SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0635 AM CST Wed Mar 03 2021 Valid 031300Z - 041200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... At least a few thunderstorms are possible today over parts of Florida and near-coastal southern California, and tonight over northern Arizona. ...Synopsis... A progressive upper-air pattern with a somewhat active, regionally splitting southern stream is forecast this period. The first perturbation in that stream -- a strong shortwave trough initially located over portions of GA/SC -- will move offshore by 18Z. Behind that feature, generally zonal flow across the south-central/ southeastern CONUS will give way to height rises over the southern Plains. Those height rises will occur in advance of a marginally synoptic- scale (roughly 1000 km width of outer closed isohypse) cyclone now apparent in moisture-channel imagery west of southern CA and northern Baja. This vortex will move inland across southern CA today, with its 500-mb trough assuming negative tilt, and a cyclonically curved, 120-140-kt 250-mb jet streak over its southern semicircle. By 12Z tomorrow, the cyclone should reach northern AZ and central/southern UT, with trough back near neutral tilt, southward to northern Sonora. ...Southern CA and AZ... Widely scattered thunderstorms are possible in the associated frontal/convergence arc immediately preceding the cyclone core, where isolated, shallow thunderstorms also may develop and move ashore. The most probable area for thunder appears to be over southern CA from portions of the L.A. area through SAN. Isolated thunder also may be noted tonight over parts of northern AZ. Strong large-scale lift (with DCVA under left-exit region of aforementioned jet max) and accompanying cooling aloft will steepen lapse rates. Despite cool surface conditions, sufficient low-level moistening should occur with those greater lapse rates to yield MUCAPE of 50-150 J/kg, extending into icing layers favorable for sporadic lightning. ...FL... 11Z surface analysis showed a low just offshore from the GA Coast with a cold front southwestward across northern FL to the central Gulf. This front should move southeastward and off the peninsula around 03Z. A prefrontal low-level convergence zone and accompanying band of convection -- initially located across parts of central FL -- should move eastward with some backbuilding possible through early/mid afternoon. With the mid/upper perturbation now north of the area forecast to move away quickly, mid/upper support and low-level mass convergence should weaken with time, albeit with some offsetting influence from diurnal destabilization and weakening MLCINH of a moist air mass inland. Still, with weak lapse rates aloft and lack of stronger lift, thunderstorm coverage should remain isolated to widely scattered before diminishing late this afternoon and evening. ..Edwards/Jewell.. 03/03/2021
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