High risk of severe storms today for northern Illinois

******UPDATE AT 2 PM CDT******
The Storm Prediction Center has issued a "Particularly Dangerous Situation" Tornado Watch for eastern Iowa, southwestern Wisconsin and northwestern Illinois this afternoon.

They expect "explosive" storm growth over the next few hours with a moderate potential for strong tornadoes and severe winds.  This watch is probably somewhat in response to the 18Z Davenport sounding:

That backing of the low-level wind field to more southerly flow has indeed happened, which is ramping up the low-level wind shear.  We also see that the cap has rapidly eroded with steepening lapse rates in the low-levels.  Sounding analogs (which look at previous soundings from severe weather events for similarities) show an 80% match with previous tornadic events.  Definitely be on the lookout this afternoon

----------------------------------------------------------------------------------

It looks like it could be a rough day for severe weather in northern Illinois.  The Storm Prediction Center has issued a relatively rare "high risk" for northern Illinois and Indiana, including the Rockford and Chicago areas.

There has actually been a fair bit of uncertainty regarding the development and evolution of thunderstorms in this area over the past 24 hours.  Yesterday several models were pointing to a powerful squall line (or quasi-linear convective system if you prefer that term) moving across northern Illinois early this morning.  Take the NCAR EnKF-initialized 3km WRF forecast from yesterday  morning--here's their forecast reflectivity over the upper midwest for 15Z this morning:

 This suggested an intense derecho-type storm (note the bow-echo appearance in this simulated radar composite) that was to move across southern Wisconsin and northern Illinois this morning bringing strong winds and heavy rain.  However, here's what the radar actually looked like at 15Z this morning:

No big thunderstorm complex---only some areas of rain.  There were some thunderstorms earlier, but they were not nearly as strong as many of the models had suggested.  We could tell even last night that this big derecho-type storm was not going to materialize as the precursor storms did not get their act together further west.

However, that brings us to today's forecast. There's a very big consequence of the lack of storms this morning--the entire purpose of thunderstorms is to re-stabalize the atmosphere by convectively removing pre-existing instability.  Had we had thunderstorms move through this morning, they probably would have done just that--cooled and dried out the lower levels of the atmosphere a bit and leave us less unstable today.  But that didn't happen.  Now we get another day of heating an already conditionally unstable airmass and this will destabilize the air even more...setting up today's potential for severe weather.

Let's look at the setup this morning.  A low pressure center has been getting its act together in the central Plains all night.  Here's the surface mesoanalysis from 12Z this morning:

You can see the cluster of storms that was there earlier, though, as I said, not nearly as strong as some models were indicating.  If we look at the Davenport sounding from this morning we see reasons for concern:

You can click on that image to make it larger.  One of the striking features of this sounding is actually the wind profile--almost uniform wind direction all the way through the troposphere.  This enhances the possibility of these derecho/straight-line wind events as storms will quickly tend to organize in a line perpendicular to a uniform wind profile. The surface dewpoint this morning is in the low 60s in Davenport and predicted to rise.  We also see relatively high dewpoints all the way up to around 850mb and near saturation above 500mb (that may have something to do with outflow aloft from the storms to the north.  All in all this sounding shows a lot of moisture in the air--meaning heavy rain is definitely a possibility.

We also have a fair bit of instability.  The sounding profile between about 800mb and 500mb shows a pattern indicative of what is called an "elevated mixed layer" (EML).  This layer of the atmosphere is marked by extremely steep lapse rates (temperatures falling off rapidly with height--basically dry adiabatic) and well-mixed moisture content.  This actually leads to this layer being relatively dry--you can see that the dewpoint is well below the temperature for much of this layer. The presence of an EML enhances the potential for very strong downdrafts--all that moisture being lofted in the thunderstorm can evaporate into that dry layer, causing a lot of cooling within that layer.  This cooled air becomes negatively buoyant and sinks down to the ground, creating powerful downdrafts and organizing a "cold pool" beneath the storms.  This kind of setup again favors organized, bow-echo sorts of storms.

Another concern is the potential for tornadoes.  The wind profile in that sounding doesn't show a lot of low-level directional shear, but this will change as the surface low moves closer.  Here's the surface analysis from 17Z this morning:

As the low has moved closer, notice that the surface winds have become more southerly across eastern Iowa and northern Illinois.  This is creating more directional wind shear in the low-level wind profile.  The low pressure center is also forecast to deepen today as we see a shortwave trough approaching from the west in the 500mb analysis, helping to give the low an added boost.

The convection-allowing models we have are a bit messy with the convection they are developing today.  Here's the Composite Reflectivity forecast from the HRRR model for 23 Z this evening:

 Lots of storms popping up all over the place.  It's difficult to tell if there's any forecast organization.  I ran WRF simulation last night with a few different parameters and had this forecast for 23Z tonight:

Similar picture with a lot of discrete storms across northern Illinois. It looks like the models are making any capping inversion relatively easy to break which may explain why we seem to have storms popping up everywhere.  My WRF run has the storms starting to organize into bowing line segments east of Chicago around 3Z tonight:

All in all it looks like we're in for a crazy day weather-wise across northern Illinois and Indiana.  We'll have to be watching to see how this develops.

A return--with some thoughts on forecasting severe thunderstorms

Hello again everyone!  It has been a year since my last blog post, and having finished a lot of work in that time I finally decided to resume posting again.  Hopefully I'll continue more steadily again from here on out.  I continue to get comments and feedback on my old posts (thanks to Google cataloging everything so effectively...) and I appreciate all of the thoughts and comments I've received.  Keep them coming!

