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Weather conditions directly above are near Lakefront. Top tabs refer to O'Hare (official).

Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Sunday, September 18, 2022

SPC Sep 18, 2022 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0238 PM CDT Sun Sep 18 2022 Valid 182000Z - 191200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHEASTERN IOWA AND NORTHEASTERN MISSOURI INTO WEST-CENTRAL ILLINOIS... ...SUMMARY... A few intense supercells, capable of very large to giant hail, along with tornadoes, one or two of which may be strong, may evolve into an MCS producing scattered severe wind gusts. This appears most likely across portions of south-central to southeast Iowa, northeast Missouri, and west-central Illinois, beginning after 5 PM CDT and continuing through tonight. ...Discussion... The prior outlook remains reflective of current expectations, with storm development expected to occur late this afternoon/early this evening across the central/southeastern Iowa/northeastern Missouri vicinity. With the airmass to continue gradually destabilizing over the next couple of hours, and an increase in both mid-level west-northwesterlies and low-level west-southwesterlies with time, risk for an all-hazards -- and locally significant -- severe-weather event remains likely. ..Goss.. 09/18/2022 .PREV DISCUSSION... /ISSUED 1130 AM CDT Sun Sep 18 2022/ ...IA/MO/IL... A low-amplitude shortwave impulse over the western Dakotas will move east across the Upper MS Valley to the south of a deeper shortwave trough shifting from MB to far northwest ON. This evolution will yield an amplifying mid-level jetlet (with 500-mb winds in excess of 50 kts) across southern MN and IA into WI by this evening. A strengthening baroclinic zone across IA, aided by robust differential boundary-layer with low 90s into northwest MO and low 70s behind the cold front in northern IA, will support initial thunderstorm development in the 22-00Z time frame centered on south-central IA as MLCIN wanes. The initial environment will be characterized by weak 0-1 km SRH, but large 0-3 km SRH with further increase in wind speeds through the mid to upper portion of large buoyancy amid rather steep mid-level lapse rates. This setup appears favorable for potentially a few long-lived discrete supercells with the primary hazard being very large to giant hail given the expected hodograph structure. The degree of tornado threat is more uncertain. Low-level southwesterlies will strengthen this evening, but primarily across northern MO and displaced largely south of expected convective development. The greatest tornado threat may evolve with the southern-most supercell which would have uninterrupted inflow and greater 0-1 km SRH near the IA/MO/IL border during the early evening. As such, have shifted the inherited 10 sig tor area south, but this threat appears more conditional than previously forecast. Consensus of latest CAM guidance still suggests potential upscale growth into a small MCS as supercells congeal near/east of the MS River, with the relatively greatest severe wind threat across southeast IA and far northeast MO into west-central IL. How far downstream this threat lasts is more uncertain, as there's a reasonable signal for low-level warm theta-e advection to support potential regenerative supercells on the upshear (northwestward) side of broader-scale convective outflow within the feed of steeper mid-level lapse rates. This may yield a persistent threat for large hail into the early morning before convection weakens further into the overnight. Read more CHECK UPDATE ZOOM GRAPHIC
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