Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 21 2025 16:32:10 ACUS01 KWNS 211632 SWODY1 SPC AC 211630 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1130 AM CDT Thu Aug 21 2025 Valid 211630Z - 221200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF EASTERN SOUTH DAKOTA INTO NORTHWEST MINNESOTA... ....SUMMARY... Scattered severe thunderstorms are possible across portions of the northern Plains and Upper Midwest this afternoon into tonight, with severe wind gusts and large hail the primary concerns. A few strong to severe storms may also occur across parts of southern Arizona. ....Northern Plains into the Upper Midwest... Enhanced mid/upper-level westerly flow will persist today across the northern Plains/Upper Midwest, sandwiched between upper troughing over the Canadian Prairie provinces and a stout mid/upper-level anticyclone centered over the Four Corners/Southwest. Multiple low-amplitude perturbations, some associated with ongoing convection across eastern MT into ND and northern MN, will move eastward today and potentially aid in additional thunderstorm development this afternoon and evening. At the surface, the primary low associated with the upper trough will remain well north in Canada, with a trailing cold front expected to continue moving east-southeastward across the northern Plains and Upper Midwest through the period. There is still considerable uncertainty regarding potential robust convective development and subsequent evolution along/near the cold front later today. This is mainly due to the stronger large-scale ascent aloft forecast to remain in central Canada, and capping concerns with southward extent along the front in the northern Plains due to fairly warm low/mid-level temperatures. Multiple potential scenarios remain plausible, ranging from little to no convection developing this afternoon/evening, as some recent convection-allowing model guidance suggests, to multiple supercells/clusters forming in a narrow zone of favorable instability and shear. Given these considerations, confidence is lower than usual regarding severe potential across the northern Plains/Upper Midwest. Still, a narrow corridor of moderate to strong instability will likely exist late this afternoon and into the evening along/near the front from northwest MN into eastern SD, aided by steep mid-level lapse rates and diurnal heating. Modest low-level flow veering to westerly at mid/upper levels will foster sufficient deep-layer shear (around 30-40 kt) to support organized convection, including the potential for a few supercells and clusters. Scattered large hail and severe/damaging winds may occur with this activity if it develops and can be sustained. So, a Slight Risk has been included with this update to account for this potential. Farther west, a post-frontal regime will exist this afternoon/evening across the northern High Plains of northeast WY and vicinity. Low-level upslope flow against the higher terrain may promote a supercell or two to develop and spread east-southeastward through the late afternoon and evening, with mainly an isolated hail threat. Some potential for clustering is also apparent this evening/overnight across parts of SD along/near the surface front. If this occurs, then a continued threat for strong to locally severe gusts may persist. Have therefore connected the Marginal Risks across SD for this possibility. ....Arizona... Modest mid/upper-level easterly flow will persist today over southern AZ, to the south of an anticyclone aloft centered over the Four Corners. Convection that initially develops over the higher terrain of southeast/east-central AZ may gradually develop westward into lower elevations through the late afternoon and continuing into the evening. Ample daytime heating of a seasonably moist low-level airmass already in place (reference observed 12Z PHX sounding) will act to further steepen low/mid-level lapse rates. At least isolated strong to severe gusts may occur with these thunderstorms given the favorable thermodynamic/lapse rate environment. The Marginal Risk has been expanded some to account for latest short-term guidance trends. ...Gleason/Supinie.. 08/21/2025 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .