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Wednesday, September 29, 2021

SPC Sep 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 Wed Sep 29 2021 Valid 291300Z - 301200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF CENTRAL TEXAS TO SOUTHERN OKLAHOMA... ...SUMMARY... Isolated severe thunderstorms are possible this afternoon or early evening from portions of central Texas to southern Oklahoma. ...Synopsis... The basic upper-air pattern will be characterized throughout the period by troughing over the Rockies and Intermountain West, a synoptic cyclone digging southward then southeastward from QC across New England, and a high-amplitude ridge over the Mississippi Valley to Hudson Bay. Ahead of the mean Western States trough, a lengthy, negatively tilted, but much weaker trough was evident in moisture- channel imagery from southeastern WY across the central/southern High Plains. The latter trough should move northeastward across the central High Plains, KS and OK through the period, becoming more poorly defined with time. Still, embedded, convectively aided/ generated vorticity maxima will help to maintain pockets of large-scale support for convective potential. At the surface, 11Z analysis showed a cold front from northern NC to eastern KY, becoming a quasistationary to warm front across south-central IL, southwestern IA, and the central Dakotas, intersecting an occluded/cold front over north-central ND. The cold front was drawn from there southwestward through a low near CDR, then across west-central CO, southern NV, and central CA. This front should reach the eastern Dakotas, central NE, western KS, and northern AZ by 00Z. By 12Z the front should reach western MN, eastern NE, central KS, northwestern OK, the TX South Plains, and southern NM. To its south, a dryline should mix eastward through mid/later afternoon, reaching portions of southwestern OK and northwest/west-central/southwest TX to northern Coahuila by around 00Z. The dryline then will retreat westward into the Panhandle and Permian Basin regions overnight. ...Southern Plains... Isolated to scattered thunderstorms are expected to develop this afternoon into early evening in a corridor of strong heating and related minimized MLCINH, along/ahead of the dryline. Isolated severe hail/gusts are possible. Steep low/middle level lapse rates are expected beneath a relatively stable layer around 500 mb, with a deep troposphere. Moisture quality is a concern, given the large area of antecedent convective outflow to the south and southeast (with considerable ongoing activity still driving it as of this writing, near and over the Gulf). Still, model-sounding modifications -- to original profiles that may contain overdone mean mixing ratios -- still present values around 12-14 g/kg with surface dew points in the low-mid 60s F. These factors yield MLCAPE around 2500-3000 J/kg. Veering of winds with height, amidst modest low/midlevel speeds, will keep hodographs well-curved but generally small, and deep shear weak. Forecast soundings suggest only around 20-30 kt effective- shear magnitudes. As such, predominantly multicellular convection is expected, with any supercell characteristics weak and/or transient. Activity should diminish late this evening into tonight as the mid/upper trough weakens and leaves the area, and as the boundary layer diabatically cools/stabilizes. ...Eastern NE to southwestern KS... Widely scattered to scattered thunderstorms are expected to develop along and behind the surface front over this region, this afternoon through the end of the period. Some of this convection may produce strong gusts, especially while still on the front or very close behind. As the front impinges on a moistening, partly modified return-flow sector, destabilization from diurnal heating and warm advection will support surface-based initiation along the front, as well as elevated convection to its west. Surface dew points generally in the mid 50s to low 60s F, beneath pockets of midlevel DCVA/cooling related to the mid/upper trough, should contribute to areas of 1000-1500 J/kg MLCAPE from eastern/southern NE through much of KS, grading to around 1500-2000 J/kg in the prefrontal sector over southwestern KS where midlevel lapse rates will be steepest. However, substantial limiting factors are apparent, including: 1. Modest low/middle-level flow prior to the onset of the nocturnal LLJ, keeping low-level modest in the afternoon and effective shear for surface-based buoyant parcels weak through tonight, and 2. Difluent but modest mid/upper winds aligned largely parallel to the cold front. This will contribute to potential for a quasi- linear, anafrontal regime whereby the front, and subsequent reinforcing outflow, undercut convection forming along it, relocating the bulk of thunderstorms atop a stable boundary layer. For now, severe potential appears too limited and uncertain for an unconditional, categorical outlook area, but the scenario will be monitored for any focused, surface-based threat for convective wind in succeeding outlooks. ..Edwards/Jewell.. 09/29/2021 Read more LIVE: spc.noaa.gov
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