Subj : DAY1SVR: Day 1 Convective To : All From : Mike Powell Date : Wed Aug 21 2024 08:50:00 ACUS01 KWNS 211252 SWODY1 SPC AC 211251 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Wed Aug 21 2024 Valid 211300Z - 221200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF EASTERN MONTANA AND WESTERN NORTH DAKOTA... ....SUMMARY... Severe hail and thunderstorm gusts are most probable over parts of eastern Montana and western North Dakota. ....Synopsis... The CONUS portion of the large-scale, mid/upper-level pattern this period will be characterized by mean troughing over the East and West Coasts, and ridging extending northward over the Great Plains States from a high over west TX. A strong cyclone -- initially centered just south of the AK Panhandle -- is expected to dig south-southeastward, with its 500-mb center moving to around 200 nm west of UIL by 12Z tomorrow. As this process reinforces the mean western trough, a series of mostly low-amplitude shortwave perturbations will eject through the foregoing southwest flow. The best-defined and potentially most important of those perturbations evident in moisture-channel imagery over portions of northern CA, northwestern NV and southeastern OR. This feature should reach southeastern ID and west-central MT around 00Z, before ejecting northeastward across the northern High Plains and losing definition tonight. Meanwhile, in the subtropical easterlies, a shortwave trough will continue to drift westward across parts of Sonora, the Gulf of California, and central/northern Baja. At the surface, 11Z analysis showed rich Gulf-origin moisture still confined near and south of a slowly weakening synoptic front over northern FL, the northern Gulf shelf waters, and southeast to north-central TX. Nonetheless, a plume of mainly 60s F surface dewpoints was analyzed from much of OK north-northwestward across western/central portions of KS, NE and the Dakotas. A lee trough was drawn farther west, over eastern portions of MT/WY/CO. This trough should intensify through the day as ridging aloft shifts somewhat eastward, and southwest flow aloft spreads over more of the High Plains. As part of that process, surface cyclogenesis is expected over the northeastern WY/southeastern MT region, with the resulting low shifting eastward into northwestern SD around 06Z. ....Northern Plains... Widely scattered to scattered thunderstorms are expected to develop initially over the higher terrain of south-central/southwestern MT by midafternoon. In this area, heating of greater elevations preferentially removes MLCINH first, as large-scale lift ahead of the ejecting shortwave begins to move overhead. This activity should move northeastward into parts of central MT atop a boundary layer that is diurnally destabilizing/mixing, yet still favorably moist. Additional thunderstorms may develop by late afternoon near the surface trough, where strong surface heating and the western rim of the corridor of greatest moisture (surface dewpoints commonly in the upper 50s to mid 60s F) will remove capping at lower elevations. Convection of both origins may merge near the trough, over eastern MT. Along the way, activity should produce at least occasional large hail (mainly with any sustained supercells) and damaging gusts. Some aggregation of convection should occur, leading to forward-propagational clustering and increased potential for swath(s) of severe to potentially significant (65+ kt) gusts from late afternoon through much of the evening, as activity moves through a lingering, deep, well-mixed layer left by diurnal heating. A well-heated and deep boundary layer west of the moist plume will support MLCAPE in the 500-1500 J/kg range, with 1000-1500 J/kg DCAPE as well. This will foster high-based convection for the first few hours, with some potential for activity to cluster and become collectively outflow-dominant before reaching the grater moisture. weak low-level winds are expected, beneath 50-80 kt flow in the upper half of the buoyant profile, contributing to favorable deep shear (40-50 kt effective) for supercells, and long, relatively straight hodographs supporting splitting storms and hail potential where convection doesn't produce too much outflow too soon. Thunderstorms will move into larger nominal CAPE and greater near- surface moisture over easternmost MT into western ND, where the moist axis should be found, with MLCAPE of 2000-3000 J/kg. Though MLCINH also will be stronger in the higher-CAPE corridor than farther west, forced ascent that is driven by cold-pool processes should sustain some of the strong-severe convection well into ND this evening. Activity should peak near the moist/buoyancy axis and weaken afterword tonight, as it moves into progressively more-stable low-level profile, both with lower ambient theta-e and nocturnal diabatic cooling/stabilization near the surface. ....Central High Plains... Widely scattered to scattered thunderstorms should develop this afternoon over higher terrain from the Black Hills to the Laramie Range and southward over the Front Range, its foothills, and adjoining ridges such as the Cheyenne Ridge and Palmer Divide. This convection should move east toward the lee trough and low-level moist axis, while crossing over a hot, deeply mixed boundary layer. Isolated severe gusts are expected, and severe hail also may be a concern with relatively discrete cells. While vertical shear will be weaker than farther north, strong veering of flow with height (especially near the trough) should contribute enough deep shear (30-40 kt effective mainly north of I-70) for at least brief supercell potential. Aggregation of outflows may foster one or more eastward-surging clusters into this evening from western KS to central SD, and a relatively concentrated local severe-wind threat will exist in any such cluster. However, guidance is too inconsistent in convective evolution and longevity, and in location/number of such clusters, to assign greater unconditional probabilities within the long "marginal" area at this time. ....Southeastern AZ... Scattered thunderstorms are expected to develop this afternoon, at first over higher terrain then across deserts/valleys as outflow/ differential-heating boundaries develop. With a relatively deep layer of south-southeasterly midlevel flow, activity should move generally northwestward. Though most progs keep the flow aloft modest (15 kt or less), some mesoscale enhancement is possible east of the track of a northwestward-moving MCV -- evident in IR satellite imagery over north-central Sonora. The most intense convection will be capable of isolated severe gusts. Clouds related both to earlier/overnight convection over southern AZ, as well as to activity over MX, are south and west of this region already, portending essentially a full day of insolation over the outlook area. Residual lower-elevation surface dewpoints in the 50s to mid 60s should decrease only slightly through the diurnal heating/mixing cycle, offset to some extent by moist advection. MLCAPE in the 1000-2000 J/kg range should develop through early/mid afternoon, atop the well-mixed subcloud/boundary layer. ...Edwards/Mosier.. 08/21/2024 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .