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Monday, August 16, 2021

SPC Aug 16, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Mon Aug 16 2021 Valid 161300Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WITH A TORNADO THREAT OVER PARTS OF THE FLORIDA PANHANDLE INTO SOUTHEAST ALABAMA AND SOUTHWEST GEORGIA... ...SUMMARY... The threat for a few tornadoes should increase across the Florida Panhandle into southeast Alabama and southwest Georgia today. Other storms may produce isolated severe wind gusts and some hail from parts of the central and southern High Plains into the Southwest. ...Synopsis... The main belt of midlatitude westerlies aloft will remain north of the CONUS this period, except across portions of the Pacific Northwest. A series of 500-mb vorticity maxima over southern/ western BC, and the northeastern Pacific west of BC, will phase late today and tonight while moving southeastward. By 12Z, a strong, positively tilted shortwave trough should result from the Canadian Rockies across south-central BC, northern/western WA and northwestern OR, with a weaker perturbation to the south over parts of northern CA and northwestern NV. In response to these developments, height falls will spread southeastward across much of CA and the northern/western Great Basin, with the ridge breaking down over the southern Great Basin and desert Southwest. A collection of weak vorticity maxima and shortwaves -- evident in moisture-channel imagery over CO, NM, AZ and eastward across KS -- should move generally southward over the remainder of CO/KS, the southern High Plains, and AZ/NM through the period. At the surface, a wavy, quasistationary frontal zone was analyzed at 11Z near an ILG-HTS-OWB-LIT-TUL line, becoming a warm front northwestward over central KS. Lee troughing will persist over the High Plains today, with mostly weak surface flow on either side. A cold front associated with the strengthening mid/upper trough will move southeastward through the Pacific Northwest and northern Rockies, reaching north-central MT, southwestern IA and northern CA by 12Z. ...Northeastern Gulf Coast -- T.S. Fred... See SPC tornado watch 441 and related mesoscale discussions for the latest near-term coverage of the tornado threat with T.S. Fred. The center of the cyclone is forecast by NHC to reach the FL Panhandle coastline near PFN sometime early this evening, then move inland and decay. Fred's relatively small circulation has a northeastward tilt with height, as well as a decidedly asymmetric, northeastward-skewed convective field, related physically to strong deep (850-200-mb) shear impinging on the system. This should continue, rendering the bulk of broader-scale to convective-scale ascent, deep convection, favorable low-level hodographs, and potentially supercell-bearing convective bands in that sector arching from inner-north through middle/outer southeast of center, as often seen in such deep-shear settings. Diurnal heating will be significantly limited over the area of most-suitable lift and low-level shear, given the thickness and northeastward spread of Fred's cloud shield. As such, destabilization will be optimized more by advective processes and moisture transport, in the most purely maritime/tropical, highest-theta-e air mass. This air mass should spread inland to within about 100 nm of the coast through this evening. Potential will be more marginal with inland extent from there tonight due to weaker instability and smaller low-level shear with northward extent. ...AZ... Widely scattered to scattered thunderstorms are expected to form this afternoon and early evening over the Mogollon Rim and nearby higher terrain, once enough time has elapsed for favorable destabilization from residual cloud cover left behind from convection last night and this morning. Isolated strong/locally severe gusts and hail are possible early in the convective cycle. Some of this activity may aggregate outflow as it moves southward to southwestward off the higher terrain, evolving a discretely forward-propagational cold pool that would concentrate wind potential on the mesoscale. Moisture will remain favorable across the area, beneath a deep layer of initially northeasterly midlevel winds that should become more northerly with time. Peak preconvective MLCAPE in the 1500-2000 J/kg range should develop, locally higher. If confidence increases in a mesoscale corridor of greatest severe-wind threat, a 15%/slight area may be introduced in a succeeding outlook. ...Central Plains to southern High Plains... Isolated to scattered thunderstorms are possible this afternoon and evening -- both near the lee trough and around outflow/ differential-heating boundaries related to clouds/precip ongoing this morning. Convection should move predominantly southward, offering isolated severe gusts/hail. Some localized clustering and concentration of severe potential may occur, conditional on mesoscale boundary effects. Activity will be encouraged by steepening low/middle level lapse rates and weakening MLCINH, related mainly to boundary-layer heating, but also to subtle large-scale lift in midlevels associated with the perturbations aloft. With surface dew points commonly in the 60s F, areas of most-sustained diurnal heating will foster preconvective MLCAPE values of 1500-2500 J/kg over much of the Plains part of the outlook area. Although strong veering with height will be noted from surface through midlevels, weak absolute speeds will keep deep-shear values and hodograph sizes small for the most part. Still, brief supercell structures may develop where favorably oriented boundaries provide an augmenting source of low-level vorticity. Otherwise, multicell modes should prevail, with some clustering and localized cold-pool generation possible. ..Edwards/Kerr.. 08/16/2021 Read more LIVE: spc.noaa.gov
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