Update on this evening’s severe weather threat.


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.

Scalloped area represents region being considered for watch issuance.
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

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