SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Fri Jun 23 2023 Valid 231300Z - 241200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE NORTH-CENTRAL PLAINS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED...RISK...AND ALSO OVER PARTS OF THE SOUTHERN PLAINS... ...SUMMARY... Scattered severe thunderstorms -- capable of producing very large hail and strong/damaging wind gusts and a tornado or two -- are forecast from southeastern Montana and eastern Wyoming into parts of South Dakota and Nebraska this afternoon through tonight. ...Synopsis... In mid/upper levels, some deamplification of the mean troughing near the West Coast is expected, as a strong shortwave trough and embedded small cyclone now over NV ejects northeastward. This perturbation should reach southeastern ID, western WY and eastern UT by 00Z. During the 06-12Z time frame, the 500-mb low should enlarge/deepen again over southern MT. By 12Z tomorrow, the low should be located near the MT/SD/ND junction, with trough across the Black Hills to near CYS. Farther east, a mean ridge will persist from northern MX northeastward across the lower Missouri Valley to the Upper Great Lakes. MCVs now apparent in composited radar imagery over north-central NE, another to the west near CDR, also northwestern SD, and the TX Panhandle, will move eastward toward or (in the southern Plains case) through the mean ridge position. The longstanding/broad cyclone in the East is centered over southern OH, and should continue to move slowly north-northeastward through the period. Weak height rises accordingly are expected amid broadly cyclonic flow across much of the Southeast. While locally damaging gusts cannot be ruled out from the most intense cells over a broad area from coastal LA to FL to the Mid-Atlantic, the severe-level gust potential has become too isolated and unfocused for a categorical outlook at this stage, though mesoscale trends will be monitored for any more-suitable concentrations of potential through today. At the surface, 11Z analysis showed a warm front from the Delmarva Peninsula across eastern VA to a weak low in central NC, becoming quasistationary across coastal AL, western LA, to eastern KS and southeastern NE. Another low was drawn between FSD-ONL, with diffuse and slow-moving frontal zone southeastward across south-central NE and eastern CO. Numerous outflow boundaries of varying strengths, lengths and ages are apparent south of the fronts across the Southeast and Plains. Broad surface cyclogenesis is expected from east-central WY to east-central CO this afternoon, consolidating gradually tonight and shifting eastward to south-central NE by the end of the period. ...North-central Plains... The most concentrated, well-organized severe threat appears to be over parts of the High Plains from southeastern MT to western NE this afternoon and evening. Scattered thunderstorms should evolve upscale from initial multicell and supercell development near the Bighorn and Laramie Ranges, with an early multimodal threat transitioning toward severe wind (locally 65+ kt gusts possible) with time this evening and overnight. The severe-wind threat may extend as far as eastern parts of NE/SD overnight, and outlook areas have been expanded east accordingly. Ongoing areas of nonsevere thunderstorms are apparent near the aforementioned MCVs in this region, and are expected to move eastward with a conditional/marginal severe threat as they encounter diurnally destabilizing boundary layer today. However, modest low/middle-level flow and lack of greater deep shear near the mean ridge may limit organization overall, with the unconditional threat level held at marginal. This does not rule out mesoscale concentrations of especially wind potential where any upscale organization can occur, but in such a subtly forced scenario, even high-res guidance understandably is wide-ranging in convective coverage and intensity. Behind the morning activity, a northwestward-aiming, narrow corridor of favorable airmass heating, warm-advection-related recovery, and upslope flow into higher terrain will support afternoon/evening severe potential. Activity should develop over and near the WY ranges this afternoon as strengthening large-scale ascent and deep shear spread over the region ahead of the ejecting shortwave trough, conterminous with favorably, diabatically minimized MLCINH. Initial mixed-mode convection will offer all severe hazards, with tornadoes and significant hail being a threat from any sustained supercell(s). Steep low/middle-level lapse rates and favorable boundary-layer moisture will contribute to peak/preconvective MLCAPE ranging from near 3000 J/kg over the northeastern CO/southwestern NE area to 1000-2000 J/kg in a small part of northeastern WY and southeastern MT. Lengthening and well-curved hodographs are forecast to develop as low-level cyclogenesis proceeds, in support of the hail/tornado threat. One or two MCSs should evolve this evening, accessing theta-e advection and moisture transport related to a 40-45-kt LLJ, with aggregation of cold pools rendering an increasingly widespread threat for damaging and severe gusts. ...Southern Plains... A secondary relative max in Great Plains severe potential is apparent over the TX Panhandle southeastward down the Red River Valley vicinity -- initially mainly south of a large area of existing clouds/precip and isolated/embedded strong-severe thunderstorms now over the northern TX Panhandle. The morning activity will leave a roughly east/west-oriented outflow boundary, along and south of which the greatest subsequent severe probabilities should focus. Initially isolated to widely scattered thunderstorms are expected to develop this afternoon over western parts of the outlook area, near what by then should be an essentially collocated dryline/lee trough as the former mixes eastward. Very steep low/middle-level lapse rates are expected as strong diurnal heating/mixing occurs near the dryline, and initial activity may be high-based with an early severe-downburst potential. As convection shifts eastward, it will encounter greater low-level moisture, with surface dewpoints into the 60s and (in some areas) 70s F, supporting preconvective MLCAPE commonly in the 2500-3500 J/kg range east of the Caprock. Though near-surface flow will be modest, strong veering with height will enable enough deep shear (effective values around 35-40 kt) for a blend of early multicell and supercell modes, offering both large hail and strong-severe gusts. Coalescence of cold pools from multiple storms is increasingly probable. Confidence has increased in the evolution of a forward-propagational MCS this evening and overnight down the Red River Valley region and toward what will be a moisture-rich corridor between central OK and north-central TX. The wind outlook has been extended even further eastward accordingly, for the overnight threat. ..Edwards/Mosier.. 06/23/2023
