Severe Outbreak Expected Tomorrow, Historic Flash Flooding For the Later Part of The Week
- Northern Illinois Severe Weather
- Apr 1
- 3 min read
A potentially significant severe weather outbreak is expected across the area tomorrow, 4/2. All severe hazards including tornadoes (a few of which may be strong/violent), very large hail (some stones in excess of 2" in diameter) and very strong wind gusts (greater than 75 mph).
The Storm Prediction Center has placed an enhanced (level 3/5) risk across the entire area. I anticipate that an upgrade to moderate (level 4/5) or even high (level 5/5) will be needed in future updates and may include some of the area.
The severe weather threat will depend completely on morning convection. A "boom or bust" scenario is in place. Some elevated thunderstorms are possible overnight tonight into tomorrow morning. If these storms leave cloud cover across the area, our severe threat will be lower (bust) than if we can clear out and get some sunshine tomorrow (boom).
The tornado threat across the area appears to be quite high, similar to what we saw on March 14th. Storm mode remains a bit unclear at this point, hence the lower tornado probabilities from the SPC as of now. If storms can remain supercellular, then the tornado threat will be increased, with the potential for several tornadoes, a few of which could be strong. If storms become linear, the tornado threat will be less, though still worthy of mentioning. Linear storms tend to pose more of a damaging wind threat and weak tornado threat, though I think a few tornadoes could still be strong, even with a QLCS mode, given the extreme shear in place.
Roughly 40-60 knots of 0-6km bulk shear is present across the area, and storms are expected to be oriented orthogonally to this shear maxima, meaning the potential for strong tornadoes exists.
Not only are tornadoes a threat, but the threat for massive hail is a bit concerning as well. Steep lapse rates of 6-8° C/km will be present along with moderate instability of 1,500-2,000 j/kg of MU-CAPE (thunderstorm fuel) will pose the threat for large hail, some of which may be 2-3" in diameter, especially if storms can remain supercells.
Finally, there will also be a threat for damaging winds across the area. This threat will really maximize if storms become linear, and wind gusts could exceed 75 mph given dCAPE values of 6=800 j/kg across the area.
Forecast soundings based off of the HRRR model are concerning across the area. Many of the soundings pulled yielded either a "TOR" or "PDS TOR" possible hazard type, indicating the atmosphere in place will be volatile Wednesday. We will continue to monitor this threat and provide updates.
Unfortunately, the cold front responsible for the severe weather anticipate tomorrow looks to stall over our area, meaning additional severe weather will be possible, though not expected to be as significant as tomorrow.
The SPC has placed a slight (level 2/5) risk across southeastern portions of the viewing area for Thursday. Significant Severe weather is not anticipated.
The SPC has placed a slight (level 2/5) risk across much of the area for Thursday as well. Significant severe weather is NOT expected.
Not only are we concerned about severe weather, but we are also concerned about significant, potentially historic flash flooding across southern Illinois. Anywhere from 4-15" of rain could fall across the area.
The latest NWS NBM (National Blend of Models) run has very high accumulation totals across the entire region, with highest totals along the Ohio River. Significant river flooding is expected. Flooding is likely to occur in areas that have never previously flooded.
A FLASH FLOOD WATCH is in effect for much of the area to accompany this threat. Again, we want to push how significant this flash flooding threat is. Historic flooding is expected. Flooding kills more people every year than any other mode of severe weather, so please, heed this warning!
PM 830a CDT 4/1/25
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