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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.


Monday, September 20, 2021

SPC Sep 20, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0218 PM CDT Mon Sep 20 2021 Valid 202000Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE UPPER AND MID MISSISSIPPI VALLEY... ...SUMMARY... Severe thunderstorms capable of damaging wind gusts and hail are expected across parts of the Upper Mississippi Valley southward to the Ozark Plateau this afternoon and evening. ...20Z Update... Overall, the forecast scenario outlined in the previous outlook remains valid, with strong to severe thunderstorms expected this afternoon through tonight along a cold front moving eastward/southeastward through the Upper/Mid MS Valley. ...Upper MS Valley... Only change was to increase tornado probabilities across northwest WI. In this region, winds are expected to stay more southeasterly throughout the afternoon and into the evening, ahead of the approaching surface low and cold front. Additionally, low-level winds are expected to strengthen ahead of the front, increasing to 30-40 kt 850 mb this evening. This increase in low-level winds coupled with southeasterly surface winds will result in a veering low-level wind profile supportive of tornadoes. Forecast soundings across the region suggest 0-1 km around 200 m2/s2 between 22 and 00Z. Buoyancy will be modest, but strong forcing along the front and attendant to the approaching shortwave trough should be more than sufficient for updraft maintenance. A linear storm mode with fast storm motion is anticipated across the region. Primary severe threat will be damaging wind gusts attendant to bowing line segments, but, as mentioned above, the environment supports the potential for a few embedded/QLCS tornadoes as well. ..Mosier.. 09/20/2021 .PREV DISCUSSION... /ISSUED 1113 AM CDT Mon Sep 20 2021/ ...MN/WI to MO... A strong upper trough is moving across the northern Plains today, while a cold front surges southeastward into the MS Valley. Ample low-level moisture is present ahead of the cold front, with dewpoints ranging from the lower 70s over MO to mid 60s over WI. Considerable cloud cover is present in the warm sector over the northern half of the risk area, limiting destabilization. However, strong forcing along the front will result in a line of fast-moving thunderstorms by mid-afternoon from southeast MN into central IA. This activity will move eastward into parts of western WI and northwest IL during the evening. Winds aloft and shear profiles are quite strong, likely resulting in bowing storm structures capable of damaging wind gusts and some hail. An isolated tornado or two is also possible. The eastern extent of the severe threat is more uncertain as diurnal cooling weakens low-level lapse rates and instability, but at least isolated strong storms may persist into eastern WI late tonight. Farther south, full sunshine is occurring over much of MO. This will help to steepen low-level lapse rates and encourage southward development of storms along the front into western MO late this afternoon. Shear profiles are weaker over this region, but better thermodynamic parameters than farther north may aid in the development of damaging wind gusts and hail in the strongest cells. Read more LIVE:
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