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Tuesday, September 21, 2021

SPC Sep 21, 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 Tue Sep 21 2021 Valid 211300Z - 221200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF ARKANSAS...NORTHERN LOUISIANA AND WESTERN MISSISSIPPI... ...SUMMARY... Isolated severe gusts and/or hail are possible today over portions of Arkansas, northern Louisiana and western Mississippi. ...Synopsis... In mid/upper levels, the pattern over the CONUS will be dominated by a high-amplitude, progressive, synoptic-scale trough, initially from northern Hudson Bay south-southwestward across portions of the central/southern High Plains. A strong, basal shortwave trough -- now apparent in moisture-channel imagery near the eastern border of CO -- will dig southeastward across KS and northern OK today, reaching near a line from SZL-TUL-ADM by 00Z. This feature will move faster than the larger-scale trough and strengthen considerably through the period, reaching southeastern IL, western parts of KY/TN, and northern MS by 12Z tomorrow. Around that time, a split- flow regime should develop over the upper Great Lakes region, as the basal trough starts to form a closed 500-mb cyclone over IN early day 2. The related surface cold front was analyzed at 11Z from the easternmost portions of Lake Superior across southern Lake Michigan, south-central MO, southeastern OK, north-central to far west TX, and southern NM. By 00Z, the front should extend from the southwestern ON/DTW area through an elongated but intensifying low-pressure area over eastern/southern IN, central KS and western/mid TN, then across central portions of MS/LA to south TX. By 12Z the front should extend from a better-defined low over OH to eastern TN, south- central AL, and the northwestern Gulf. Isolated strong thunderstorms may occur almost anywhere from IN to east TX today as the front impinges on a diurnally destabilized warm sector, but a more-focused potential for marginal severe appears to be taking shape mainly over parts of AR/LA to near the Mississippi River. ...ArkLaMiss and vicinity... Scattered showers and widely scattered non-severe thunderstorms were evident this morning across much of the Ozarks to central AR, as a remnant of part of last night's MCS. The related prefrontal outflow boundary over central AR is expected to decelerate, stall, and perhaps retreat slightly northward today over central/southern AR ahead of frontal passage. The outflow/front intersection will be the most ready focus for afternoon initiation, due to relatively maximized low-level convergence. Other potential foci include: nearby parts of the front to the southwest, the outflow/ differential-heating zone, and fresher outflow subsequent to the first couple hours of afternoon convection. Scattered to numerous multicellular thunderstorms should move eastward to southeastward over the outlook area this afternoon, offering potential for isolated, marginally severe hail/gusts early in the convective cycle. As activity merges and grows upscale -- with additional development possible into northern LA -- damaging to isolated severe gusts will become the main concern. Strong heating of a richly moist boundary layer is forecast southeast of the front and along and south of the outflow boundary, with temps in the 90s F, low/mid 70s F dew points, and PW near 2 inches. Related large boundary-layer theta-e, along with a deep troposphere, will offset modest mid/upper-level lapse rates enough to yield 2000-2500 J/kg warm-sector MLCAPE. These processes will foster convective potential under a regime of height falls aloft, strengthening upper/anvil-level flow and upper difluence, even with the greatest large-scale ascent progged to remain behind the surface front. Weak, nearly unidirectional low/middle-level flow render only around 15-25-kt effective-shear magnitudes in forecast soundings. Regardless, deep/water-loaded downdrafts and localized cold-pool effects will contribute to potential for strong to isolated severe gusts. A general areal/ intensity decrease is expected near southeastern fringes of the outlook area by mid/late evening, as outflows spread and the boundary layer nocturnally stabilizes in the warm sector. ..Edwards/Jewell.. 09/21/2021 Read more LIVE: spc.noaa.gov