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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 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1227 PM CDT Tue Jul 20 2021 Valid 211200Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN MID ATLANTIC INTO SOUTHERN NEW ENGLAND... ...SUMMARY... Scattered strong to severe thunderstorms are possible Wednesday afternoon from New Jersey northward through southern New England. Hail and damaging winds are expected to be the primary threats. ...Mid Atlantic/Northeast... A seasonably strong mid/upper-level trough is forecast to move eastward across the Mid Atlantic into New England during the day on Wednesday, and then move largely offshore by late Wednesday night. In conjunction with this feature, a surface low over Maine is forecast to gradually move eastward off of the coast by Wednesday evening, as a trailing cold front moves through much of the Mid Atlantic and New England. Some convection may be ongoing across the region Wednesday morning, which may hinder destabilization for some portion of the area, but where heating does occur within a moist low-level environment, cooling midlevel temperatures will support moderate buoyancy along and ahead of the front. Scattered thunderstorms are expected to develop along the front, and possibly along any other boundaries resulting from early morning convective outflow and/or differential heating zones. Low-level flow will remain rather weak, but increasing midlevel flow will support effective shear of 30-40 kt, sufficient for organized convection. Forecast soundings depict long, straight hodographs, suggesting the potential for strong multicells and perhaps splitting supercells with initial development, before storm mergers result in more of a cluster mode as storms move offshore by late afternoon/early evening. Hail and damaging winds will be the primary threats with the strongest storms. While some concerns remain regarding the relatively early frontal passage and potential for overnight convection and morning clouds to limit destabilization in some areas, the severe potential appears sufficient for a Slight Risk upgrade. ...Arizona... Another round of convection is expected across much of the Southwest on Wednesday, within the summer monsoonal regime. Midlevel flow is expected to become more easterly across portions of southern AZ, which may favor some potential for outflow-dominant clusters to move into the lower elevations with some gusty wind potential, though uncertainty remains since the flow and deep-layer shear is expected to remain quite weak. Wind probabilities may eventually be needed for some portion of this area, depending on shorter-term guidance and observational trends within the Day 1 outlook cycle. ...ID/western MT... Widely scattered thunderstorms will again be possible across portions of ID/western MT Wednesday afternoon/evening. Convection may be somewhat more organized than previous days as a mid/upper-level trough approaches, though the strongest flow/shear may tend to be displaced westward as storms move northeastward with time. Severe probabilities may eventually be needed for some portion of this area. ..Dean.. 07/20/2021 Read more LIVE: spc.noaa.gov