SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0647 AM CST Fri Dec 18 2020 Valid 181300Z - 191200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorm development will be possible late tonight from parts of the Texas Hill Country northeastward to the Arklatex. No severe thunderstorms are expected. ...Synopsis/Discussion... In mid/upper levels, a positively tilted, synoptic-scale trough is apparent in moisture-channel imagery from eastern WY across western CO, northern AZ, and northern Baja. An associated/primary vorticity lobe extends northeast-southwest across the Four Corners area. This trough will move eastward through the period while maintaining positive tilt. By 12Z, it should extend from eastern IA across southeastern KS and west TX to north-central MX. Continental/polar air left behind the last frontal passage will remain the dominant boundary-layer condition over most of the central/eastern CONUS. The 11Z surface analysis showed a cold front from central/southern MB through a low near MBG, to another low between HLC-GLD, and southwestward over northeastern NM. By the end of the period, this front should move to Hudson Bay, eastern Lake Superior, eastern IA, southwestern MO, north-central TX, to near LRD. Ahead of that front, a plume of elevated, low-level warm advection and moisture transport will spread north-northeastward over the outlook area, accompanying a 35-45-kt LLJ. Isentropic lift of parcels with increasing mixing ratios to LFC will occur in a regime of steepening low/middle-level lapse rates above the boundary layer. Largest/deepest buoyancy, and greatest potential thunderstorm coverage, should be over roughly the southwestern 1/3-1/2 of the outlook area after 09Z -- or across portions of central to southeast TX. MUCAPE 500-700 J/kg may develop into icing layers suitable for lightning generation. Weaker buoyancy with isolated thunder potential will extend farther north during that time frame, and between 06-09Z throughout the area. Sufficient over-water airmass modification to support surface-based effective-inflow parcels still appears to be improbable until after the end of the period on the middle TX Coast, if at all. See SPC day-2 outlook for more details. ..Edwards.. 12/18/2020
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