient there, sharpening the cold front even further.  We see a response to this sharpening temperature gradient below in the 500mb wind forecast.  Notice how just 6 hours later in the GFS forecast at 00Z tonight, the jet streak over the southern plains looks much stronger:
As that jet streak strengthens, the upper-air flow becomes favorable to support the surface low deepening even more as it moves eastward across central Illinois.  By tomorrow morning, the surface low is forecast to be well-organized, with clear warm and cold fronts:

We're expecting a lot of different precipitation types with this storm.  First--snow to the north.  You'll notice on the surface map progression that north of an Iowa-central Illinois-Detroit sort of path the surface temperatures don't look to get much above the low 30s.  While surface temperatures would seem to be cold enough to support snow, we need to look at the temperature of the atmosphere above to be sure that the whole mass of air above is also cold enough to support snow.  We might suspect this is so, given that the jet stream shown in the maps above stays to the south of this particular line, and usually the jet stream is a good indicator of the separation between cold, polar air to the north and warmer, subtropical air to the south. But, we can still check the forecast for 1000-500mb thickness.  Here's the forecast for this afternoon:
The "critical" thickness line (the 5400m thickness line) runs right through southern Iowa, central Illinois, and into northern Indiana and Ohio.  North of that line, the atmosphere should be cold enough to support snow.  near and south of that line, we're looking more at rain.  So we can start to narrow down the areas we'd expect to see snow.  First, it's only cold enough north of this critical thickness line.  Secondly, we're pretty sure there will be a lot of lift, as a well-defined warm front and a surface low are moving northeastward from western Missouri into central Illinois.  The only remaining question is the availability of moisture.  This gets back to that strengthening pressure gradient on the southeastern side of the low that I mentioned a while ago.  As southerly winds increase to the east of the low, more moist air will be brought up from the south.  This air should rise over the warm front, cool, and reach saturation in a band just north of the warm frontal boundary.  We can see this in the GFS forecast for 700mb relative humidity for this evening:

Remember that at lower levels (near the surface), southerly winds are bringing up warmer, moister air from the south.  That air will only really rise once it encounters a mechanism that will force it to rise, like the presence of a warm front.  We saw that the warm front would be moving through central Illinois and the Ohio River valley by this evening.  And, sure enough, as we look at 700mb, suddenly we see a band of near 100% relative humidity just north of the warm front.  That's where that warm, moist air below has lifted over the warm front, cooled, and is now saturated.  This is the best area for precipitation to form, and it so happens that most of this looks to be in the area where it's cold enough to support snow.

In fact, if you look at the National Weather Service watch/warning map from early this afternoon, it outlines much of this same banded area with winter storm watches and warnings, particularly on the northern edge of the moisture:
The thinking prevalent in most of the forecast discussions I've seen is that by the time we get into southern Iowa and central Illinois, we'll be too close to that "critical" thickness line, and the atmosphere may not be cold enough to support snow.  Further north there's a better chance of snow.

As this storm moves east, there will also be the possibility of severe weather.  At the moment, that warm air advection to the east of the low is rapidly bringing the atmosphere throughout parts of the southeast to a conditionally unstable environment.  This is particularly true slightly further north away from the Gulf where the upper-air temperatures are colder, making the atmosphere more unstable.  However, until a strong lifting mechanism comes through, this conditional instability will remain untapped.

That cold front looks to quickly blast through the southern plains and is even expected to move across the Mississippi river by the evening.  How fast of a blast is this going to be?  Notice the large areas of pinks in Oklahoma and Texas on the watch/warning map above.  Those are Red Flag Warnings, indicating that they're expecting unusually strong winds accompanied by dry air. This is all the result of a fast cold frontal passage.

As that cold front moves into the southeast, a strong lifting mechanism will be present that can tap into that conditional instability.  It's no wonder that the Storm Prediction Center's day one outlook has a slight risk of severe weather for the Ohio River valley down into Tennessee and Kentucky.

So we get two different kinds of "severe" weather with this--snow to the north and thunderstorms to the south.

Thursday, February 16, 2012

Will winter every fully arrive?

Last weekend's massive high pressure system brought some cooler air to the middle of the country, getting us slightly closer to more winter-like weather.  Compare the temperature anomalies from the first week of February nationally (from NCDC)...
...to the temperature anomalies from the second week of February.
This shows a significant change in temperatures across the central part of the country--from well above normal (over 15 degrees Fahrenheit above normal for much of the northern plains) to near normal or slightly below normal for much of the high plains by the next week.  That dome of cold, high-pressure air really had an impact.

Cooler weather allowed snow to fall across the central and southern plains, generating one of Oklahoma's few snowstorms even in a relatively warm year.  Here was the snow depth map as of the morning of the 14th (from NOHRSC):
A good 1-2" across much of Oklahoma, with higher amounts to the north in Nebraska.  Of course, now two days later the snow has almost completely melted.  Here is this morning's snow analysis:

Much of the snow is gone--even back up into the Chicago area. Warmer weather looks to be returning...

The longer-term forecasts also seem to be pointing toward keeping the warm weather around for much of the eastern US at least through this week.  Compare the following three GFS forecast maps.  One for 500mb at 12Z tomorrow:
Notice a trough over eastern Canada, but there is no well-defined jet stream or any particularly outstanding jet streaks.  A small cut-off low in far southern Arizona and New Mexico may lead to severe weather in south Texas later on Friday.  Here's the 500mb forecast map for Saturday morning:

Still not a very well-defined jet stream, and even the trough and ridge pattern is not well defined.  This is not the signature of any strong push of cold air coming out of Canada any time soon.  Finally for Monday morning:
Still nothing impressive.  There looks to be large-scale troughing over much of the western US, but there are no organized jet streaks around the trough.  This implies that the temperature contrasts in the atmosphere beneath are pretty weak--very little in the way of strong organized fronts, probably.  Without strong pushes of cold air from the north, temperature in the pre-existing air mass should have time to moderate and warm a little.  I'm expecting the temperatures to remain at or slightly above normal for much of the eastern US for the next few days.  Winter is still on hold...

Friday, February 10, 2012

A really big high brings winter back

A lot of snow and rain is currently impacting the eastern third of the country.  This morning's radar composite sums it up nicely.
Heavy rain for southern Louisiana with scattered heavy showers throughout the southeast.  Further north where it's colder, snow (enhanced by the lake effect) is being reported in the Chicago area and northern Michigan.

All this active weather...but where is the surface low?  Here was this morning's 12Z GFS surface analysis:
The only really organized center of low pressure is analyzed well to the northeast in eastern Quebec.  There is, however, a general trough of low pressure extending back through the Great Lakes and down into the southern plains.  This trough looks to be simply a consequence of being caught between two high pressure centers--one weaker high off the Carolina coast, and another, stronger, sprawling high pressure center in the Canadian Prairies.  Notice the very cold temperatures associated with this high pressure center--well below zero Fahrenheit a the surface this morning in parts of central Canada and the northern plains.  There's also a pretty sharp boundary between this colder air and slightly warmer air being pulled north in that low pressure trough. So, even in the absence of a strong surface low, we still have such a strong high pressure center that good enough thermal gradients are set up to help produce some significant weather.

This high pressure center looks to be here to stay--by tomorrow morning it is forecast to have set up shop in the central plains, bringing down much colder air than we've been seeing in the central and eastern US as of late.  Here's tomorrow morning's GFS forecast:

Notice the very strong temperature contrasts still on the leading edge of this high pressure center.  Furthermore, as this cold air moves down over the land surface, it encounters much warmer air over the open waters of the Gulf of Mexico and off the east coast.  This is setting up some very strong-looking temperature gradients that follow the coastline.  It also looks to be driving some cyclogenesis off the east coast.  Furthermore, with such a strong high pressure center, the pressure gradients are also very strong, and this means strong winds--you can see some 15-20 knot winds forecast over the upper Mississippi valley and into the southeast.  This means when the cold air arrives, it's going to arrive with quite the blast.

This high continues to linger into Sunday, according to the 48-hour GFS forecast:
Nos the strongest pressure gradients look to be across the middle Atlantic states.  Should be quite the blowdown.

With this huge high pressure center crashing the party, it looks like the weather pattern is finally turning to something more winter-like.  Seattle is once again back to having its rounds of rain with relatively cool weather, frigid temperatures are building back across the northern Plains, and with the presence of this high pressure center things look to dry out a bit from the rainier-than-normal conditions some places in the south and east have seen lately.  Perhaps these La-Nina-based seasonal predictions will come true after all...

