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Weather conditions on bar directly above are near Lakefront. Tabs under Moon Phases refer to O'Hare (official).

Archives for SPC Convective outlook are updated daily with a live map a the beginning of the article. Touch or click map to get possible update, and follow link at end of article for referral to NOAA/NWS Storm Prediction Center. Archives for Chicago's hourly weather data were updated 24-48 hours after end of each day prior to Dec. 2020.

Tuesday, July 20, 2021

SPC Jul 20, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1204 AM CDT Tue Jul 20 2021 Valid 201200Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE LOWER GREAT LAKES AND NORTHEAST... ...SUMMARY... Scattered storms producing isolated severe wind or marginal hail are possible over parts of the lower Great Lakes and Northeast. ...Synopsis... As an upper jet dives southeastward across the upper Great Lakes, height falls will occur across the Northeast with midlevel westerlies increasing to around 35 kt. At the surface, a cold front will stretch from Lower Michigan into northern NY by late afternoon, with mid/upper 60s F dewpoints along it and to the south. Heating will result in an unstable air mass, with scattered storms along the front, some possibly producing marginally severe wind gusts or small hail. Meanwhile, an upper high will remain centered over the Four Corners area, with ridge extending north into SK. A weak disturbance is forecast to round the ridge over ND at 00Z and into MN during the evening. Sufficient moisture and instability may be present over parts of the Upper MS Valley Tuesday evening to support a few strong storms, but severe is not anticipated at this time. ...The Thumb of MI eastward across NY and into VT... Storms are likely to form along a cold front from Lake Huron into southwestern Ontario by 18Z. Heating will lead to a well-mixed boundary layer, with MLCAPE of 1500-2000 J/kg. Although winds in the low levels will be weak at 10-20 kt, storms should organize along the front, with forward-propagating line segments possible. Scattered wind damage is possible, with isolated gusts over 50 kt as storms spread across NY and surrounding areas. Cellular storm mode may yield marginal hail as well with deep-layer shear up to 30 kt. ...Upper MS Valley... Cool midlevel temperatures will exist across the region beneath northwest flow aloft, with 40-50 kt deep-layer shear. Heights will fall after 00Z due to the aforementioned wave, now over western MT. Although a high pressure ridge will extend from the central Plains to Lake Superior, strong heating will weak CIN, and weak lift during the evening could lead to isolated thunderstorms mainly over MN. MLCAPE around 1000 J/kg and steep lapse rates may support small hail, but the threat is too conditional to add probabilities currently. ..Jewell/Squitieri.. 07/20/2021 Read more LIVE: spc.noaa.gov