Afternoon heating triggers widely-scattered showers/thunderstorms over northern Illinois – few strong storms possible
As temperatures warm into the middle 80s this afternoon, building cumulus clouds have continued to develop into widely scattered showers/thunderstorms over northern Illinois. Because of the unseasonable heating and the unstable atmosphere, a few of the strongest storms could produce brief downpours and damaging winds. Individual storm cells will be moving north-northeast at 30 to 40 miles per hour.
The National Storm Prediction Center has issued a Mesoscale Discussion for this area (escalloped area outlined on the headlined map) posted below – they do not anticipate issuing a Severe Thunderstorm Watch unless overall storm activity intensifies.
Mesoscale Discussion 1551 NWS Storm Prediction Center Norman OK 0245 PM CDT Tue Oct 09 2018 Areas affected...Far east Iowa...far northwest Illinois...southern Wisconsin Concerning...Severe potential...Watch unlikely Valid 091945Z - 092145Z Probability of Watch Issuance...20 percent SUMMARY...Isolated damaging wind gusts and possibly a brief tornado may accompany the stronger storms. A WW issuance is not expected at this time. DISCUSSION...Isolated, small, relatively discrete storms have developed just south of a differential heating zone within the last 1-2 hours, where temperatures have climbed into the 80s. A few of these storms have exhibited occasional low-level rotation (as was the case recently in Dubuque County, Iowa). With mid 60s dewpoints and modest lapse rates in place, up to 1000 J/kg MLCAPE has recently been realized across the discussion area. Still, buoyancy is rather modest, as is deep-layer shear, with 35 knots of effective bulk shear noted. Deep layer ascent and stronger low-level shear are currently located well upstream of the region and are not expected to arrive until later this afternoon/early evening. As such, in the short term, a few of the stronger cells may become organized, perhaps with brief periods of moderate to strong low-level rotation. A few marginally severe wind gusts may occur and a brief tornado cannot be ruled out. Given the isolated and marginal extend of the short term severe threat, a WW issuance is not expected at this time, though convective trends will be monitored for any increase in threat.