Good afternoon everyone! It is January 21st and I am going to be talking to you about the potential for severe weather tomorrow. Seems a little out of the ordinary, but recent history shows that we can and certainly do see severe weather during the winter season. Recent events like February 20th, 2014 where a few weak tornadoes occurred across the I-72 corridor, and February 28th, 2017 where an outbreak of tornadoes impacted the state from north to south, come to mind. Our overall threat should mimic the February 20th analog versus the February 28th outbreak. We aren’t expecting a major outbreak of wide spread severe weather, but this set up has stood out to me and the other forecasters for 36 hours now. We are going to break it down for you all and explain which areas may see the highest threat for small hail, a strong wind gust or two, and even a brief tornado.
Looking to the left we see current temperatures across the state. Notice the areas across northern and eastern Illinois where temperatures remain in the 40’s? This is where some of the snow pack still exists whereas areas around the Mississippi River are in the 50’s and 60’s! Foggy conditions dominate the northern and eastern section of the state which is a tell tale sign of moisture advection in advance of our weather maker. Foggy conditions dominate the northern and eastern section of the state which is a tell tale sign of moisture advection in advance of our weather maker. Below is a forecast of surface visibility at 1 am overnight tonight showing very low visibility under 1/4 of a mile across parts of northern Illinois.
Focusing on tonight into tomorrow, showers and thunderstorms will develop and spread northward across the state. These storms will have the potential to produce very efficient rainfall, but little to no severe weather threat. I am a little concerned with the flooding potential across east-central Illinois as ice jams may form on the area rivers. These showers and storms should move out of southern Illinois by mid-morning, central Illinois by early afternoon, and northern Illinois by mid-afternoon. Temperatures are expected to remain in the 40’s and 50’s through the morning as the rain moves through.
Winds this evening and overnight should increase in response to the surface low across Iowa. These southerly winds will continue to advect moisture into the region. By afternoon I anticipate highs in the mid 50’s across northern Illinois to the mid to upper 60’s across southern Illinois. Dew point temperatures in the low to mid 50’s will provide more than enough low level moisture for the development of storms later on. For many of the of images below I will be using a blend of the 18z NAM and 18z NAMnest. I feel the NAM is performing reasonably well with this system albeit under doing the thermodynamics.
What’s really going to drive this severe weather threat is the very cold air aloft. Temperatures of around -30 degrees at H5 will promote very steep lapse rates and create instability. 0-3km lapse rates are expected to be in -6 to -8 range with 0-3km CAPE [shown below] with nearly 300 j/kg. This low level instability is often overlooked by those forecasters simply looking at SBCAPE or other cursory parameters. In this case, many would write this threat off based on instability without really realizing that the heating is under done and there’s more than enough low level CAPE. We aren’t dealing with the 60,000 foot tall Plains supercells. 20,000 foot tall “showers” with very favorable low level shear will easily support the threat for rotation. The reason the 0-3 km region of the sounding is important to study is because this is generally where updraft initiation for thunderstorms occurs and where the best speed and directional shear is located.
Speaking of CAPE, the 18z nest does show a favorable axis of MUCAPE from I-80 through I-64. What makes this such a localized threat is the relatively narrow axis of instability. These storms should develop on the axis of highest instability and rapidly move north/northeast before eventually outrunning their juice. The threat tomorrow won’t be terribly long lived, but it will be in a fairly expansive area. I believe the most favored area will be tied to where the coldest air aloft coincides with the steepest low level lapse/CAPE. This area in my opinion would be confined to west-central Illinois. Our products below show the overall risk area.
The key cog to this mechanism is whether or not we achieve clearing to get the minimal amounts of surface heating needed to kick things off. It has been well advertised by almost all the model blends that a very pronounced dry slot will pivot in by mid-afternoon across western Illinois. Sunshine should break out immediately following the rain and warm things nicely. By mid to late morning, cumulus should rapidly develop and grow into showers and storms in the dry slot right on the axis of instability. If the dry slot fills back in with strato-cumulus clouds too quickly, then the overall threat will not materialize. Many of the high resolution models promote at least a couple hours of heating which should be enough to kick off these showers/storms.
The overall severe weather threat is low. Many areas may see more non-severe thunderstorms tonight than they will see tomorrow. Even the greatest severe weather threat [brief tornado] could come from convection without any lightning. Nevertheless the area of highest potential to see a brief, weak tornado will be from just south of the Quad Cities down through the area north of Springfield. I feel this portion of the risk area has the best overlap of parameters to see some shenanigans tomorrow. The threat for small hail also exists with any convection due to the very cold air aloft. This set up loosely reminds of a set up you would see in Colorado during the mid-Spring. In those cases, a larger more thermodynamically driven severe weather threat exists further south while the upper level low region seems to produce based on kinematic fields and bottom of the scale thermos.