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Monday, September 27, 2021

SPC Sep 27, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0804 AM CDT Mon Sep 27 2021 Valid 271300Z - 281200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE OHIO VALLEY AND PA... ...SUMMARY... Isolated severe thunderstorms may generate hail across portions of the Ohio Valley and Pennsylvania late tonight. ...Synopsis... In mid/upper levels, a progressive and highly amplified northern stream will persist, with troughs crossing the northwestern CONUS/ southwestern Canada region and southeastern Canada/northeastern U.S. mainland. A shortwave trough -- evident in moisture-channel imagery over parts of southwestern MN, western IA and eastern NE, is forecast to move through the broader cyclonic-flow field toward the eastern trough. By 12Z tomorrow, the shortwave trough should reach parts of PA, WV and OH. Farther west, synoptic-scale ridging will shift eastward out of the central Rockies and northern High Plains, reaching the lower/mid Missouri Valley and MB by the end of the period. A strong upstream trough -- now positioned over the Pacific south of a cyclone now west of Vancouver Island -- will move inland across northern CA/OR/WA between 00-06Z. This trough should reach the northern Great Basin and ID by 12Z. As associated height falls spread eastward toward the northern/central Rockies, a cut-off low now over western NM near the AZ border will move slowly northeastward. Its cyclonic-flow field will spread across the CO/NM Rockies and central/southern High Plains. A convectively augmented shortwave perturbation -- apparent over the TX Trans-Pecos/Big Bend region -- may pivot northward then northwestward around the broader gyre today into this evening, and become the primary 500-mb low overnight over the San Juans near the CO/NM line. At the surface, 11Z analysis showed a cold front from western QC across Lake Huron, southern WI, and southern NE. This front should reach Lake Ontario, southern Lower MI, and southern IA by 00Z, becoming quasistationary westward across the southern NE/ northeastern CO region. By 12Z, the front should reach central New England, northwestern PA and central IL. Substantial surface moisture (dew points mid 60s and upward) remains confined to the Gulf, southern FL and southern AZ. ...OH Valley/PA overnight... Isolated to scattered, elevated thunderstorms are expected tonight -- primarily after 06Z and ahead of the surface cold front. Marginally sever hail is possible from the most intense cores relatively early in the local convective cycle, with relatively discrete cells, before modes become more clustered/messy. Large-scale ascent -- both as warm advection and DCVA preceding the shortwave trough -- will contribute to steepening mid/upper-level lapse rates across this area. Low-level moisture advection and transport above the boundary layer will increase buoyancy and reduce MUCINH, as increasingly buoyant parcels lift to an LFC with time. Forecast soundings suggest around 800-1200 J/kg MUCAPE, a substantial fraction of which will be in hail-growth zones in mid/upper levels. However, usable vertical shear will be modest with nearly unidirectional flow through the cloud-bearing or effective layer. Forecast soundings indicate 25-35 kt effective- shear magnitudes, which will be somewhat greater with westward extent. However, convective coverage may be larger and more certain eastward, contributing to uncertainty on severe potential. ...Southwest TX... Widely scattered to scattered thunderstorms are possible this afternoon into early evening from the Permian Basin to near the lowest Pecos River, in a zone of weak but sufficient low-level convergence. Lift will be aided (and MLCINH minimized) by diabatic heating in the area behind the ejecting Trans-Pecos trough, whose backside DCVA field should be well north of the area by mid/late afternoon. A well-mixed boundary layer may support strong gusts or small hail. However, weak low/middle-level flow, and related lack of shear (effective-shear magnitudes 20-30 kt) indicate poor organization. At this time any severe potential appears too uncertain and conditional for a categorical outlook. ..Edwards/Jewell.. 09/27/2021 Read more LIVE: spc.noaa.gov