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Tuesday, June 22, 2021

SPC Jun 22, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook CORR 1 NWS Storm Prediction Center Norman OK 0922 AM CDT Tue Jun 22 2021 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF IOWA AND EASTERN NEBRASKA... CORRECTED FOR NDFD WIND GRID ...SUMMARY... The greatest severe-thunderstorm threat will be for gusts and hail this afternoon into tonight over portions of Nebraska and Iowa. ...Synopsis... In mid/upper levels, the CONUS will remain characterized by a high-amplitude pattern of progressive eastern troughing and western ridging, and a slowly meandering cyclone of the norther CA coastline. A series of shortwaves will pivot through the expansive area of cyclonic flow related to the eastern trough -- including one now apparent in moisture-channel imagery from near the IN/OH line south-southwestward to MS. That perturbation should eject northeastward across the central Appalachians to the Mid-Atlantic and western New England by 00Z. A weak shortwave trough evident over the northern CA/southwestern OR region should move slowly north-northeastward over OR today and penetrate the broader-scale ridge over ID tonight. The 11Z surface analysis showed a seasonally strong cold front from Maine to central VA, western NC, central AL, central LA, south- central TX, and northern Chihuahua. This front should move offshore from New England by 00Z, while reaching eastern NC, central GA, southern AL and southern LA, while decelerating/stalling from LA westward over south TX. Overnight, the front should stall over southern SC, southern GA and southern AL, while moving back north as a warm front and weakening farther west over TX. A separate, developing warm front was drawn from a weak low over eastern WY across the Pine Ridge area of southern SD, to east-central NE and the MKC area. This boundary should move slowly northeastward through this evening. ...NE/IA region... Isolated to scattered thunderstorms are possible from late this afternoon into this evening, mainly along and northeast of the surface warm front, with the larger potential coverage (but amidst weaker surface-based instability) being over the eastern parts of the outlook in IA. Hail and isolated strong to damaging gusts are possible. A more-conditional yet potentially greater-magnitude hail/wind threat exists near the Missouri River and westward over eastern NE, where strong surface heating and residual moisture (dewpoints reaching the 50s F) will erode most or all of the MLCINH and create well-mixed boundary layers amendable to maintaining hail and intense downdrafts to the surface. MLCAPE 500-1500 J/kg should develop across much of the outlook area. The eastward shift of the major synoptic trough, related height rises in this region, and lack of substantial northwest-flow perturbations to contribute large-scale ascent aloft, collectively impart some concern about enough convective coverage/longevity for a severe threat. Low-level flow generally will be weak, until a 30-40 kt LLJ ramps up after 00Z to support nocturnal convective potential in IA. Still, progs have been consistent in the idea of generating at least isolated surface-based convection near the warm-frontal/ surface convergence zone, and in a supercell-favorable wind profile. Forecast soundings reasonably suggest 21-00Z effective-shear magnitudes in the 45-55-kt range, given the veering with height in the lowest 300 mb and the favorably strong mid/upper-level northwesterlies. ...Southeastern CONUS... An ongoing swath of precip -- with embedded convection including widely scattered thunderstorms -- should continue to shift eastward across the southern GA/eastern FL Panhandle/northern FL region through the afternoon. Other isolated to scattered thunderstorms are possible today, northeastward across parts of the Carolinas to the Delmarva/Tidewater area, and perhaps in a zone of near-frontal airmass recovery behind the morning activity over parts of GA/southeastern AL and the FL Panhandle. The latter area appears to be the most conditional, given the extensive clouds/precip now over that region. Isolated damaging to marginally severe gusts are possible. A richly moist prefrontal air mass is in place, with surface dew points in the upper 60s to mid 70s F. Modest midlevel lapse rates will limit buoyancy, however, with MLCAPE generally in the 800-1200 J/kg range. Weak low-level shear will exist over most of the area, though portions of the outlook from northern NC northeastward will have strong flow in upper levels and greater cloud-layer shear to aid in local storm organization. Convective/severe potential should diminish gradually after dark. ...Northeastern NM and vicinity... Isolated to widely scattered, high-based thunderstorms may form through the afternoon over the eastern slopes/foothills of the Sangre de Cristo Mountains of NM,as well as eastward over the Raton Mesa. Activity should move southeastward a short distance across the adjoining plains, mesas and canyons, offering isolated severe gusts. Relatively warm midlevel temperatures near the upper ridge will keep lapse rates aloft modest. Still, available/marginal low-level moisture and strong diabatic surface heating of higher terrain should minimize MLCINH and support higher-elevation convective development beginning early this afternoon. Deep, well- mixed subcloud layers beneath 500-800 J/kg peak MLCAPE, along with enough directional shear to support effective-shear magnitudes around 35-40 kt, will support some organization before activity weakens near the eastern parts of the outlook area. ...Interior Northwest... Isolated to widely scattered, high-based thunderstorms are forecast to develop across the outlook area this afternoon and move northward to northeastward, offering a threat for isolated severe downdrafts. Low/middle-level moisture will be favorable, with PW values near an inch, while diurnal heating and large-scale lift related to the slow-moving shortwave trough contribute to favorably steep low/ middle-level lapse rates. The result should be an area of preconvective MLCAPE in the 500-800 J/kg range, atop a deep and well-mixed boundary layer conducive to downdraft acceleration. Effective-shear magnitudes of 30-40 kt in forecast soundings support some potential for organized convection. ..Edwards.. 06/22/2021 Read more LIVE: spc.noaa.gov
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