SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0650 AM CST Wed Dec 23 2020 Valid 231300Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL GULF COAST REGION... ...SUMMARY... A threat for damaging gusts and a few tornadoes will shift eastward across the central Gulf Coast region, this afternoon through tonight. ...Synopsis... The dominant upper-air feature over the CONUS will remain a high- amplitude, progressive, synoptic-scale trough -- initially apparent in moisture-channel imagery from MB south-southwestward across the central High Plains to Chihuahua. Reinforced by several shortwaves and vorticity maxima, the associated cyclonic-flow field will shift westward and expand through the period, covering most of the land between the Rockies and Appalachians by 00Z. By then, the major trough should extend from MN-NM, where it will be reshaped by a strong, basal shortwave perturbation now digging southeastward over UT. By 12Z, the synoptic trough should extend from the upper Mississippi Valley across MO to central/southwest TX. At the surface, a primary low was analyzed over southern MN. A cold front extended from there over western IA, eastern KS, western OK, the TX Panhandle, and central NM. The low should reach northern WI or western Upper MI by 00Z and begin occluding, with cold front extending across southern IL, northeastern AR, and east to south- central TX. By 12Z, with the occluded low over eastern Lake Superior, the cold front should be sweeping eastward to southeastward near an axis from DTW-JKL-MOB and across the northwestern Gulf. A secondary surface low may develop along the cold front tonight, over parts of MS and AL, as the basal mid/upper- level shortwave perturbation pivots eastward over central TX. ...Central Gulf Coast and vicinity... Widely scattered to scattered thunderstorms should form by mid/late afternoon and expand in coverage this evening, in two primary early episodes: 1. A cold-frontal band across parts of AR and northeast/east TX, shifting eastward across AR/LA. This activity initially will offer a marginal severe threat as it impinges on a boundary layer somewhat destabilized by both muted diurnal heating and synoptically forced theta-e advection. 2. A nearly front-parallel but warm-sector low-level convergence zone from southeast TX to northern LA, likewise shifting eastward but not as quickly as the front. As such, the first should overtake the second tonight, probably across portions of MS to southern LA, forming a single primary thunderstorm band. The combined, primary thunderstorm band, in turn, should shift eastward along or just ahead of the front over southeastern LA, MS, and portions of AL and the western FL Panhandle by 12Z. Late tonight, a few thunderstorms also may move ashore in the western FL Panhandle within a newer prefrontal convergence zone developing over the adjoining Gulf. Damaging gusts will be possible throughout the outlook area, along with at least a marginal supercellular and/or QLCS tornado threat for most of the distance inland. The greatest juxtaposition of a favorably modified moist boundary layer with strengthening ambient shear and low-level hodographs will be across the "slight" area this evening and tonight. Surface dew points in the low/mid 60s F -- apparent in the partly modified warm sector over the northwestern Gulf now, should spread inland as far north as southern AR this afternoon and southern portions of MS/AL tonight, offsetting marginal lapse rates enough to yield around 300-800 J/kg MLCAPE. Meanwhile the approaching trough and its mass response will lengthen the deep-shear vector and enlarge hodographs, with effective shear/SRH values around 45-50 kt and 150-300 J/kg respectively, where sustained surface-based inflow appears most probable. Already modest boundary-layer lapse rates and low-level CAPE will diminish with northward/inland extent across the central Gulf Coast region, but isolated severe gusts and/or a tornado cannot be ruled out even near the north edges of the "marginal" area where MLCAPE less than 200 J/kg is expected. The blended severe regime should reach the western FL Panhandle by 12Z, then continue into day-2 from there eastward and southeastward across parts of FL/GA. ..Edwards/Smith.. 12/23/2020
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