Thursday, February 2, 2012

Another slow-moving storm forming in the southwest

Last week I was talking about a slow moving storm that was pushing through the southern US.  This week we're looking at another cyclone, currently developing over the desert southwest, that looks to slowly push eastward into the central plains into this weekend.

Let's start by looking at the GFS model 500mb forecasts.  At the moment, the Oklahoma HOOT site where I usually get my model graphics is migrating to another server and as such their output has fallen a bit behind.  For something completely different, I decided to use San Jose State's website for today's model graphics.  Here's the GFS 500mb height and vorticity forecast for 18Z today...about now.

Without going into the details of what vorticity means, one of the things it is useful for is tracing the location of shortwave troughs aloft.  For instance, you can see that in the base of the deep shortwave trough on the left side of the map above, the vorticity values are very high--those bright yellows, reds and purples indicate strong positive absolute vorticity, which is indicative of a cyclonic (counter-clockwise) horizontal shear of the winds.  We typically see such wind motions in a shortwave trough.  Let's use this to trace the forecasted path of this trough over the next few days...

By 18Z Friday, the trough is forecast to move eastward into the high plains.
Notice there still is a streak of high vorticity values associated with this trough.  Let's go another 24 hours out.  This is now Saturday at 18Z:
By this point, the trough has become much more poorly defined--looking at the 500mb height lines (the black contours), you may not necessarily think there is much of a trough there.  But, there is still a big vorticity maximum present in the central plains.  With the right temperature structure, this means that the flow could still be unstable enough to keep developing any surface cyclones.

So lets look at the surface forecast as this storm moves across the plains.  Here's the forecast for 18Z today at the surface (about now):
Looks like an area of low pressure trying to get its act together over New Mexico.  Notice that the path of the 500mb trough we saw above tends to follow the zone where there is a stronger horizontal temperature gradient at the surface.  This is not a coincidence--the steering winds aloft are controlled by low-level temperature gradients.  Also notice the very warm temperatures throughout the southern US and the southeasterly winds blowing from the Gulf of Mexico into the southern plains.  That indicates a lot of moist, warm air moving in, setting the stage for an unstable atmosphere.

Going forward 24 hours, we see that the surface low, while not very well defined on the map below, appears to be slowly moving eastward with the shortwave aloft.

While we can see the beginnings of a cold front in the Texas panhandle and New Mexico, the cold air behind it really isn't that deep, nor is the temperature transition really that abrupt.  Since we're looking at surface temperature here and the surface temperature naturally gets colder as you go up into the Rocky Mountains, the cold air behind the front is not as cold as you might think.  Furthermore, there's a strong pressure gradient developing in eastern Colorado that could help pull air down from the high mountains out onto the plains.  As air descends down the mountain slopes its pressure increases and it warms up--another factor working against a strong push of cold air.

And now the forecast surface map for Saturday:
Notice that even though our shortwave trough aloft had become somewhat less defined in the 500mb height field, the surface low pressure center has continued to consolidate (though it hasn't deepened that much.  However, in this image things don't look that good for the low.  The cold front is weak at best, and any attempt at a warm front is having difficulty getting through the Appalachian Mountains.  Still no sign of a strong push of Arctic air down from Canada, so temperatures will probably continue to be seasonably mild for much of the eastern US.

As far as precipitation goes, we're looking at showers and thunderstorms in the warm sector of this storm, which will generally stay in the southern plains and the deep south.  Lots of stratiform rain is expected to the north and east as the storm is able to pull that warm moist air up and around the low.  Some areas may see some snow, particularly in the central plains north of the low where the air is just cold enough.  Here's the forecast 6-hour precipitation accumulations for 6Z tonight:
That strong pressure gradient I mentioned over eastern Colorado will help increase the wind speeds quite a bit in that area.  Combined with the expected snow, this has led to blizzard warnings being posted for much of the west-central plains.

Moving to tomorrow, the precipitation comma follows the low nicely.
The purple contours on these maps are the 1000-500mb thickness lines.  Often we use the 5400m 1000-500mb thickness line as a separation between snow and rain.  We see here that this line runs from western Kansas up through Iowa and into the Chicago area.  While it seems clear that places like northern Colorado and western Nebraska will get all snow, the precipitation type forecast becomes a bit more difficult the further south and east you go.

On one final note, with such warm moist air coming off the Gulf into the southern plains, the Storm Prediction Center has slight risks for severe weather in Texas and Oklahoma tonight and tomorrow.  So, we may see a few strong thunderstorms with this storm as well.