ACUS01 KWNS 181954 SWODY1 SPC AC 181952 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0252 PM CDT Tue Aug 18 2026 Valid 182000Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF THE UPPER MISSISSIPPI VALLEY INTO THE GREAT LAKES...ACROSS PORTIONS OF THE PLAINS...AND ACROSS PARTS OF THE NORTHERN AND CENTRAL HIGH PLAINS... ...SUMMARY... Thunderstorms with severe wind gusts and isolated large hail will be possible this afternoon and evening from the central Plains into the lower Missouri Valley, as well as the Upper Midwest. Other storms capable of severe wind gusts and hail will also be possible from the Northern Rockies into the central High Plains this afternoon into tonight. ...20z Update... The primary adjustment for this update was the introduction of additional 15% wind probabilities across southern Montana into northern Wyoming and far western South Dakota. Latest GOES imagery and lightning trends show deepening convection within the higher terrain of southern to central MT. An approaching shortwave trough will promote additional ascent and east/southeastward propagation off the terrain through late afternoon and early evening. As this occurs, thunderstorms will spread into a warm and deeply-mixed environment featuring 0-3 km lapse rates on the order of 8-9 C/km and sufficient buoyancy to sustain convection. Given this favorable thermodynamic profile and ample shear across the region (sampled by 18 UTC RAOBs from RIW and UNR), the potential for severe winds appears higher than previously anticipated. Recent HRRR/RRFS time-lagged ensemble guidance appears to be capturing this trend and depicts higher severe wind potential than earlier solutions. ...Southeast Wyoming into Colorado and Nebraska... The early stages of deepening convection is noted near the Casper, WY area ahead of a weak mid-level disturbance and where daytime heating has resulted in surface temperatures climbing into the low 80s. While this initial activity will likely remain sub-severe, the potential for intensification should steadily increase as it spreads southeast into a more moist air mass. Similarly, cumulus is noted across north-central CO on the western fringe of the higher-quality moisture. Guidance continues to show considerable spread regarding convective coverage and evolution across both regions, but given these observed trends and a fairly supportive environment downstream of emerging convection, expanding hail and wind risk probabilities appeared prudent. ...Northeast Kansas/western Missouri... Latest guidance continues to show additional thunderstorm development later tonight within a warm advection regime across northeast KS into west/northwest MO. However, there has been a notable northward trend in recent HRRR/RRFS solutions compared to 12z guidance. 15% hail and wind probabilities have been adjusted northward to account for this trend. ..Moore.. 08/18/2026 .PREV DISCUSSION... /ISSUED 1212 PM CDT Tue Aug 18 2026/ ...Central Plains into Lower Missouri Valley... Scattered weakening thunderstorms continue across northeast Kansas/southeast Nebraska into northern Missouri, reinforcing a convective boundary that extends across the region. Thunderstorm redevelopment/intensification is conditionally possible through the diurnal heating cycle near the boundary, which would carry with it the potential for at least isolated hail/damaging winds. A more certain scenario is expected this evening, when thunderstorm development/re-invigoration is more probable as the low-level jet increases. Steep lapse rates and moderate vertical shear support the potential for thunderstorms capable of large hail during the initially more cellular mode. These storms will likely cluster quickly, with a trend towards upscale growth into a forward-propagating MCS with an increasing damaging wind potential as it spreads toward/across Missouri. ...Northern Rockies through the Central High Plains... A shortwave trough will progress southeastward from British Columbia/Alberta into the northern Rockies. Thunderstorm development is anticipated across the northern Rockies as this wave interacts with the destabilized and buoyant airmass across the region. Moderate westerly flow aloft with support the potential for more organized storm structures and the development of several line segments. These widely scattered thunderstorms are forecast to progress east-southeastward this afternoon/evening, with additional development possible from Wyoming into the central High Plains. The moderate westerly flow aloft, high storm bases, and steep low-level lapse rates support the potential for strong to severe wind gusts and possibly some increased hail potential through evening/early overnight as these storms progress east-southeastward toward the richer reservoir of moisture/buoyancy across the central High Plains, including western Nebraska/northeast Colorado/northwest Kansas. ...Upper Midwest including portions of Iowa/Wisconsin/Michigan... Low-level moisture is expected to increase across the Midwest ahead of an approaching cold front, resulting in moderate buoyancy by late afternoon. Near-frontal convergence will influence convective initiation, with the combination of moderate buoyancy and seasonally strong westerly flow aloft supporting strong to severe storms including some initial/semi-discrete supercells, which could yield large hail. A transition into linear segments is expected by evening, with damaging wind gusts becoming the more prevalent severe risk as storms spread generally southeastward. ...Mississippi/Alabama to northern Florida... Scattered thunderstorm development is expected this afternoon, particularly across portions of Mississippi into western/southern Alabama and northern Florida. These storms will develop amidst a very moist and unstable environment. Vertical shear will be weak, with an outflow-dominant/multicellular mode expected. Some clustering into one or more line segments appears probable, resulting in scattered thunderstorm coverage. Steep low-level lapse rates and relatively high cloud bases support the potential for strong downdrafts capable of locally damaging winds. $$