SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Sun Aug 01 2021 Valid 011300Z - 021200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER EASTERN PARTS OF VIRGINIA AND THE CAROLINAS...AND OVER PARTS OF CENTRAL/WESTERN NEW YORK AND CENTRAL PENNSYLVANIA... ...SUMMARY... Strong to severe storms with damaging wind and perhaps a tornado may impact southern portions of the Mid Atlantic today. A few strong to severe storms are also possible over the Northeast and the Gulf Coast States. ...Synopsis... In mid/upper levels, the large-scale pattern will continue to be dominated by: 1. High-amplitude ridging over western North America, near which weak shortwave troughs now over western ID and southern NV will move slowly northward through the period; 2. Cyclonic flow over most of the eastern CONUS, northeastward to a large, complex cyclone covering much of northern QC/Labrador and adjoining waters. A strong shortwave trough -- now apparent in moisture-channel imagery over Lake Huron, eastern Lower MI and northwestern OH -- is forecast to pivot eastward to the lower Great Lakes and upper Ohio Valley by 00Z, then to western New England and southeastern NY/NJ by 12Z tomorrow. The surface cold front associated with the Great Lakes perturbation was drawn at 11Z from a low over Lake Ontario through extreme southern ON, the Michiana area, to north-central IL and southern IA. The low is expected to move generally east-northeastward across the Adirondacks today and northern New England overnight. The front should reach eastern PA, central WV, and the lower Ohio Valley by 00Z. Thereafter, the front will become more diffuse in a broader field of northwesterly low-level winds and cold advection occurring behind a deepening cyclone that will be moving offshore from the Mid-Atlantic. The associated southern frontal zone was drawn across northern/western NC, northern AL/MS, southeastern OK, northwest TX, and southeastern NM. This boundary -- its baroclinicity reinforced in places by outflow -- should move slowly southward across the Carolinas, GA, AL, MS, AR/northern LA, southern OK, and north- central, central and southwest TX through the period. ...Carolinas/Southeast VA region... Ongoing convection across parts of northern NC and extreme south- central/southeastern VA may pose a local, marginal threat across all severe modes this morning, limited by lack of greater low-level lapse rates/buoyancy. Additional convection may develop southward into more of NC as well, before the more-substantial boundary-layer destabilization occurs from midday into afternoon. By then, scattered thunderstorms are expected near the front, a prefrontal surface trough, and perhaps sea-breeze boundaries. Other convection may develop behind the morning activity near the Blue Ridge of VA and move southeastward into higher theta-e as that air mass recovers behind the morning activity. Mostly multicells are possible, though a few supercells may also occur. Damaging gusts, a slight tornado threat, and isolated/marginally severe hail are expected. Rich low-level moisture, with surface dew points in the upper 60s to mid 70s F, and diurnal heating will offset modest mid/upper-level lapse rates enough to yield a corridor of MLCAPE in the 1500-2500 J/kg range roughly corresponding to the 15%-wind/"slight risk" area. Vertical shear (both low-level and deep-tropospheric) should be relatively maximized near the front, where surface flow is backed, and weaken with southward extent under progressively less mid/upper flow. Effective-shear magnitudes of 35-45 kt are possible. Consolidations/merger into a band or clusters of convection should occur with time as activity nears the coast, with the most intense embedded cells still capable of strong-severe gusts and perhaps a tornado. ...Northeast... Scattered to locally numerous thunderstorms are expected to develop from late morning through the afternoon along/ahead of the cold front -- first in northern/western portions of the region, then spreading/expanding eastward. Sporadic damaging winds and isolated severe gusts/hail are possible. While magnitude of damaging gusts should be mostly subsevere to marginally severe, given the lack of both greater moisture/buoyancy and low-level flow, the expected dense coverage of convection may result in enough events to justify a 15%/categorical upgrade to wind probabilities at this time. Convection will be supported by a combination of weak MLCINH, diurnal heating, modest but adequate low-level moisture, boundary- layer convergence near the front and low, and large-scale lift aloft ahead of the shortwave trough. Moist advection should increase surface dew points over most of this area to the upper 50s and low 60s F. This will contribute to MLCAPE mainly in the 300-800 J/kg range. Strong mid/upper (anvil-level) winds will aid in organization, though low-level flow will remain too weak to enlarge hodographs appreciably. Well-mixed sub cloud layers will aid in localized strong-severe gust potential, until evening stabilization of the boundary layer removes enough low-level CAPE/lapse rates to weaken convection substantially with proximity to the Mid-Atlantic Coast and western New England. ...Gulf Coast States to GA... Isolated to scattered thunderstorms - sometimes in clusters -- should develop near the surface front and move roughly southward to southeastward across the outlook area. Damaging to isolated severe gusts are possible. A combination of very rich boundary-layer moisture (surface dew points commonly in the 70s F, strong diurnal heating, and a deep troposphere will offset modest lapse rates aloft enough to generate favorable buoyancy, with peak afternoon MLCAPE in the 2000-3500 J/kg range across this corridor. Deep-layer flow and vertical shear will be weak, however, with multicell organization and strong-severe pulse downdrafts as the main concerns. Some aggregation and upscale growth of cold pools may occur, further concentrating potential for strong-severe gusts, but such processes will be strongly dependent on meso-alpha to storm-scale processes not reliably predictable at this time. ..Edwards/Leitman.. 08/01/2021
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