026 ACUS01 KWNS 221245 SWODY1 SPC AC 221243 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0743 AM CDT Fri Mar 22 2019 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE SOUTHERN HIGH PLAINS... ...SUMMARY... Severe thunderstorms are most probable across parts of the Texas/Oklahoma Panhandles and South Plains regions, this afternoon into early evening. Large hail and locally severe wind gusts should be the primary threats. ...Synopsis... A high-amplitude upper-air pattern will prevail over the CONUS this period, with very high-amplitude ridging from south TX across the Dakotas to AB, across AK and over the adjoining Arctic Ocean. A deep synoptic trough exists to its east, with one embedded/compact cyclone initially over NJ and a larger, intense shortwave trough over the upper Great Lakes and northeastern ON. The NJ cyclone will eject across New England through the period, weaken, and merge/phase with the larger cyclone forecast to evolve from the upstream wave, over the lower Great Lakes and Mid-Atlantic region. The combined vortex then should pivot northeastward across New England to the southern Maritimes of Canada by 12Z. Meanwhile, the Plains part of the aforementioned ridging will break down somewhat, as a strong trough associated with broad cyclone now centered over the Great Basin shifts eastward across the CO Rockies to the central High Plains. The existing 500-mb low over UT should eject northward and weaken. Meanwhile, a new low should develop tonight across southeastern CO/western KS from a basal shortwave trough now apparent in moisture-channel imagery over AZ. At the surface, 11Z analysis showed a cyclone over Atlantic waters south of western Long Island. This feature, which is nearly stacked with the mid/upper cyclone already, should move northeastward across the Cape Cod area toward the ME coast by 00Z, then northeastward past western NS. Though the boundary layer over the Mid-Atlantic/ Delmarva region will be well behind the associated/offshore cold front today, strong, rapid midlevel cooling ahead of the next mid/upper perturbation should enable steep low/middle-level lapse rates and isolated thunder potential. Meanwhile, a dryline initially drawn from central Coahuila to eastern NM is expected to become better-defined through the day across extreme eastern NM, moving to near the TX/NM line around 00Z. ...Southern Great Plains... Widely scattered to scattered thunderstorms should develop this afternoon along and ahead of the dryline, moving generally northeastward across the southern High Plains into the evening before weakening. Large hail and damaging gusts are expected from the most intense cells. The tornado threat is more marginal and conditional, but cannot be ruled out. Quicker evolution to quasi-linear mode is likely with northward extent, where buoyancy will be weaker, deep-layer forcing stronger, and winds aloft less orthogonal to the dryline. Meanwhile, more-sustained presence of relatively discrete modes with a few supercells are possible in the central/southern Panhandle and perhaps parts of the South Plains, where somewhat stronger CINH and still-favorable vertical shear are expected. As such, the 15% wind potential is skewed north, hail south, with some overlap over parts of the TX Panhandle. With continued moist advection in the boundary layer through afternoon, a narrow corridor of upper 40s to mid 50s dew points should form ahead of the sharpening dryline. Warm advection and pockets of diabatic heating, in a narrow corridor behind morning/early afternoon cloud cover, should contribute to the development of around 1000-1500 J/kg MLCAPE from the southern Panhandle southward, decreasing to pockets of around 500 J/kg over southeastern CO and southwestern KS. Effective-shear magnitudes of 40-50 kt are expected, while effective SRH increases past 300 J/kg late afternoon into early evening under the west rim of the intensifying LLJ. With southward extent through the Permian Basin toward the Big Bend region, moisture will be somewhat greater, but so will MLCINH, while deep-layer forcing and deep shear each weaken. Still, a short-lived supercell or two may form this afternoon near the dryline and/or outflow/differential-heating boundaries from morning activity, and move a relatively short distance eastward/northeastward, offering large hail and damaging gusts. The severe potential in this region is briefer, more isolated and more conditional than farther north, and a slightly extended marginal outlook area appears most appropriate at this time. The eastern extent of the severe threat will be limited by the narrowness of the favorable moist sector, and by substantially strengthening MLCINH with both time and eastward extent tonight. Convection emanating from the Panhandles may last somewhat longer and farther east relative to elsewhere, given the stronger storm-relative low-level flow above the surface related to a 50-55-kt LLJ. However, the severe threat likewise will diminish eastward across western OK because of near-surface diabatic stabilization and messier/less-suitable mode for hail production by elevated convection. Convection forming on the Serranias del Burro of Mexico may reach the Rio Grande late this evening, but this potential (and any accompanying strong/severe gust threat) appears too isolated/conditional for an outlook at this time. ..Edwards.. 03/22/2019 $$