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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, September 21, 2021

SPC Sep 21, 2021 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0200 AM CDT Tue Sep 21 2021 Valid 231200Z - 241200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THURSDAY ACROSS PARTS OF SOUTHEASTERN NEW YORK STATE...ADJACENT PORTIONS OF MASSACHUSETTS...CONNECTICUT...AND LONG ISLAND...MUCH OF NEW JERSEY...EASTERN PENNSYLVANIA...DELAWARE...MUCH OF CENTRAL AND EASTERN MARYLAND...THE DISTRICT OF COLUMBIA...EASTERN VIRGINIA AND NORTHEASTERN NORTH CAROLINA... ...SUMMARY... Scattered strong thunderstorms may impact parts of the northern Mid Atlantic region Thursday, accompanied by at least some risk to produce potentially damaging wind gusts. ...Discussion... Models indicate that amplification within the westerlies will continue through this period, with mid-level ridging building across the Canadian Rockies into the Canadian Prairies, and downstream troughing digging across the central international border area into the northern U.S. Great Plains. In response to these developments, and as blocking persists in the downstream pattern across the mid-latitude Atlantic, a significant mid-level low initially over the lower Ohio Valley is forecast to accelerate north-northeastward, across and northeast of Lake Huron by 12Z Friday. The lead mid-level perturbation will be accompanied by an occluding surface low, with a trailing cold front advancing east of the lower Great Lakes and Appalachians, toward the Atlantic coast. A significant reinforcing cold front likely will surge southeastward through much of the northern Great Plains by late Thursday night. In the wake of the lead cold front, generally dry and stable conditions are expected across most areas. However, some low-level moisture return through the Gulf of California, beneath a southwestward propagating pocket of relatively cool mid-level air, may contribute to widely scattered thunderstorms across parts of the Southwest. Ahead of the lead cold front, 30-50+ kt southerly flow in the 850-700 mb layer may linger through much of the day Thursday across the northern Mid Atlantic. It appears that this will coincide with a corridor of seasonably moist air that may develop weak to moderate CAPE and support scattered thunderstorm development with daytime heating. Although lapse rates likely will be generally weak, some of this activity may be able to organize into lines or clusters, with precipitation loading contributing to downward momentum transfer and the potential for locally damaging wind gusts, before convection weakens Thursday evening. ..Kerr.. 09/21/2021 Read more LIVE: spc.noaa.gov