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Thursday, July 29, 2021

SPC Jul 29, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Thu Jul 29 2021 Valid 291300Z - 301200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF CENTRAL/EASTERN PA...NORTHERN MD...DE...AND NJ... ...SUMMARY... Damaging thunderstorm winds and a tornado or two will be possible today over parts of the Ohio Valley, central Appalachians and Mid-Atlantic. ...Synopsis... The CONUS portion of the large-scale mid/upper pattern will change little through the period, anchored by a stout anticyclone whose core region of 594+ dm 500-mb heights will extend from the central Rockies to the Mid-South. To its northeast, a belt of northwest flow aloft will extend from SK/MB across the upper Great Lakes, becoming cyclonically curved through a trough over the Northeast. The most important feature embedded in that flow, for this forecast, is a shortwave trough initially located over portions of southern ON to northwestern OH and IN. Embedded MCVs -- related to the prior/overnight convective complex in the upper Great Lakes region -- were apparent east of southern Lake Huron, and over northwestern OH. This shortwave trough will pivot southeastward to eastward over the Mid-Atlantic region and southern New England today, moving offshore mid/late afternoon. At the surface, a low influenced by the aforementioned shortwave trough was analyzed at 11Z near APN, somewhat ahead of a cold front extending from eastern Upper MI across southwestern WI, northern IA, western NE and southeastern WY. The low and front should link today, with the low crossing Lake Ontario by 00Z and reaching near PWM by 12Z. The trailing cold front should reach northern OH, central portions of IL/IN, northern MO, southern/southwestern NE and south- central WY by 00Z. During the day, a warm front initially drawn across NJ/PA will move northeastward, its western segment near the low reaching southeastern NY and coastal southern New England late this afternoon into this evening. ...Mid-Atlantic and vicinity... Isolated to scattered thunderstorms should develop ahead of the surface cold front and along or south of the warm front today, with damaging gusts as the main threat, and a tornado or two possible. The greatest potential concentration of both convection and severe threat should be in a zone of favorably destabilized air ahead of the MCV/shortwave trough. The latter will provide large-scale ascent to boost ambient midlevel lapse rates somewhat, while also enhancing vertical shear to its southeast over parts of the Mid- Atlantic. Low-level shear should be favorable near the warm front, where surface winds will be relatively backed; however, buoyancy will be less than farther south amidst greater cloud cover and related slowing of diurnal heating. Rich moisture will be present over the area east of the Appalachians, with warm-sector surface dew points commonly in the upper 60s to mid 70s F. The associated high theta-e and diabatic heating will help to offset the modest lapse rates aloft, enough to support peak preconvective MLCAPE in the 1000-1500 J/kg range (briefly approaching 2000 J/kg locally). Greatest buoyancy should reside across the Chesapeake Bay/Delmarva and Delaware Valley regions eastward over NJ, with some extension across small parts of southeastern NY into southern New England possible prior to convective passage. Thunderstorms should concentrate most densely in and near the 30% wind area, shifted slightly northward to be closer to the low-level convergence max associated with the low, and under somewhat stronger deep-layer flow/shear. Forecast soundings indicate 35-45-kt effective-shear magnitudes in this area, generally decreasing southward. ...OH Valley to MO/southeast NE... Widely scattered to scattered thunderstorms are expected to develop along the surface cold front and prefrontal outflow boundaries, mainly over the OH Valley region, but potentially westward across northern MO/southeastern NE area. Damaging, isolated severe gusts and sporadic large hail are possible. The low levels south of the boundaries will remain very moist, with strong diurnal heating and a deep troposphere helping to drive MLCAPE into the 3000-4500 J/kg range over the corridor from NE to western IL, and 2000-3000 J/kg across portions of IN/OH that can recover in time from the ongoing clouds/precip. As buoyancy will increase westward, mid/upper-level flow and deep shear will decrease, though the latter still should be sufficient to aid some organized multicell to transient supercell structures, particularly over the OH Valley region. Severe potential, in reality, will not be uniformly distributed across the lengthy 15%/slight area, with upscale clustering and related concentration of wind potential in particular being possible on the mesoscale. ...Central High Plains... Isolated to widely scattered thunderstorms are expected to form this afternoon, mainly on higher terrain from the southern Bighorns into the Laramie Range and vicinity, and more conditionally, east- southeastward near the front. Isolated severe gusts/hail are possible from this activity as it moves northeastward to eastward late this afternoon and evening, before dissipating in a nocturnally stabilizing boundary layer. Overall coverage and longevity generally should diminish westward from MO into NE, hence the narrowing outlook area. A substantial easterly component of low-level winds will become established behind the front, enabling upslope lift, westward moisture/theta-e advection and greater storm-relative boundary-layer flow. This also will contribute to substantial veering of flow with height to augment deep shear, despite the lack of stronger deep- layer wind speeds near the mid/upper anticyclone. Insolation of higher terrain will weaken MLCINH as low-level moisture increases, contributing to steepening low-level lapse rates and strengthening buoyancy, with MLCAPE reaching 1000-2000 J/kg in a corridor of roughly front-parallel instability extending from east-central/ southeastern WY across the NE Panhandle and southwestern NE. Meanwhile, 35-40 kt effective-shear magnitudes are possible, supporting occasionally organized convective structures. Clustering and cold-pool forcing also may occur on the mesobeta scale for a few hours, locally augmenting the wind potential, though spatial uncertainties for such processes are too great to focus larger unconditional probabilities at this time. ..Edwards/Gleason.. 07/29/2021 Read more LIVE: spc.noaa.gov
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