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Friday, August 13, 2021

SPC Aug 13, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Fri Aug 13 2021 Valid 131300Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE OHIO VALLEY AND CENTRAL HIGH PLAINS... ...SUMMARY... Severe thunderstorm gusts and/or hail are most probable today over parts of the central High Plains and nearby foothills, as well as portions of the Ohio Valley. ...Synopsis... Although a mid/upper-level cyclone will eject from northern Hudson Bay across Baffin Island while weakening, a series of shortwaves will help to maintain a belt of pronounced cyclonic flow to its south over the Upper Midwest, Great Lakes and much of the Northeast. Farther south, a persistent cyclonic-shear zone aloft, with embedded lows/vorticity maxima in the 300-700-mb layer, should persist across parts of central/southern NM and the AZ/Sonora borderlands, with easterly midlevel flow to its north. At the surface, 11Z analysis showed a cold front from a low over Lake Huron's Georgian Bay southwestward across southern lower MI, northern IL, the southeastern corner of NE, western KS and eastern CO. The western part of this boundary will become diffuse today amidst southeasterly flow on both sides, but with more diurnal drying/mixing to its south over southern/eastern CO. By 00Z, the remainder of the front will move to Lake Ontario, northern/western OH, southern IL and southern MO. Over parts of the lower Ohio Valley to Ozarks and OK, the front will be preceded by an interconnected series of outflow boundaries. ...Central High Plains/Rockies region... Widely scattered thunderstorms may develop this afternoon over higher terrain of southeastern WY and northern CO, moving east-southeastward to southeastward across adjoining Plains toward the NE Panhandle and northeastern CO. Damaging gusts and large hail will be the main concerns with the more persistent/organized convection. Activity should be supported by southeasterly near-surface flow, with related moisture advection/transport offsetting diurnal mixing enough to maintain favorable theta-e in the inflow layer. Diurnal heating will minimize MLCINH over the higher terrain beneath steep midlevel lapse rates, with a corridor of 1000-1500 J/kg MLCAPE developing from southeastern WY southeastward. The surface southeasterlies also will contribute to favorable storm-relative low-level flow, and strong veering with height. Weak midlevel winds will limit deep shear to some extent, though effective-shear magnitudes of 35-45 kt are evident in forecast soundings. This suggests multicell and at least brief supercell modes are possible. ...Ohio Valley to Ozarks and Northeast... Isolated to scattered thunderstorms in clusters are possible this afternoon into tonight within a long corridor from northeastern OK to the northeastern CONUS, mainly along prefrontal boundaries, but with some frontal development possible, especially over the Northeast this evening/tonight. Severe potential overall will be very nonuniform and erratic in organization across this area, with strong mesobeta- and smaller-scale forcing/boundaries being the greatest determinant of coverage. However, the greatest confidence in at least 15% concentration/clustering of threat appears to be over the Ohio Valley region this afternoon and evening. Widely scattered thunderstorms also may develop near the Blue Ridge of western VA, northeastward to southern PA, with locally damaging to marginally severe gusts possible as activity moves northeastward to eastward. A plume of rich boundary-layer moisture is in place ahead of the front and outflow boundaries, with dew points commonly in the upper 60s to low 70s F. Although modest midlevel lapse rates are expected across much of the swath, areas of sustained diurnal heating and a deep troposphere will help to boost peak preconvective MLCAPE to around 1500 J/kg. Modest lower middle-level and upper-level winds -- and related lack of vertical shear -- will temper storm organization, supporting mainly outflow-dominant multicellular modes. ...AZ... Widely scattered to scattered thunderstorms are expected to develop this afternoon around the Mogollon Rim and southward across the mountain ranges east and southeast of TUS. Convection should shift generally southwestward, with outflow yielding strong/isolated severe gusts. Diurnal heating of favorably moist higher terrain will weaken MLCINH and support convective development, with a favorable deep midtropospheric layer of easterly to east-northeasterly flow for movement toward the lower deserts. The lower-elevation subcloud/ boundary layer ahead of the early convection should become well- mixed, yet still remain moist enough to support 1000-2000 J/kg MLCAPE, with DCAPE also exceeding 1000 J/kg. Discretely propagating multicell convective development over aggregating outflow boundaries should occur, further reinforcing the outflow and sending it southwestward well the PHX/CGZ/TUS corridor and over parts of western/southwestern AZ before weakening tonight amidst strengthening CINH. As such, the threat area has been expanded across west-central to southern AZ. If confidence increases in a more tightly focused corridor of wind threat within this region, an area of greater unconditional probabilities may be needed in a later outlook. ...Southeastern mainland FL and Keys...late overnight... Tropical Depression Fred is forecast by NHC to intensify somewhat and become a tropical storm by the time it reaches the Straits overnight tonight, then crosses the Keys sometime during the day Saturday, per latest track forecast. A great deal of uncertainty remains in the tornado potential with this system the last few hours of this period into early day 2, given its presently disrupted/ disorganized structure and forecasts of strong deep shear limiting intensification rate, as noted in NHC discussions. The timing and intensity of recovery north of central/eastern Cuba will influence potential for supercell development, and remains unclear. Also, under current track/wind-radius forecasts, the most-favorable downshear/northeastern sector of this small system also will be limited in area, and may arrive after 12Z. Given these uncertainties, potential appears too low and conditional to introduce a marginal tornado line for the last few hours of the period -- but will be monitored in succeeding outlooks. ..Edwards/Kerr.. 08/13/2021 Read more LIVE: spc.noaa.gov