Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 16 2025 06:02:00 ACUS01 KWNS 160601 SWODY1 SPC AC 160600 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0100 AM CDT Sat Aug 16 2025 Valid 161200Z - 171200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO TONIGHT ACROSS SOUTH DAKOTA AND PERHAPS ADJACENT PORTIONS OF NORTHERN NEBRASKA...NORTHEASTERN WYOMING...SOUTHEASTERN MONTANA...AND SOUTHWESTERN NORTH DAKOTA... ....SUMMARY... Scattered strong thunderstorm development, including a couple of evolving clusters posing a risk for strong to severe wind gusts, are possible today into tonight across parts of the northern Great Plains into Midwest and Great Lakes region. ....Discussion... Stronger westerlies in the northern mid-latitudes are forecast to generally remain confined to the north of the Canadian/U.S. border. Within this regime, mid-level ridging, now building across and northeast of the Canadian and adjacent northern U.S. Rockies, is likely to undergo further amplification across and east of the Canadian Prairies later today through tonight, downstream of an initially vigorous short wave trough pivoting northeastward inland of the British Columbia coast. Models indicate that larger-scale troughing across the northeastern Pacific will be maintained, as another vigorous short wave trough digs southeast of the Gulf of Alaska. Downstream of the building ridge, large-scale eastern Canadian mid-level troughing is forecast to undergo amplification, as another notable short wave trough digs across and southeast/east of the northern Ontario/southern Hudson and James Bay vicinities. Even as this occurs, it appears that modest mid-level height rises will develop across eastern portions of the northern U.S. Great Plains through the Upper Midwest and adjacent Great Lakes region later today through tonight, downstream of mid-level troughing within weak, broadly confluent mid-level flow across the Wyoming/Colorado Rockies into the western Dakotas. The remnants of an initially notable convectively generated or enhanced perturbation may dig southeastward within the northeastern periphery of the building ridging, across the Midwest/adjacent Great Lakes vicinity into Ohio Valley. However, there is considerably greater spread evident in model output concerning this feature. ....Northern Great Plains into Great Lakes/Ohio Valley... South of the westerlies, convective potential may depend largely on sub-synoptic developments with still relatively low predictability. The NAM and Rapid Refresh, among other model output, continue to suggest a notable perturbation evolving with a consolidating cluster of storms now across parts of central South Dakota into north central Nebraska. By 12Z this morning, it appears that this may be in the process of spreading east of the mid Missouri through upper Mississippi Valleys, above a boundary-layer stabilized by preceding convective activity. A conglomerate surface outflow boundary currently extends from parts of central Illinois through central Iowa and Minnesota. It seems probable that the southern flank of this boundary will undergo at least some modification/northward return into and beyond daybreak, but the extent remains unclear. Outflow impacted air across eastern Minnesota and northeastern Iowa into Wisconsin will probably be reinforced by the approaching cluster, which probably will tend to weaken while overspreading the Upper Midwest into Great Lakes region. However, depending on how quickly the boundary-layer is able to modify on the trailing flank of the reinforced outflow, it appears possible that renewed thunderstorm development and intensification could occur, and eventually grow upscale into another organizing cluster with a tendency to propagate southeastward toward the Ohio Valley. HREF output suggests that a corridor across southern Wisconsin toward areas near and south of southern Lake Michigan may need to be considered for higher severe probabilities in later outlooks for this period. However, given uncertain boundary-layer destabilization, coupled with a tendency for warming aloft/rising mid-level heights, in the presence of generally weak ambient mean flow, there seems a reasonable possibility that the corridor of higher convective potential evident in the HREF may be displaced, if not overdone. Otherwise, there remains a signal in model output that the approaching mid-level troughing could contribute to another upscale growing cluster of storms with potential to produce strong to severe wind gusts across parts of South Dakota this evening. However, this signal is more tempered than prior days. ...Kerr/Lyons.. 08/16/2025 $$ = = = 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) .