Although storms to the north and west of the Chicago area have diminished considerably in coverage and intensity, the severe weather threat for our area remains. The air mass over northern Illinois and NW Indiana has recovered from the passage of storms earlier in the day and is moisture-rich with dew points in the mid and upper 70s.
In addition, very strong WSW winds just a few thousand feet off the surface are expected to initiate rapid thunderstorm growth across our area later this evening and a severe thunderstorm watch is being considered by the SPC. Satellite imagery and surface observations noted mid-level cloud growth suggestive of sufficient instability aloft and potential for nocturnal storm development.
Below is the technical discussion from the SPC: Mesoscale Discussion 1492 NWS Storm Prediction Center Norman OK 0735 PM CDT Wed Aug 11 2021 Areas affected...Eastern IA...Northern IL...Far Southern WI...Northwest IN...Southwest Lower MI Concerning...Severe potential...Watch possible Valid 120035Z - 120200Z Probability of Watch Issuance...40 percent SUMMARY...Storm coverage is expected to increase this evening, with at least an isolated severe threat into late tonight. New watch issuance is possible by 01-02Z. DISCUSSION...Earlier supercells over eastern IA, southwest WI, and northwest IL have weakened over the last hour, likely due to generally weak large-scale ascent and modest capping noted in 00Z DVN sounding. However, regional soundings and VWPs also depict a rather strong southwesterly low-level jet across the area, which is generally forecast to intensify further tonight as it veers to a more westerly direction. Storm coverage is expected to increase with time later this evening within this warm advection regime, as airmass recovery continues across northern IL/IN and a surface boundary moves southeastward across the region. Even if storms remain somewhat elevated tonight, steep midlevel lapse rates and rich low-level moisture will still support large MUCAPE, with sufficient deep-layer shear for some organized storm structures. A mix of cells and clusters will be possible, posing a threat of at least isolated hail and damaging wind. A conditional tornado threat will also be present with any storm that can become rooted in the boundary layer, which would be most likely for a few hours this evening before MLCINH becomes increasingly prohibitive late tonight. While some uncertainty remains regarding the convective evolution and coverage of the severe threat later tonight, watch issuance is possible by 01-02Z, in advance of the expected increase in storm coverage.