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Weather conditions directly above are near Lakefront. Top tabs refer to O'Hare (official).

Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Friday, September 16, 2022

SPC Sep 16, 2022 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Fri Sep 16 2022 Valid 161300Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE CENTRAL PLAINS... ...SUMMARY... Severe thunderstorms are possible across portions of the central Plains this afternoon and evening. ...Synopsis... Mid/upper-level troughing now over the Gulf of Alaska will dig south-southeastward across the Northeast Pacific. By 12Z tomorrow, this will form the southwestern node of a binary closed cyclone, well offshore from the Columbia River mouth, while the northeastern center meanders near the middle of the BC/AB border. This complex process will lead to height falls and amplified cyclonic flow over the Pacific Coast States, Great Basin and lower Colorado River Valley. Downstream west-southwest to southwest flow aloft will continue, with the leading perturbation now extending across central parts of SD, NE and KS. This feature should eject northeastward across the upper Mississippi Valley and Lake Superior region through the period, weakening as it moves through a larger-scale ridge. Behind that, the main feature for this forecast will be a shortwave trough evident in moisture-channel imagery from the northern Rockies to near the NV/UT border. This perturbation should reach central parts of MT/WY and north-central CO by 00Z, the trough should be located near an axis from GGW-RAP-AIA-AKO. The 11Z surface analysis showed a low over northwestern MN between FAR-TVF -- related to the leading shortwave trough. A surface trough and weak cold front extended from the low across eastern SD and central NE to northeastern CO, and should weaken through the day as mass response occurs to strong lee troughing over the High Plains. By 00Z, the primary lows should be located over southeastern CO and southeastern MT. These lows -- and the trough connecting them -- should migrate eastward tonight. By 12Z, the lows should reach western SD and northwestern KS, along a trough from northeastern MT to central NE to the eastern TX Panhandle. ...Central Plains... Isolated to widely scattered showers and thunderstorms are evident this morning over portions of UT, western CO, and western/southern WY, in a plume of large-scale ascent that precedes the NV/UT shortwave-trough segment. As the associated UVV field proceeds eastward today, it will overlie a diurnally destabilizing boundary layer to support early/mid-afternoon convection -- developing both over the higher terrain of central WY and northern CO, and near the lee trough on the adjoining High Plains. Initially, this activity will be high-based, forming over weak but sufficient moisture and deep/well-mixed subcloud layers. Wind profiles west of the lee trough will be nearly unidirectional, limiting effective shear, despite gradually increasing mid/upper-level speeds. Isolated severe gusts and marginal hail will be the main concerns at first. However, thunderstorms forming on or crossing the lee trough will gain greater severe potential while impinging on a more-favorable boundary layer with: 1. Surface dewpoints in the upper 40s to 50s F, but still steep lapse rates and deep mixing, supporting MLCAPE near 500 J/kg; 2. Weak but relatively backed flow, enabling enough veering and strengthening of wind with height for long hodographs and around 35-45 kt effective-shear magnitudes. This will support some supercell potential with activity that has not already become too outflow-dominant/cold-pool-driven in character atop the deep mixed layers farther west. Greater low-level moisture will be available to convection reaching west-central/southwestern NE late this afternoon and this evening, from parcels originating east and southeast of the old, decayed front. Some of the activity may persist into the evening with a continued wind/hail potential, before overnight stabilization in the foregoing inflow layer reduces the severe threat. However, elevated thunderstorms may persist or develop anew from southern/central NE to southeastern SD and eastern NE tonight, along the northwestern rim of a LLJ. The severe threat gradually becomes lower and more conditional with eastward extent and time tonight. ..Edwards/Leitman.. 09/16/2022 Read more CHECK UPDATE ZOOM GRAPHIC
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