Severe thunderstorms, flooding rains threaten Chicago-area Monday

Data pix.

CHICAGO — A moisture-laden atmosphere, coupled with a low pressure center and cold front along and just south of the Chicago area, is providing fuel for several rounds of thunderstorms that will affect the Chicago area later Monday afternoon and evening.

The greatest threat for severe weather will be from Chicago south into central Illinois, but the entire area is at risk for severe thunderstorms.

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The greatest severe weather threat is just south and west of Chicago.

Severe thunderstorms are likely Monday over the middle Mississippi Valley to the Ohio Valley. Damaging gusts are the primary risk but a few tornadoes are possible.

...Mid MS/OH Valley Region...

An impressive amount of deep convection has developed over the central Plains early this morning ahead of a disturbance that should approach western IA by sunrise. A reservoir of untapped buoyancy currently extends ahead of the MCS over eastern KS which should aid eastward progression of organized convection into the beginning of the upcoming day1 period. Latest NAM model guidance suggests leading edge of this activity will be near the MO River at sunrise; however, current speed/movement of the convection is considerably faster and storms could advance into western IA by 09z. It's not entirely clear whether the MCS will slow its forward propagation so confidence in timing/placement of this complex of storms is somewhat in doubt.

Current outlook is predicated on MCS slowing a bit toward sunrise.

For these reasons have adjusted severe probs west across IA/northern MO to account for possible severe at the beginning of the period. If boundary layer is able to warm/destabilize ahead of this activity, as NAM currently depicts, environmental parameters would seem supportive of organized convection continuing downstream, with some upward evolution expected by late morning. If convection maintains MCS structure then damaging winds will be the primary threat.

However, forecast soundings across southeast IA into west-central IL exhibit favorable CAPE/shear for supercells and possible tornadoes. Will introduce low tornado probs to account for possible discrete updrafts by early afternoon.

Heavy rain that could lead to flash flooding will also be a problem for the entire area.

Heavy rain that could lead to flash flooding will also be a problem for the entire area.

There is a slight risk of excessive rainfall for portions of North Dakota & the Midwest/Great Lakes.

Convection remaining tied to the warm frontal zone across eastern Missouri and southern Illinois, we have expanded the Slight Risk to capture this early morning activity. Flash flooding around Saint Louis and points east is likely to persist a few more hours. Follow mesoscale discussions for more details.

...Midwest/Great Lakes...
A mesoscale convective vortex over Nebraska has been tracking east-northeast early this morning, growing upscale within a favorable environment ahead of an ejecting upper low near the central International Border with Canada. The guidance QPF (global and mesoscale) started off too far north when compared to early morning radar imagery, except for the 00z NSSL WRF which fit fairly well and strongly resembled continuity. For this forecast period, started with a non-GFS blend before making targeted changes. The expectation is that the heavy rainfall currently across northern KS early this morning will track east-northeast and co-locate with the 850 hPa warm front across central IA this morning as instability and the stronger portion of the mid-level capping inversion shift northward. Thereafter, the heavy rainfall axis should lie rather close to the 12C isotherm at 700 hPa as the system progresses eastward.

The guidance shows dispersion on the exact axis; believe central IA across northern IL into northern IN is the best placement. Inflow at 850 at 30-40 knots (near the magnitude of the mean 850-400 hPa wind) imports MU/ML CAPE of 2000-4000 J/kg which should continue to foster active convection as the system moves east-northeast. Precipitable water values near 2" would allow for hourly rain totals to 2.5". The guidance has a good signal for local amounts of 4-6" in the rain band which
could be due to cell training over 2-3 hours or a couple of aligned embedded mesocyclones. As two week precipitation in this
area is only 10-25% of normal, confined the slight risk area to a narrow axis where the most rainfall is anticipated. The marginal risk was kept broad to account for the dispersion seen with the heavy rain axis amongst the available guidance.

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