SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0656 AM CST Sat Nov 14 2020 Valid 141300Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE OZARKS REGION INTO PARTS OF THE MID-SOUTH... ...SUMMARY... Thunderstorms will offer damaging to severe gusts and the threat of a few tornadoes, this afternoon into tonight, from the Ozarks region into parts of the Mid-South. ...Synopsis... On the large scale, broadly cyclonic mid/upper-level flow over the western CONUS transitions downstream to zonal in the East. With time today, the contributing series of numerous vorticity maxima and embedded shortwave troughs -- now evident in moisture-channel imagery across the Rockies, Great Basin and High Plains -- will phase better. The result will be a sharpening synoptic trough, moving eastward over much of the Great Plains by 00Z. By 12Z tomorrow, this trough should extend from most of the length of MB across the eastern Dakotas and eastern NE, to eastern OK and east TX. At the surface, 11Z analysis showed a marine warm front over the northern Gulf and northwestward into north-central TX, and a developing synoptic warm front across parts of central/southwestern NE, northwestern to southeastern KS, and the western Ozarks. The northern boundary will become better-defined with time today as precip shifts off to its northeast, and as increasingly modified Gulf trajectories moving through the frontolytic marine boundary approach the region. Meanwhile, cold frontogenesis over the central Rockies/High Plains will advance eastward across the central Plains today. The warm front will move northeastward across parts of MO and the lower Ohio Valley from this evening through overnight. A frontal-wave low should develop from midday into the afternoon, near or just north of the warm-front/cold-front triple-point over the central Plains. The deepening low should eject northeastward over IA around 00Z, with cold front trailing across northwestern MO, northern/western OK, and the TX Panhandle. By 12Z, the center of a well-developed synoptic surface cyclone should reach central Upper MI, with occluded/cold front arching equatorward through lower MI, IN, western/mid TN, central parts of MS/LA, and south-central TX. ...Ozarks and vicinity to Mid-South... Multiple rounds of convection will cross the outlook area through the period, with the morning activity generally nonsevere and in an elevated, warm-advection regime. Small hail cannot be ruled out. One or two bands of thunderstorms are expected to develop along and ahead of the cold front this afternoon through evening over western/ central parts of the outlook area and shift eastward, with damaging gusts, isolated marginally severe hail, and the potential for a few tornadoes. More-isolated convection forming in the warm sector, from the Ozarks southward to southern AR, also may become surface-based before outrunning the destabilization plume. This pattern will present the classic cool-season tradeoff between stronger deep-layer flow and low-level shear to the north, and greater moisture/instability but weakening large-scale lift to the south. By afternoon, cooling midlevel temperatures should begin over western parts of the "slight" and "marginal" areas as the mid/upper trough amplifies and approaches, preceded by related DCVA and strengthening deep shear. This process will spread eastward into tonight above and just ahead of the cold front. Forecast soundings in the north part of the surface-based buoyancy across MO (see below) contain very large hodographs (effective SRH at least briefly exceeding 500 J/kg) amidst 55-65-kt effective-shear magnitudes. This supports supercell and QLCS_embedded vortices each capable of producing tornadoes, as well as severe gusts. LEWP/bow configurations also will contribute to the wind threat. Meanwhile, the warm-sector boundary layer should destabilize from southwest-northeast behind morning clouds/convection from southwest-northeast over eastern OK AR and the Ozarks, from a combination of diurnal heating and theta-e advection. A northward-narrowing corridor of upper 50s to 60s F surface dew points will support a similarly narrowing plume of 500-1000 J/kg MLCAPE. Albeit briefly (before being overtaken by the front), surface-based effective-inflow parcels may extend as far northeast as central and southeastern MO this evening, and as far east tonight as near the Mississippi River vicinity in the Mid-South tonight. Given the strength of the wind fields before convection runs out of supportive instability late in the period, the 5% and 15% wind areas each have been expanded somewhat southeastward over more of the Mid-South. Concerns over initiation timing and sparse convective coverage, and fast northeastward translation of any cells forming in the area before they can mature to severe levels, support a modest eastward nudge to severe probabilities in eastern OK. ..Edwards/Leitman.. 11/14/2020
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