SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0742 AM CDT Mon Oct 26 2020 Valid 261300Z - 271200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Organized severe thunderstorms are not expected across the U.S. mainland today or tonight. ...Synopsis/Discussion... The main mid/upper-level feature influencing this outlook is a positively tilted, synoptic-scale trough -- apparent in moisture- channel imagery from SD across southern WY and UT to southern CA. The southwestern part of this trough -- with an embedded/closing low currently located over UT -- will break southward through the period, with the 500-mb low hooking south-southwest then east to southern AZ by 27/12Z. As that occurs, the associated cyclone aloft will deepen and become more symmetric, with strong mid/upper-level difluence over the southern/central High Plains. Downstream, a broad fetch of low-level warm advection and moisture transport will combine with elevated frontal lift to support thunder potential north of the surface front described below, and that will include some thunder-accompanied freezing/frozen precip. At 11Z, the surface analysis revealed a wavy frontal zone offshore from the Carolinas and GA, extending across northern FL, then westward over the northern Gulf to near GLS and CLL. In the latter area, this boundary was being overtaken by an intense but slow- moving cold front, drawn from western KY across southeastern AR and south-central TX, to the Big Bend and Trans-Pecos regions and central NM. This front extended into a low between ABQ-GUP, then southwestward across southern AZ to northwestern Sonora, northeastern Baja and southwestern CA near SAN. This front will penetrate southward well into northern MX through the period, while moving slower across south TX and the lower Mississippi Valley. ...West-central/northwest TX to near the Red River... Steep midlevel lapse rates above the nearly saturated elevated frontal surface will foster MUCAPE around 1000-1500 J/kg over parts of this region through midday, before columnar cooling in low/middle levels reduces buoyancy. With 40-55 kt effective-shear magnitudes evident in modified RAOBs and forecast soundings, any sustained/ discrete cells may produce small hail, and brief hail approaching severe limits cannot be ruled out. However, the very isolated/ transient nature of this potential, with generally messy convective modes, precludes an unconditional severe-hail outlook. ..Edwards/Mosier.. 10/26/2020
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