231 ACUS01 KWNS 281252 SWODY1 SPC AC 281251 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Tue Sep 28 2021 Valid 281300Z - 291200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID-ATLANTIC REGION... ...SUMMARY... Strong to severe gusts and hail are possible this afternoon and evening across parts of the Mid-Atlantic region. ...Synopsis... The mid/upper-level pattern over the CONUS will be progressive and highly amplified this period, featuring mean troughs over the East Coast and just inland from the Pacific Coast. In between, ridging will amplify as it shifts eastward from the Plains States to an axis from the western Gulf of Mexico to Hudson Bay. A shortwave trough -- embedded in a broad field of cyclonic flow around the mean trough -- was evident in moisture-channel imagery over the upper Ohio Valley. This perturbation will pivot southeastward to eastward today across WV, southern PA, VA, MD, and the Delmarva Peninsula, moving offshore between 18-00Z. In the West, the synoptic trough will shift eastward across the northern Rockies and over the Great Basin by 12Z tomorrow. A formerly cut-off low now over the San Juan Mountains has a broad but mostly weak field of associated cyclonic flow, covering the Four Corners States and central/southern High Plains. The low will devolve to an open-wave trough amidst larger-scale height falls preceding the progressive Pacific system. By 00Z, the opened trough should extend from central CO across eastern NM and far west TX. By 12Z, it should weaken further, with an axis near a CYS-LHX-MAF line. By then, convectively generated/enhanced vorticity lobes may exist in downstream southwest flow, across north and central TX. The 11Z surface analysis showed a low over eastern NY, with cold front southwestward over central parts of OH/IN/IL, becoming a warm front over northeastern MO, central NE and western SD. The eastern part of the front will move southeastward across New England and the Mid-Atlantic region through the period, reaching the Carolinas and northern GA by 12Z. ...Mid-Atlantic region... Widely scattered to scattered thunderstorms are expected to develop near the mid/upper trough and along/ahead of the cold front, offering occasional damaging to isolated severe gusts, and sporadic hail. Favorable lapse rates aloft near the 500-mb height/thermal troughs will overlap a diurnally heated boundary layer, containing 60s F surface dew points, with PW around 1/25 to 1.5 inches. Nearly unidirectional and not particularly strong low/middle-level wind profiles will maintain modest shear and support assorted discrete to clustered multicellular modes. Forecast soundings and planar progs reasonably indicate that similarly favorable/prefrontal diurnal destabilization and moisture as farther south will extend into the preconvective boundary layer across more of southeastern PA and NJ, so the 15% area has been expanded north somewhat, while being trimmed on the west end in central VA due to concerns over lack of greater lift/convective coverage farther behind the mid/upper trough. ...Southern Plains... Widely scattered to scattered thunderstorms should develop this afternoon in an arc across northwest/west-central/southwest TX in a zone of relatively maximized low-level ascent near the dryline, and diurnally weakened MLCINH. Isolated severe hail and gusts are possible from the early, relatively discrete stages (mainly multicellular). The afternoon/preconvective environment will be characterized largely by 60s F surface dew points beneath favorable low/middle-level lapse rates for MLCAPE around 1500-2500 J/kg, locally/briefly near 3000 J/kg. Though strong veering of flow with height is expected, modest speeds should keep hodographs small and effective-shear magnitudes generally under 35 kt. Some potential exists for upscale aggregation this evening into a loosely organized MCS near the eastern Hill Country and/or I-35 corridor of central/ south-central TX. Though low/middle-level winds and low-level lapse rates each will be weak over the area by that time, the wind probabilities have been shifted east somewhat over the area, to account for potential of isolated marginally severe gusts related to cold-pool-driven processes. ..Edwards/Jewell.. 09/28/2021 $$
