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Thursday, September 23, 2021

SPC Sep 23, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0746 AM CDT Thu Sep 23 2021 Valid 231300Z - 241200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID-ATLANTIC... ...SUMMARY... Strong thunderstorms associated with a marginal tornado and wind-damage threat are expected today across parts of the Mid-Atlantic. ...Synopsis... A progressive, amplified upper-air pattern will persist across much of central and northern North America through 12Z tomorrow. The most important feature for this forecast will be a prominent deep- layer cyclone -- currently centered over southeastern Lower MI. Its stacked, surface-to-500-mb low is progged to move north- northeastward across Lake Huron through the period, as a separate cyclone and associated strong trough pivot across SK/MB, with height falls over the Dakotas and MN. The surface cold front was analyzed at 11Z from an occlusion triple point over the north shore of Lake Ontario, across west-central NY, east-central PA, central MD, southeastern VA, eastern NC, and central FL. A warm front was drawn roughly east-northeastward near the St. Lawrence Valley. By 00Z, the cold front should reach eastern NY, NJ, coastal Delmarva Peninsula, the NC Outer Banks, the south-central FL Peninsula, and the east-central/south-central Gulf. ...Mid Atlantic... Isolated damaging gusts and a marginal tornado threat remain possible over the outlook area for most of the period. A band of showers and widely scattered to isolated embedded thunderstorms is ongoing along and ahead of the front from southern ON across Lake Ontario, central/eastern PA, central/eastern MD, and eastern VA. The associated zone of deep ascent (frontal lift and warm advection, with moisture transport) will persist through the period and make only slow eastward progress as the deep-layer cyclone shifts northward. Convection will continue to train northward within the precip belt, in an environment characterized by: 1. Rich low-level moisture and low LCL, with surface dew points commonly in the upper 60s to low 70s F and PW around 1.75-2 inches, but also, 2. Continued abundant/training precip, thick cloud cover and weak boundary-layer lapse rates. Buoyancy generally will decrease northward, with peak MLCAPE ranging from 250-700 J/kg in about the northwestern 1/3 of the outlook area to 1000-1500 J/kg across the Delmarva area east of the current location of the band. With the strongest, largely meridional flow aloft remaining behind the front, vertical shear will be more favorable in the boundary layer (0-1-km shear 30-40 kt north to 20-25 kt south) than in deep layers (effective-shear magnitudes around 40 kt north to 15-20 kt south). Effective SRH today should continue a pattern already noted this morning of increasing northward into more-backed surface winds, with values around 100-200 J/kg in higher CAPE over the Delmarva and 250-350 J/kg in eastern PA, NJ and eventually southern NY. In such an environment, isolated embedded mesocirculations may tighten up every several hours enough to cause a brief tornado or nontornadic wind-damage concern. The more widespread, consequential threat with this system should be from heavy rainfall; see WPC excessive-rain outlooks for more details. ..Edwards/Jewell.. 09/23/2021 Read more LIVE: spc.noaa.gov
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