Today I just want to offer a few thoughts or notes on forecasting severe thunderstorms with some examples surrounding the Moore tornado.

• It has been well-established by many people that the National Weather Service did an excellent job at warning the people of Moore before the storm hit--as much as 36 minutes of lead time by some estimates, which is well above the national average of around 10 minutes of lead time.  I happened to be in central Oklahoma last weekend (though I left on Sunday evening when there were a different set of tornadic storms moving through) and it was my experience, just as when I was living down there, that the people of Oklahoma are extremely weather literate and aware.  It seemed everywhere I went, the people I encountered would remark about how we're, "in for some rough weather this afternoon" or to, "get that rental car back before a hailstorm moves in...or worse".  Everywhere TVs were tuned to the local news and the Weather Channel and I overheard non-meteorologists talking about "moderate risks" and "mesoscale discussions".  This weather event was not something that was completely unexpected at all, and I admire and credit the people of Oklahoma for taking such an active interest in their weather forecasts so that they can stay safe.
• As someone interested in mesoscale numerical modeling, I follow the work being done by the Hazardous Weather Testbed Spring Experiment teams who are looking at evaluating our model performance and nowcasting tools and abilities for severe weather events.  They have multiple blogs, for example the GOES-R Proving Ground group or the Experimental Forecast Program.  These groups have some fascinating new tools and observations that show what's on the forefront of our ability to predict severe convective weather events.  Below are some things that I noted from their tools surrounding the Moore event.
• To illustrate just how hard it is for our models, even at high resolution, to predict when and where individual thunderstorms will strike, below is a comparison of two model runs from the NSSL 4km WRF, one starting at 00Z on May 20 and another at 12Z on the 20th.  Both are simulations of  what the radar composite would look like for the hours leading up to 20Z--the time when the tornado was entering Moore.  You can see that the 00Z model run (the left column) seemed to pick up more on storms in southwestern Oklahoma and some in central Oklahoma, but it completely missed the development further to the northeast along the cold front.  The 12Z run (middle column) really picked up on the development along the cold front, but kind of missed the storms in central Oklahoma.  The right column shows the actual observed reflectivity composite for comparison.  It's often been noted in convective events that the 12Z model guidance doesn't always provide a better forecast than the 00Z guidance, even though it was run later with more recent information.  This shows just how hard it is for our deterministic models to forecast storm development, even only a few hours in advance.
  
• One way to try and work with the difficulties that any single model will have at trying to predict when and where storms will form is to use an ensemble--many models runs with slightly different initial conditions and/or different model formulations.  We can use these to estimate probabilities of certain events occurring by seeing how many of the ensemble members have that event occurring.  Below shows an example of forecast probabilities of updraft helicity (i.e., rotating updrafts) exceeding various thresholds from the Storm Prediction Center's Storm-Scale Ensemble of Opportunity for a three hour period including the time of the Moore tornado.  Not too bad--the ensemble suggests a high probability of  rotating updrafts throughout central Oklahoma.

          Of course, some ensembles can also be misleading or still not capture things well.  Below is an example prediction from the CAPS ensemble of where a certain parameter (the Significant Tornado Parameter) will be greater than 3 (indicating a likelihood of strong tornadoes) during the Moore event.  The highest probabilities are indicated far to the northeast of Moore.  However, low probabilities are still present in the Moore area, indicating that by this parameter a significant tornado was still possible in this area...

• There are also efforts underway to support a project called "Warn-on Forecast".  The idea is that once cumulus clouds start growing, we can first identify which particular clouds have the highest potential of growing upscale into stronger thunderstorms.  Tools are being developed to do this, including satellite products that estimate the rate of cloud growth by how fast the tops of the clouds are cooling, or lightning-based methods that will identify storms where lightning frequency is ramping up.  Once these growing storms have been identified, the plan would be to build a high-resolution ensemble of models centered on that storm.  We could then forecast the evolution of that storm and get uncertainty information from the different ensemble members.  This would let forecasters evaluate the ensemble to get probabilities for the storm producing large hail, strong winds or even tornadoes.  We could also get uncertainty in the path of the storm, allowing forecasters to make much more accurate warnings.  Furthermore, as our confidence in these forecasts grows, we could issue warnings for the storms before they even become severe, greatly increasing lead times.
• Unfortunately an active, real-time warn-on forecast system like this still a ways off, but some of the tools to support it do exist.  Cloud-top cooling from satellites and lightning mapping arrays are very real things and are actively being tested for their ability to identify storms that will grow.  Some groups are also working on using radar data to make high-resolution model analyses of the wind, temperature, pressure, and moisture fields surrounding these storms as they are developing.  This is the first step in trying to make these storm-scale ensembles.  Below is an example 1km analysis produced from radar data as the Moore tornado was developing.  Vorticity is contoured in black, and you can see that the analyzed wind fields have a lot of vorticity (rotation) in the right areas of these storms.

One problem with this warn-on forecast methodology is that it is inherently limited--we can only identify when and where the growing storms will be after they have already formed and are beginning to grow.  Forecasting convective initiation--that is, when and where storms will form before they've even formed is still incredibly difficult (see the NSSL WRF above for an exmaple).  But we're working on that too...

So, in brief, the meteorological community is working on ways to better predict severe convective events like what happened in Moore.  Forecasters already are doing an amazing job with the resources they have, but new tools are in the works to more accurately refine our forecasts to reduce false alarms and give people more lead time.  It's an exciting time to be researching in this field!

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:

http://www.atmos.washington.edu/~lmadaus/research/barotropic

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...