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Wednesday, September 1, 2021

SPC Sep 1, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0246 PM CDT Wed Sep 01 2021 Valid 012000Z - 021200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID-ATLANTIC... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS... ...SUMMARY... The threat for several tornadoes and scattered damaging winds will continue across parts of the Mid-Atlantic as the remnants of Ida move through the region into this evening. The greatest tornado threat remains centered from central Maryland to southern/central New Jersey. ...20Z Update... Only minor changes have been made to the categorical severe risk categories across the Mid-Atlantic into southern New England based on recent convective trends. A broken band of low-topped supercells with a history of producing tornadoes developed earlier this afternoon over central MD. Strong low-level shear noted on the 16Z sounding from IAD and recent VWPs from KLWX remains centered from northern VA into central/eastern MD and continuing into southern NJ. Recent upscale growth into a cluster has occurred with this leading broken line, but the favorable shear environment will continue to support a threat of tornadoes with embedded circulations through the rest of the afternoon and into this evening. Scattered to possibly numerous damaging winds may also occur with convective downdrafts as low-level lapse rates continue to steepen with diurnal heating along/south of the surface front. Additional storms may strengthen in another confluence band moving from northern VA into central MD. The tornado potential should gradually shift northeastward across the northern VA/MD/DE region and into southeastern PA and southern NJ in tandem with Ida's remnant circulation and a southerly low-level jet. Storms along/near the front may have the best chance of producing tornadoes given surface winds backed to a more east-northeasterly direction, locally enhancing 0-1 km SRH. See Mesoscale Discussion 1671 for additional details on the near-term severe threat across the Mid-Atlantic. A Marginal Risk for occasional damaging wind gusts has been added to parts of the Southeast for a small cluster of storms that has developed across central AL this afternoon. Modestly enhanced low/mid-level winds and steepened low-level lapse rates may allow for enough downdraft acceleration to produce mainly isolated tree damage. Additional clusters may form farther west across parts of MS, AR, and LA. See recently issued Mesoscale Discussion 1672 for more information. ..Gleason.. 09/01/2021 .PREV DISCUSSION... /ISSUED 1117 AM CDT Wed Sep 01 2021/ ...Mid-Atlantic States... The remnants of TC Ida will move east-northeast from southeast WV across northern VA through the afternoon to the southern New England coast overnight. Mid 70s boundary-layer dewpoints are prevalent across eastern VA to southern NJ to the south of a northward drifting warm front. Here to, surface temperatures have warmed into the 80s amid scattered to broken cloudiness which is already supporting a plume of MLCAPE from 500-1500 J/kg. Low to mid-level vertical shear will increase into this evening as the remnants of Ida become further absorbed into the deepening longwave trough emanating south from QC. An initial arc of low-topped showers has developed across parts of north to southeast VA, largely ahead of schedule compared to much of the CAM guidance. As large-scale ascent and instability continue to further increase, it seems probable that this arc will eventually deepen into a broken band/cluster with several embedded supercells that will spread east-northeast towards the Atlantic coast into this evening. The relatively greatest tornado threat should exist along the baroclinic zone from central MD into south NJ with several tornadoes and occasional damaging gusts probable. See MCD 1668 for further near-term discussion. Low-level static stability and the exact cyclone track will determine the extent of the tornado threat this evening into tonight across Long Island and the immediate southern New England coast. Otherwise, damaging winds will be possible given the rather strong low to mid-level wind fields. Vertical shear will also be progressively weaker farther to the south of the warm front. Still, it should be adequate given the buoyancy for some threat of an isolated tornado or two into eastern NC, as well as the potential for a few damaging gusts with scattered storms along and just ahead of the primary wind shift trailing southward from the Ida remnant cyclone. ...NE Panhandle into western/central SD... Lee cyclogenesis is expected this afternoon across northeast CO, on the southern fringe of southwest mid-level flow. An inverted trough will extend north of the lee cyclone, across the NE Panhandle into western SD, and a slow-moving northwest/southeast-oriented warm front will intersect the trough across the NE Panhandle. With daytime heating/mixing, convergence and low-level ascent should be sufficient to weaken convective inhibition and allow at least isolated thunderstorm development late this afternoon across the NE Panhandle. The storm environment will favor cluster with a couple embedded supercells moving north-northeast along the trough, with an attendant threat for large hail and severe gusts into early tonight. Read more LIVE: spc.noaa.gov
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