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Saturday, August 14, 2021

SPC Aug 14, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Sat Aug 14 2021 Valid 141300Z - 151200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF NEW ENGLAND ACROSS THE MID-ATLANTIC TO NORTH CAROLINA...AS WELL AS OVER PORTIONS OF THE CENTRAL HIGH PLAINS...ARIZONA TO THE LOWER COLORADO RIVER VALLEY...AND SOUTHERN/SOUTHWESTERN FLORIDA.... ...SUMMARY... Isolated strong to severe gusts are possible across parts of the Mid Atlantic and New England. Isolated damaging gusts and large hail are possible over portions of the central High Plains, as well as Arizona. Gusty thunderstorm winds and a tornado or two may be possible across portions of central and southern Florida. ...Synopsis... In mid/upper levels, a synoptic trough -- initially extending across Hudson Bay to southern ON and OH -- is forecast to move eastward across much of QC through the period. This will occur as the strongest basal shortwave perturbation -- evident in moisture- channel imagery over Lake Huron and adjoining parts of ON -- pivots northeastward across southern/eastern QC, outrunning the synoptic trough. The southern part of the shortwave trough will move down the St. Lawrence Valley. Meanwhile, a broad weakness in the height field over the central Plains, mid/upper Mississippi Valley and southeastern CONUS will persist, with northwesterly to northerly flow on its back side over the central High Plains. The surface analysis at 11Z showed a cold front from southern QC southwestward over western NY, northwestern PA, southwestern OH, extreme southern IL, and the MO/AR Ozarks, becoming diffuse over OK behind a batch of convective outflows. By 00Z the front should reach or pass Downeast Maine and Cape Cod, then extend southwestward over southeastern VA, western NC, and northern/central portions of AL/MS. A lee trough was drawn from central MT across southeastern WY, east-central CO, and northeastern NM. By afternoon, the lee trough will shift eastward and sharpen somewhat over the central/ southern High Plains, either collocated with a developing dryline or very nearly so. ...Northeast/Mid-Atlantic... Widely scattered to scattered thunderstorms are expected to develop today along/ahead of the cold front from northern New England through the coastal northern Mid-Atlantic to NC. The main concern over both lobes of the outlook, as well as the narrower but now connected area between, will be damaging to isolated severe gusts. In the northern outlook lobe: frontal lift, as well as large-scale ascent/destabilization aloft preceding the ejecting shortwave trough, will contribute to convective potential. Low-level instability and overall buoyancy will be modest compared to farther southwest, but still sufficient in a weak-CINH environment with MLCAPE generally in the 500-1000 J/kg range (locally/briefly higher), and relatively maximized upper/anvil-level winds. Deep-layer flow/lift will decrease with southwestward extent, but greater heating/buoyancy will offset that, with MLCAPE maximized into the 2000-3000 J/kg range over parts of the southern Delmarva/ southeastern VA/northeastern NC region. Though deep shear will be weak, a combination of water-loaded pulse downdrafts and clustering into mesobeta-scale cold pools will support strong to marginally severe gusts capable of damage. ...Central High Plains... Isolated severe hail cannot be ruled out with an ongoing cluster of elevated convection across portions of west-central/southwestern NE roughly between LBF-OGA. Steep midlevel lapse rates and increasing moisture in the 700-850-mb layer are supporting up to about 1500 J/kg MUCAPE, with 30-40-kt effective-shear magnitudes, based on the 12Z LBF RAOB and modified RAP soundings. This activity may produce an outflow/differential-heating zone upon which afternoon and evening convection can focus or interact, in addition to the lee trough/dryline. Isolated to widely scattered thunderstorms are possible this afternoon into evening, predominantly along and east of the lee trough/dryline. Afternoon heating and adequate low-level moisture are expected to support development along the boundaries, with surface dew points in the upper 50s to mid 60s F. This, and a well-mixed boundary layer, will underlie steep midlevel lapse rates (850-500-mb values around 7.5-8 deg C/km), supporting 1000-2000 J/kg MLCAPE. Strongly veering winds with height will contribute to effective-shear magnitudes around 40-50 kt and hodographs supporting both organized multicells and supercell potential. The main question will be coverage, with a good deal of inconsistency found in convection-allowing models valid for that time frame. This is not surprising, given mesoscale uncertainties on both location and strength of the boundaries involved. Until those uncertainties are better-resolved, will hold the unconditional severe potential at 5% wind/hail levels.. ...AZ...lower Colorado River Valley... Widely scattered to scattered thunderstorms are possible late this afternoon into tonight, initially over higher terrain along and near the Mogollon Rim. Some clustering and southwestward/outflow-driven discrete propagation possible southwestward across the western through southern deserts, with isolated severe gusts possible. Hail near severe limits also may be noted relatively early in the convective cycle. Coverage is uncertain given the precip/clouds over parts of AZ this morning, and related recovery period needed through much of the day as those dissipate. Favorable destabilization will be delayed following the previous night's severe MCS over portions of central/ southwestern AZ, but still should occur amidst a moist air mass this afternoon into evening. Morning RAOBs and GPS data indicate 1.5- 2.25-inch PW values across south-central/southwestern AZ, and values generally in the 1.5-2-inch range should hold through most of the period. Predominantly easterly flow component is forecast through the 800-400-mb layer over most of the outlook area, supporting movement over the deserts of convection initiated in the higher terrain, as well as outflow propagation encouraged by the ambient flow. ...FL... As of outlook time, T.D. Fred is forecast by NHC to move away from the disruptive influence of Cuba today and strengthen back to a tropical storm overnight into Sunday across the eastern Gulf, after its re-formed center passes west of the Keys. As NHC track forecasts for this system have trended westward, so will the eastern/downshear rim of the expected marginal tornado threat across western/southern FL and the Keys. As Fred organizes over the Gulf, the hodographs should increase in size from the Keys up the southwest/west-central coast of the peninsula and perhaps inland, along with low-level convergence suitable for convective development in that fringe of the favorable eastern sector that brushes across land. If the track trends even further seaward, the already marginal tornado potential will shrink still further spatially. See latest NHC advisories for track/intensity guidance and tropical watches/warnings related to Fred. ..Edwards/Kerr.. 08/14/2021 Read more LIVE: spc.noaa.gov
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