WIDESPREAD SEVERE STORMS

SIGNIFICANT SEVERE WEATHER IS EXPECTED OVER THE MIDWEST THE NEXT 48 HRS

A complex storm system with potent dynamics appears set to bring several rounds of severe weather to the central Midwest through Thursday. The Storm Prediction Center expects numerous to widespread severe thunderstorms both Wednesday and Thursday with potential for large hail up to 2 inches, a few strong tornadoes and damaging winds over 75 mph begining today as multiple rounds of thunderstorms develop and track northeast the next 48 hours.

....TODAY OVER THE UPPER MISSISSIPPI VALLEY/MIDWEST...

At mid-levels, a trough will move eastward into the central and northern Plains today, as the exit region of a jet streak overspreads the upper Mississippi Valley and Midwest. At the surface, a cold front will advance eastward across the Plains, with a warm front moving into the western Great Lakes. Between these two fronts, a very moist unstable airmass will be in place locally with surface dewpoints in the lower to mid 70s. By early morning a line of strong thunderstorms is expected to develop in western Iowa near the western edge of the moist airmass and an associated cold front. The potential for severe wind gusts will be possible with this line as it advances east.

Eastward, across the the open warm moist sector that includes my area, moderate to strong instability is expected to develop by midday as surface temperatures warm. This will help the line in western Iowa grow upscale, leading to a linear MCS (potentially with derecho like characteristices), as it moves across into eastern Iowa towards southern Wisconsin and northern Illinois early this afternoon. Severe wind gusts and QLCS tornado spin ups will be possible along the strongest parts of this bowing line segment. Due to mesoscale details still in the developing stages, the precise location of the strongest updrafts and storms is yet to be determined, but eventually some part of the local area has a good chance of being impacted by mid-afternoon.

In the wake of this initial line segment, instability is expected to increase again as moisture advection and surface heating take place behind it. A rather strong instability axis is forecast to develop in the afternoon from eastern Missouri northward into eastern Iowa, where CAPE (instbility) should increase into the 3000 to 3500 J/kg range. Thunderstorm development will be likely during the mid to late afternoon along a zone of enhanced low-level convergence from northern Missouri into central and parts of southeast or eastern Iowa.

Additional storms are expected to form further north into parts of far eastern Minnesota and western Wisconsin. RAP and HRRR forecast soundings in the late afternoon from eastern Iowa into southwest Wisconsin have curved hodographs, with 0-6 km shear near 40 knots, and 0-3 km storm-relative helicity around 225 m2/s2. If realized, such an environment would support supercells with tornado potential. Even a strong tornado or two will be possible as supercells move eastward in the strengthening low-level jet during the late afternoon and early evening. Large hail and wind damage will also be possible with any supercells. From this cluster, a second severe line segment with damaging wind gusts is expected to develop. This line would likely impact the area from the Mississippi River east into north-central Illinois Wednesday evening.

All of my area is under an enhanced level 3 risk of 5 severe weather Wednesday afternoon and evening.

...A GREATER TORNADO THREAT THURSDAY OVER THE CENTRAL MIDWEST...

After things calm down Thursday evening, the atmosphere recharges again Thursday with another round or two of severe weather possible. SPC again forecasts scattered to numerous severe storms capable of potentially strong tornadoes, large hail greater than two inches in diameter, and destructive winds with gusts in excess of 75 mph likely from portions of the mid Mississippi Valleys into the upper Great Lakes.

...Synopsis...

A potent short-wave trough initially over the central High Plains Thursday morning is forecast to accelerate through the upper Great Lakes with a trailing energy moving through the central Plains into Iowa and Illinois by early Thursday evening. Both disturbances, are located on the cyclonic side of an unseasonably strong mid-level jet with 500-mb wind speeds of 60-80+ kt.

At the surface, low pressure initially over north-central Kansas Thursday morning will develop northeast into Iowa by evening. The surface low will be deepening along an outflow generated warm front that will be rapidly lifting north through the Midwest into the upper Great Lakes. Meanwhile, a trailing cold front will advance southeast towards it over eastern Iowa.

There is good model agreement that a cluster of severe storms, including supercells, will be ongoing early Thursday across the mid MO Valley, immediately northeast of the surface low and within a zone of strong, low-level warm advection. The early-day storms are expected to grow upscale into an organized MCS with embedded supercell and bowing structures across IA by mid to late morning amidst a rapidly destabilizing air mass coincident with strong low-level jet and deep-layer shear. As such, the potential for tornadoes (some strong) and corridors of destructive winds in excess of 75 mph are expected to increase significantly as the morning progresses. The intensifying MCS and its related destructive wind and tornado threat is expected to move into eastern Iowa, southern WI and northern IL by early to mid afternoon. An additional wave of supercells and/or bowing structures is expected to develop along the trailing outflow from the lead MCS, as well as along the cold front from eastern IA and northern IL into the mid MS Valley during the afternoon and early evening. Current indications are that these storms will be capable of tornadoes, large to very large hail, and damaging winds with significant gusts, which could become widespread as storms transition to a bowing line. SPC again has an enhanced level 3 of 5 risk of severe storms. If some of the high-resolution CAM models verify to their current severity, it is possible some part of the region could be upgraded to a moderate risk level 4.

Currently, my area is shown in a 10-14 percent risk of a tornado withing 25 miles of a point Thursday.

Rapidly changing conditions can be expected both Wednesday and Thursday as scattered to widespread storms move in and out of parts of the area. Pay close attention to forecasts, watches, and warnings. Make sure you have a plan what to do if your area is threatened by severe storms. Hopefully, we end up with a weaker version of severe storms than what most models are indicating. Roll weather...TS