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                   Wednesday, April 9, 2008 - 12:03AM

Thursday storm forecast 1

By DAN ROBINSON
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25 Years of Storm Observing
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Due to this week's sales going well, I've decided to give Thursday a chance - even if it ends up being a little out of my 'home range'. My tentative plan is to leave after the afternoon model runs and the SPC's 1730Z Day 2 outlook are out after noon Wednesday, making a leisurely drive westward toward the St. Louis area for the night. The early departure will give me the opportunity for a good night's sleep before what will likely be one heck of an exhausting day to follow.

So on to the forecast. Any experienced storm observers reading this blog probably get tired of me explaining the obvious on here, but since I have many readers who are new to observing or just have a casual interest in weather, I like to go into the hows and whys a little more. Severe thunderstorms, more specifically tornadic storms, need four basic ingredients - 1.) instability, 2.) moisture, 3.) lift, and 4.) vertical wind shear. There are other factors to consider, but for now we'll focus on those four. First let's look at the wind shear parameters for Thursday. Remember how I talked about the different layers of the atmosphere a few posts ago? What is happening in those upper layers comes into play in a big way for Thursday's outlook. Below is a three-panel composite of the latest WRF model forecast for Thursday evening. The first panel shows the midlevel (500mb) forecast, the second panel shows the lower levels (850mb) and the third shows the surface forecast. The darker colors indicate areas of stronger winds:


WRF 500mb, 850mb and surface forecasts for Thursday evening

Now pick any point on that map and compare the wind direction on each panel (the direction the little barbs are pointing) - notice how the winds are moving in three different directions at each level over Illinois, for example. That's what we call vertical wind shear - or more specifically in this case, directional wind shear. What this means is that any thunderstorm cell developing in that area will rise up through those layers and get 'twisted' as a result. Now admittedly that's an oversimplification, but in essence, that's a prime condition needed for tornadic supercells to form - and is one of the major things storm observers look for on a potential outbreak day. Also notice the surface winds over Illinois - out of the southeast, pointing more head-on into where the storms will be. This is what is referred to as 'backed' surface winds - another prime ingredient for tornadoes. A supercell with backed surface winds feeding into it tends to have a higher chance of tornadoing than one with surface winds more parallel to its motion.

So we've covered the wind profile for Thursday - and as you saw above, it looks great for tornadoes. So what else do we need to look at? Winds are only part of the equation. Secondly, we need deep low-level moisture - and it looks like we'll have plenty of that, with dewpoints well into the 60s across the risk area:


WRF surface dewpoint forecast for Thursday evening

The third ingredient tornadic storms need is instability - simply put, warm air at the surface (usually heated by the daytime sun) that becomes bouyant relative to the air above it. Instability (CAPE or Convective Available Potential Energy) will not be in short supply either on Thursday:


WRF CAPE for Thursday evening

The last ingredient is lift - in other words, a mechanism to cause storms to fire. This is the one factor that is a sticking point for me for Thursday. With such a powerful large-scale system, we may have too much lift for individual supercells to maintain their isolation. Here is the QPF (precip) forecast for Thursday as depicted by the WRF model:


WRF QPF for early Thursday afternoon

Look at all that rain - and early in the day, too. In a setup like Thursday's, that much precip on the models can either mean good or bad things for storms. Either all that precip is due to numerous strong supercells (which would mean an insane tornado outbreak) or it is due to widespread storm clusters firing early on in the volatile environment. Too many storms too early mean that individual cells won't get organized as easily, as they'll be competing with each other for energy.

Due to the strength of all the other ingredients for tornadoes being present, I expect the SPC will issue a high risk for a large area in the mid and lower Mississippi valley when the new Day 2 outlook comes out in a couple of hours. However, storm coverage will be a critical issue on how chaseable this setup ends up being. We'll have to watch that factor as the event draws closer. If it becomes apparent that isolated supercells will be choked by too much convection, I may hold off on the expedition plans and wait for the eventual squall line to reach West Virginia.

My tentative starting target for Thursday morning is 50 miles west of St. Louis on I-70, with the intent to move west - while keeping a move to a southern target along the Mississippi river within reach.

25 Years of Storm Observing
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