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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.

Wednesday, May 19, 2021

SPC May 19, 2021 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook Day 2 Convective Outlook RESENT 1 NWS Storm Prediction Center Norman OK 0112 PM CDT Wed May 19 2021 Valid 201200Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS... ...SUMMARY... Severe thunderstorms with large hail and wind damage will be possible tomorrow across parts of the northern High Plains. Marginally severe thunderstorms may occur outside of the Slight risk area in parts of the Northern Plains into the Upper Mississippi Valley. ...Synopsis... Broad upper ridging and associated large-scale subsidence will prevail across much of the eastern CONUS, as a mid-level trough gradually progresses eastward across the western CONUS. Across the Plains states into the Upper Mississippi Valley, deep-layer meridional flow will define the upper air pattern, with a wide corridor of 30+ kt low-level flow advecting moisture northward through the day. Low-level moisture return will become relatively more meager with westward extent across the central CONUS. Still, upslope flow along the higher terrain to the lee of the Rockies, along with glancing upper support from the approaching mid-level trough, will contribute to deep-layer ascent supporting the development of strong storms with some severe risk across the northern High Plains. Deep-moist convergence at the terminus of the stronger low-level flow across the Upper Mississippi Valley is also expected to foster organized storm development during the afternoon hours, with a couple of severe storms possible. ...Northern High Plains... Convective initiation is expected by mid afternoon to the lee of the Rockies. Storms are expected to increase in coverage and intensity atop a dry, deeply mixed boundary layer (extending up to 700-500 mb in some spots) given relatively lackluster low-level moisture recovery, with surface dewpoints remaining generally below 50F. Nonetheless, sfc-3km AGL lapse rates are expected to exceed 8.5 C/km by peak heating across much of the northern High Plains, overspread by 8+ C/km mid-level lapse rates supporting 1500+ J/kg MLCAPE. Vigorous convective development is expected along the lee of the Rockies, that combined with 30+ kts of sfc-6 km speed shear, will support updraft organization into multicellular clusters and transient supercells. Large hail may accompany the stronger updrafts. However, relatively mediocre low-level shear and evaporative cooling within the deep, very dry sub-cloud layer is expected to result in cold pool mergers and subsequent upscale growth into one or more MCSs by early evening. Damaging wind gusts may accompany the stronger cores embedded within any organized MCS. ...Northern Plains into the Upper Mississippi Valley... While widespread showers and isolated thunderstorms are expected within the axis of stronger 925-850 mb flow at the start of the period across parts of IA into MN, gradual clearing is expected later in the day. While low and mid-level lapse rates will not be as steep compared to areas farther west, richer low-level moisture is expected to compensate. As such, 1000-1500 J/K MLCAPE is expected by afternoon across the Upper Mississippi Valley, northwestward to the U.S./Canada border, where a cold front is expected to be slowly sagging southward. Damaging gusts and a couple severe hail stones may accompany the stronger storms embedded in the low-level wind maxima, and along the front itself. Given relatively modest speed and directional vertical wind shear (with bulk effective shear values remaining around or below 35 kts), the severe threat is expected to remain relatively isolated, with a Marginal risk in place to address this threat. ..Squitieri/Goss.. 05/19/2021 Read more LIVE: spc.noaa.gov