The Storm Prediction Center (SPC) has issued a Moderate Risk today for parts of the Great Lakes region. They have also issued a Public Severe Weather Outlook for today as well.
Associated with the Moderate Risk is a 10% hatched tornado ring just west of the Chicago area. This is an incredibly dangerous situation today.
The SPC is basing this threat on a warm front that will creep northward, with a strong, humid, low-level jet that will bring moisture to the warm sector. This will allow for dew points in the 70s and 2000-3000 J/kg of Convective Available Potential Energy (CAPE) by this evening. While there is a capping inversion, it is expected to be broken by late afternoon, allowing for strong thunderstorms. Also, there is a jetstreak moving into the area that will boost the deep-layer shear.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows mostly zonal flow through the risk area, but a moderate jetstreak will be present this evening. This is expected to boost the deep-layer shear.
The 500 mb NAM chart shows some moderate Positive Vorticity Advection (PVA) into the threat area, but the vorticity is not very strong, and the winds blow at a shallow angle to the gradient. I was expecting a little more PVA by 18 Z.
The 700 mb NAM chart shows some strong Upward Vertical Velocities (UVV) by 18Z across northern Illinois.
The 850 mb NAM chart shows the northern advancement of the warm front. Warm Air Advection (WAA) is present throughout the threat area as winds blow across the thermal gradient from warm to cold.
The 12Z upper air sounding from Omaha, NE, shows a huge capping inversion with -1173 J/kg of Convective Inhibition (CINH). The dewpoint was already 62 F, and, given the possible temperature increase with the Warm Air Advection (WAA), perhaps this capping inversion will not be as strong over Iowa and Illinois as it is over Nebraska.
The interesting part of the Omaha sounding was the shear. The deep-layer shear was 44 kts, and the low-level shear was 38 kts. In terms of shear, this is enough to support rotating storms and tornadoes.
The 12Z upper air sounding from Davenport, IA, shows the influence of the ongoing morning convection. The atmosphere is quite damp, and a little cooler than Nebraska this morning. There was very little CAPE, but this will increase due to diurnal heating once the morning convection has dissipated.
The deep-layer shear was 70 kts, and the low-level shear was 26 kts. Once again, this shear is strong enough to support rotating storms and tornadoes.
TwisterData.com shows the moisture will push up the Mississippi River valley throughout the afternoon. A dryline, currently stationed over central Nebraska will push east towards western Iowa and Minnesota by this evening; the dryline will weaken slightly in terms of moisture, but the gradient will tighten.
TwisterData.com also shows the CAPE breaking 3000 J/kg over eastern Iowa.
Currently, the surface observations (from the SPC Mesoscale Analysis Map) show winds turning into the low pressure system centered over the Iowa/Nebraska border this morning. Dewpoints are in the low 70s just south and east of this low pressure system. The warm front is a little hard to see, but there is a shift in wind direction from the northwestern corner of Iowa into southwestern Minnesota.
The Supercell Parameter is expected to increase to 28 over the next six hours in north and central Illinois.
The visible satellite imagery shows some ongoing convection that lingers over the area. This is expected to mix out over the next few hours.
The enhanced infrared satellite imagery shows that much of this convection is moderately thick.
The water vapor satellite imagery shows relatively dry air to the south and west of the threat area. However, the strong low-level jet is bringing moisture into the Moderate Risk area from the southeast.
Overall, I expect that today will consist of strong storms across Illinois, and perhaps becoming linear by the time they reach Indiana and Michigan. The HRRR has storms firing farther north and I’m simply not buying it. I think that the combination of the warm front and the moisture will cause storms to form in the northwestern corner of Illinois and track east across the state.
Thank you for reading my forecast.
The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from NASA – MSFC
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