Subj : DAY1SVR: Marginal Risk To : All From : Mike Powell Date : Mon Sep 25 2023 10:01:00 ACUS01 KWNS 251248 SWODY1 SPC AC 251247 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Mon Sep 25 2023 Valid 251300Z - 261200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF SOUTHWEST TEXAS TO SOUTHWESTERN LOUISIANA...AND OVER PORTIONS OF PENINSULAR FLORIDA... ....SUMMARY... Isolated severe thunderstorms are possible across parts of Texas, southwestern Louisiana and peninsular Florida today. ....Synopsis... In mid/upper levels, the central region of a major synoptic-scale cyclone will remain offshore from the Pacific Northwest and Canadian Pacific Coast through the period, though height falls will shift progressively farther inland as the low moves northeastward. Downstream, a blocky pattern is evident over north-central/ northeastern North America, with an anchoring high expected to move slowly southeastward from the Hudson Bay region. A much smaller cyclone than the one in the northeast Pacific is evident in moisture-channel imagery over the Upper Mississippi Valley to the eastern Dakotas and southern MB. The cyclone -- cut off from the prevailing northern stream -- is centered over northwestern MN. That center is forecast to move southeastward to northeastern IA through 12Z tomorrow. To its south, a series of weak shortwave troughs and convectively induced/enhanced vorticity maxima will slowly traverse the broadly cyclonic flow field covering much of the Gulf Coast States. A deep-layer low in the easterlies -- initially over the Yucatan region with troughing northeastward over the eastern Gulf -- is expected to move slowly westward through the period. The 11Z surface analysis showed a slow-moving cold front from central AR across north-central to southwest TX. As this front continues to move slowly southward, its baroclinicity will merge with that of an outflow boundary across southeast through south- central TX. The combined boundary should decelerate and become nearly stationary over southeast through south-central/southwest TX by mid/late afternoon. Elsewhere, a weak, residual, quasistationary front was drawn across central FL, and is forecast to drift northward today while weakening further. ....Southwest TX to southwestern LA... A small plume of convection, that has persisted for several hours south of the front in a low-level convergence zone from the SJT area to west of ERV, still may contain cells capable of marginally severe hail for another couple hours before it weakens. Isolated to scattered thunderstorms are expected to develop this afternoon along and south of the front, over areas of orographic heating/lift in western parts, and along outflow/differential-heating boundaries produced by ongoing/morning activity in the Edwards Plateau region and southeast TX/southwestern LA. Environmental trends will include: 1. Area-wide diurnal destabilization with time today that will weaken MLCINH to negligible amounts, albeit at differing rates depending on cloud cover and boundary-layer moisture content; 2. Increasing low-level moisture eastward across the area, rendering deep/well-mixed subcloud layers over the Big Bend, lower Pecos and southern Edwards Plateau regions, but upper 60s to 70s surface dewpoints and PW 1.75-2 inches from the DRT area eastward; 3. A modest version of the subtropical jet in upper levels, running parallel to the outlook area, with slightly strengthening upper/anvil-level flow and cloud-layer shear to the west; 4. Veering with height, but of modest wind speeds, in low levels. These factors should contribute to the potential for a few organized multicells/multicell clusters with damaging to marginally severe downdrafts possible, and few supercells of at least transient nature over western areas capable of large hail and strong-severe gusts. Given considerable mesobeta-scale dependency of convective initiation/growth away from the higher terrain, coverage still is too uncertain to highlight greater (15%) unconditional severe probabilities within the lengthy marginal area. ....Central/southern FL... Scattered thunderstorms are expected to develop this afternoon across much of central/southern peninsular FL, mainly along and south of the front. Damaging to isolated severe gusts are possible, along with isolated, marginally severe hail in the most intense cores. To the east of the eastern Gulf troughing, southerly to southeasterly low-level flow and moisture transport will continue over much of the Florida Peninsula. This will foster widespread 70s F surface dewpoints and PW of 1.75-2.25-inches. In such a moist airmass, strong diurnal heating will erode already meager MLCINH, beneath a deep troposphere, offsetting modest midlevel lapse rates enough to yield 1500-2500 J/kg peak/preconvective MLCAPE. Sea- breeze boundaries, the front, outflow boundaries from initial activity, and subsequent boundary intersections will focus convection. Though deep-layer winds generally will be less than 30 kt with weak speed shear, veering with height will contribute to 20-30 kt effective-shear magnitudes, supporting multicellular organization. Convection should diminish in coverage and strength after dark as a combination of nocturnal cooling and spreading/ deepening outflow air stabilize the overland boundary layer. ...Edwards/Kerr.. 09/25/2023 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .