SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0643 AM CST Sun Feb 28 2021 Valid 281300Z - 011200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST TEXAS TO MIDDLE TENNESSEE AND SOUTHERN KENTUCKY... ...SUMMARY... Damaging thunderstorm gusts, a few tornadoes and isolated large hail are possible from northeast Texas to Middle Tennessee and southern Kentucky. ...Synopsis... In mid/upper levels, a positively tilted trough is apparent in moisture-channel imagery from central Canada across the northern/ central Plains and central Rockies to AZ and Baja. A shortwave trough initially over SD/NE will eject/stretch northeastward across the Upper Great Lakes through the period, losing amplitude, but remaining connected to a very lengthy vorticity banner extending southwestward around the northern/western sectors of a developing closed cyclone over parts of NM/AZ. The associated 500-mb low should reach southern NM by the end of the period at 01/12Z. Farther equatorward, a weak but potentially influential southern- stream perturbation over Chihuahua should eject northeastward across west-central/north-central TX today. At the surface, 11Z analysis depicted a surface low near the IA/MN border with cold front southwestward over southeastern KS, southwestern OK, and the TX South Plains. By 00Z, the low should reach ON, just north of Lake Huron, as the center of a well- developed and deepening synoptic cyclone, with cold front trailing across Lower MI, IN, the lowest segment of the Ohio Valley, the Arklatex region, and portions of south-central/southwest TX. By 12Z, the cold front should reach eastern NY, central VA, northwestern GA, central MS, central LA, the mid/upper TX Coastal Plain, and deep south TX. ...North TX to TN/KY... Scattered thunderstorms may develop along/ahead of the cold front as early as mid/late morning over parts of north TX and southeastern OK, increasing in coverage and extending northeastward through the day as the regime both backbuilds southward, shifts eastward and expands northeastward. An organized belt of scattered to numerous thunderstorms is expected to evolve as a result, by mid/late afternoon, from the Ozarks across southeastern OK to central/ northeast TX. The foregoing warm sector will destabilize and build buoyancy through the day from a combination of warm advection, moisture advection/transport, and modest/erratic surface heating under broken cloud cover. Surface dew points in the 60s to near 70 F should be common near and south of a decaying warm front now located over parts of TN, AR and eastern OK, and forecast to become more diffuse with time. This will contribute to a pre-cold-frontal plume of MLCAPE 1500-2000 J/kg over the TX part of the outlook, where midlevel lapse rates will be steepest, and 800-1500 J/kg elsewhere. Low-level and deep shear will be favorable, with effective SRH commonly in the 150-250 J/kg range despite somewhat veered flow just above the surface, and 60-80 kt effective-shear vectors. The latter will be aligned with a substantial component parallel to the main belt of convective forcing, such that a combination of embedded supercells and QLCS mesocirculations are possible. Those will yield potential for tornadoes and damaging to severe gusts. Hail will be more of a concern relatively early in the convective cycle and in western parts of the outlook area, while modes still are less messy, and amidst greater lapse rates. The severe potential may continue through much of the evening before weakening late tonight, except for the next regime described below. ...South TX tonight... Isolated to scattered thunderstorms are possible overnight -- mostly from about 06Z onward -- across portions of the Edwards Plateau, and adjoining parts of the Rio Grande Valley, northeastward over the Hill Country. This activity may develop on either side of the cold front, as strong moist advection via easterly flow occurs mainly in the 850-950-mb layer. Forecast soundings indicate this process, underlying steep low/middle-level lapse rates, will yield weak MUCINH, with MUCAPE ranging from around 1500-2000 J/kg over southern parts of the outlook area to 500 J/kg in the north. The substantial easterly component in the inflow layer will contribute to strong deep shear, with effective-shear magnitudes 60-80 kt possible. Activity developing on or just south of the front will be undercut quickly, but may persist long enough to produce large hail. At this time, coverage of the severe convection is uncertain, so the unconditional outlook will be maintained at marginal probabilities. ..Edwards/Jewell.. 02/28/2021
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