Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 18 2023 12:46:50 ACUS01 KWNS 181246 SWODY1 SPC AC 181245 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0745 AM CDT Fri Aug 18 2023 Valid 181300Z - 191200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF NEW ENGLAND...THE DESERT SOUTHWEST AND MONTANA... ....SUMMARY... Isolated, marginally severe thunderstorm gusts are possible over parts of New England, the Desert Southwest and Montana. ....Synopsis... A high-amplitude synoptic pattern will remain over the CONUS in mid/upper levels, anchored by a high over the southern Plains. Associated ridging will extend northward over the northern Plains for much of today, shifting eastward toward the upper Mississippi Valley late in the period. A deep downstream trough -- with intermittently closed cyclone now near the neck of ON, extends southward across the central Appalachians. This feature will progress eastward, so that by 00Z, the 500-mb low is over southern QC, with trough southward over the Mid-Atlantic then southwestward over the Carolinas. By 12Z, the low should reach the St, Lawrence Valley region near Montreal, with trough near an axis near BTV-JFK-RDU. Upstream from the ridging aloft, a longstanding cut-off low off the central CA coast will slow its already modest eastward advance, then meander erratically offshore, with cyclonic flow extending inland across parts of southern CA and NV. A strong northern-stream shortwave trough -- evident in moisture-channel imagery over central/southwestern BC -- is expected to pivot across the northern Rockies today, then deamplify and eject eastward over SK tonight. At the surface, the 11Z analysis showed an occluded/cold front related to the eastern mid/upper trough, extending across eastern NY, western CT, NJ, eastern VA, east-central NC, and southeastern GA, becoming quasistationary westward over the Gulf Coastal Plains to LA, and a warm front over north-central/northeast TX. Another warm front was drawn from a triple point near central Long Island Sound, eastward along the coast to just south of ACK. The warm front is expected to move northward/inland over southeastern New England through the midday, albeit modulated by intervening convection, and being overtaken by the occluded/cold front from west to east. These fronts should exit eastern ME by 12Z tomorrow morning. The trailing inland segment of this front should reach southeastern VA, the central Carolinas, and southern GA/AL/MS by 00Z, while moving northward as a warm front across western LA, northeast TX, and OK. Gradual cold frontogenesis is expected today over MT, with the boundary becoming well-developed across the Dakotas into eastern WY by 12Z. ....New England... Scattered thunderstorms are ongoing over western/southern New England in a loosely organized, nearly north/south swath ahead of the cold/occluded front, and astride (but mostly north of) the warm front. With this activity moving into a field of around 500-1000 J/kg MLCAPE in the warm-frontal zone, and 35-45-kt effective-shear vectors aligned orthogonal to more meridional segment(s), a few damaging to isolated marginally severe gusts or a brief tornado are possible in the next couple hours before activity moves offshore. Especially over central/northern parts of the outlook area, where deep-layer forcing and frontal lift are stronger, another band of convection may form along the cold/occluded front behind the morning activity. This potential second round will have very limited, if any, chance for downshear surface destabilization, given its relatively small spacing/quick timing following the first round. Still, with fast cell motions, damaging gusts approaching severe levels cannot be ruled out with the most intense embedded cells, before the convective regime outruns what is left of the modified boundary-layer warm sector. ....MT... Isolated to widely scattered thunderstorms are possible this afternoon near and behind the developing cold front, with potential for a few gusts near severe limits. Convection should develop around midday to early afternoon in vicinity of the Bitterroots, and ranges between there and I-15, where diurnal heating and moisture at those elevations will support deep convective growth. A fetch of monsoonal-origin, low/middle-level moisture will be maintained into this region, supporting peak/ preconvective MLCAPE near 500 J/kg in the mountains -- decreasing somewhat downshear. As it moves northeastward to east-northeastward across southwestern/central MT, activity will be high-based and somewhat moisture-starved, with a deep, well-mixed boundary layer containing nearly dry-adiabatic lapse rates. Rather fast storm motions will augment downdraft potential, though convection should diminish past central MT as it moves into a more-stable air mass. ....Desert Southwest... Ongoing cloud cover and precip over most of northern AZ will delay appreciable insolation into the afternoon. Still, enough diabatic heating/destabilization of higher terrain should occur to preferentially minimize MLCINH and support widely scattered to scattered, mid/late-afternoon thunderstorm development. This will occur amid increasing low/middle-level moisture amid a deep net southerly flow component. Recent GPS PW data, observed RAOBs, and forecast soundings all suggesting 1-1.5-inch PW values across the region. Activity should move generally northwestward to northward, with well-mixed subcloud layers beneath 200-800 J/kg MLCAPE supporting potential for isolated strong/briefly severe gusts. ...Edwards/Jewell.. 08/18/2023 $$ = = = 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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .