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Thursday, September 9, 2021

SPC Sep 9, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Thu Sep 09 2021 Valid 091300Z - 101200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Organized severe thunderstorms are not expected across the U.S. today through tonight. ...Synopsis... In mid/upper levels, a high-amplitude western-ridge/eastern-trough longwave pattern will continue. The anchoring 500-mb high initially over the Four Corners region will move slowly east-southeastward to the southern Rockies, as a trough now well offshore of the West Coast approaches coastal northern CA, OR and WA late in the period. Downstream, a strong shortwave trough -- initially apparent in moisture-channel imagery from parts of IN to the Tennessee Valley region -- will pivot through the mean trough's cyclonic-flow field. This perturbation will cross the central/southern Appalachians today, then deamplify and eject northeastward over the Mid-Atlantic region to New England tonight. The associated surface cold front was drawn at 11Z from easternmost ON across central PA, western VA, northern GA, south-central MS, and south-central TX. Through early afternoon, as the shortwave trough approaches, a frontal-wave low may develop over the Delmarva region and ripple northeastward along the boundary toward Cape Cod, with considerable mesobeta-scale uncertainty still apparent over whether the low will pass over Cape Cod and adjoining islands, and then coastal Downeast ME, or just offshore from each, during the evening and tonight. Flow around the back side of T.D. Mindy also may influence frontal position and offshore-exit timing farther south across the Carolinas/GA segment of the Atlantic Coast. Either way, by the end of the period, the front should be offshore from Cape Cod to northern FL, and across the northern Peninsula to the northeastern Gulf. ...Northeastern CONUS... Widely scattered to scattered thunderstorms should cross both offshore waters and portions of the Long Island/southeastern New England corridor through this afternoon, near and ahead of the front. Prefrontal outflow boundaries from antecedent convection are evident from just east of Cape Cod across the ACK area southwestward toward waters just east of Cape May. Some westward retreat of the prefrontal boundary is possible today across southeastern MA, as the surface low develops to the southwest and approaches. The position of the boundary will be the main factor in whether a severe threat (localized damaging gusts, perhaps a tornado) brushes across the area or remains just out over water, amidst somewhat-favorable ambient flow and deep shear (e.g., effective-shear magnitudes around 30-40 kt). Given present offshore boundary trends, will refrain from introducing an unconditional severe area ahead of the synoptic front, though the situation will be monitored for succeeding updates. ...Coastal GA/SC...T.D. Mindy... The center of T.D. Mindy is near the GA coast this hour, with weakly favorable low-level wind fields in the downshear/ northeastern sector taking a narrow, short-lived path over portions of coastal SC. The spatial margin between offshore and onshore potential is razor-thin. Surface mesoanalysis shows a confluence line with weak, likely outflow-reinforced collocated baroclinic zone very close to the shoreline, northeast of HXD and coastward of CHS. Northerly surface winds are common north of that boundary, which is not likely to progress much (if at all) inland given the forecast track of the cyclone center. The main concern is thermodynamic, with the 12Z CHS RAOB just inland of the boundary showing a roughly 70-mb/2300-ft stable-layer depth above surface for a surface temperature of 73 F. While a strong convective gust or waterspout/brief tornado cannot be completely ruled out at or south of the boundary for another couple hours, overall severe potential is greatest offshore, and too isolated/conditional to introduce a categorical area. See NHC advisories for latest track/intensity forecasts and tropical watch/warning information related to Mindy. ..Edwards/Gleason.. 09/09/2021 Read more LIVE: spc.noaa.gov
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