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Sunday, August 15, 2021

SPC Aug 15, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Sun Aug 15 2021 Valid 151300Z - 161200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF THE CENTRAL PLAINS...NORTHWESTERN ARIZONA...AND FLORIDA GULF COAST... ...SUMMARY... Isolated strong to severe storms are possible over portions of the central Plains, northwestern Arizona, and Florida Gulf Coast. ...Synopsis... With the departure of a mid/upper-level trough over QC, upstream northern-stream flow will become nearly zonal across southern Canada. the strongest winds aloft will remain over Canada, except for an area of the northern Pacific Northwest to northern Rockies, south of a strong shortwave crossing northern parts of BC/AB through tomorrow morning. A broad, ill-defined weakness in the 500-mb height field will persist across the central/southeastern CONUS, attracting what is forecast to become T.S. Fred northward toward a day-2 landfall near PNS (per latest NHC forecast charts). Farther west, a weak shortwave trough -- now evident in moisture-channel imagery over central/eastern WY -- is expected to move south-southeastward over central/eastern CO, around the eastern rim of an anticyclone over CA/NV. Another small perturbation -- initially located over southern AZ -- should move west-southwestward toward northern Baja through the period, around the southern fringe of the same anticyclone. At the surface, 11Z analysis showed a cold front over the Atlantic east of the Mid-Atlantic region, becoming quasistationary inland over VA, KY, the Ozarks, northeastern OK, and central portions of KS/NE. This boundary should move little through most of the day, then slowly retreat poleward overnight. ...Central Plains... Isolated to scattered thunderstorms are possible this afternoon and evening along/east of the lee trough, and along outflow and differential-heating boundaries related to this morning's convection/precip over parts of the area. Isolated severe hail/gusts will be possible. A low-level moist corridor, with surface dew points commonly in the upper 50s to mid 60s F will persist west of and parallel to the front. Forecast soundings suggest sufficient moisture for strong-severe convection will remain in a well-mixed boundary layer by mid/late afternoon, with steep low-middle-level lapse rates. This should result in peak/preconvective MLCAPE in the 1500-2000 J/kg range, atop 500-1500 J/kg DCAPE. Though mid/upper winds will be modest, strong veering of flow with height is forecast to enable 30-40 kt effective-shear magnitudes, as well as favorable low-level storm-relative flow. A blend of multicell and at least brief supercell modes is possible. The main concern remains storm coverage, with a good deal of inconsistency in progs that appears to be related to both handling of mesoscale low-level features and lack of more-robust forcing aloft. ...AZ... Widely scattered thunderstorms may develop this afternoon over higher terrain from the western Grand Canyon area southeastward near FLG, then intensify while moving southwestward into a favorably unstable air mass. Isolated strong-severe gusts and marginal hail will be the main concerns. IR imagery shows cloud cover from the previous day's convection has broken up across this region prior to sunrise. Strong diabatic surface heating should steepen low-level lapse rates and erode MLCINH throughout the day. Related deepening and vertical mixing of the boundary layer will reduce surface dewpoints from their current mid-50s to low-60s F values across the outlook area, but still with enough buoyancy to support sustained convection, with 500-1500 J/kg MLCAPE (locally/briefly higher south of I-40) in the immediate convective environment. A deep layer of low/middle-level northeasterlies -- enhanced somewhat by the perturbation over southern AZ -- will support southwestward movement of the convection across lower elevations before weakening occurs this evening. ...FL Gulf Coast... The remnants of Fred are forecast by NHC to reinvigorate to tropical-storm strength over the northeastern Gulf through tonight, while developing a more-organized circulation, with wind fields skewed northeastward by ambient southwesterly deep (850-200-mb) shear. As this occurs, the bulk of the supercell potential should remain over Gulf waters until day-2. There is considerable doubt as to whether the eastern rim of the intensifying (but still relatively small) circulation of Fred can spread favorable low-level shear and convection-supporting convergence far enough east to support supercells to form and mature along or inland from the west coast of FL. However, given the somewhat more easterly track in latest NHC guidance compared to previous, will leave a narrow sliver of that area in a marginal unconditional risk area. As the outer northern/northeastern rim of the strengthening circulation envelope approaches the coastal FL Panhandle late overnight into tomorrow morning, hodographs should enlarge along the coast from the western Apalachee Bay region past PFN, but with tornado potential still marginal, given that the most favorable areas of convection should be mostly offshore before 12Z. ..Edwards/Kerr.. 08/15/2021 Read more LIVE: spc.noaa.gov
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