Severe Storm Outlook Narrative (AC)
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Issued by NWS
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495 ACUS01 KWNS 231259 SWODY1 SPC AC 231257 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Thu May 23 2024 Valid 231300Z - 241200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE CENTRAL PLAINS... ...SUMMARY... Severe thunderstorms are possible across a broad swath of the Great Plains this afternoon into tonight, with a few tornadoes and isolated very large hail. A concentration of severe winds -- some 75 mph or more -- is expected across a portion of the central Plains this evening into tonight. ...Synopsis... In mid/upper levels, a progressive flow belt will cover much of the CONUS, to the southeast through southwest of a persistent, somewhat flattened, quasistationary cyclone over the Canadian Prairie Provinces. This cyclone is loosely attached to another that is located over northern ON, and forecast to devolve into an open-wave trough as it moves into western/southern QC by 12Z tomorrow. The southern part of a preceding vorticity lobe will brush the St. Lawrence Valley and northern New England through 00Z. Meanwhile, another mid/upper low -- now over the northern Rockies -- should become part of a strong, open-wave trough by 00Z from eastern MT to eastern UT. This trough should pivot eastward to the Dakotas and NE by 12Z. To its south, a series of low-amplitude perturbations and vorticity lobes (some convectively generated/ enhanced) occupied the extensive west-southwest flow field from the Desert Southwest to the Appalachians. One of these -- now over southern OK and north TX -- is influencing ongoing convection from southeastern OK across AR -- and will move across the Mid-South and Tennessee Valley regions into tonight. Behind it, similar perturbations are apparent in moisture-channel imagery over Chihuahua, and over southeastern AZ/southwestern NM. These should reach central and northwest TX by 00Z. At the surface, 11Z analysis showed a low over east-central/ southeastern WY near TOR, with synoptic warm front over northern SD and southern MN. As the shortwave trough approaches, the low should deepen sharply and move to near PIR by 00Z, with cold front over western NE and northeastern CO. By 12Z, the low should reach eastern ND, with the cold front to northwestern IA, southeastern NE, south-central KS, and the TX South Plains. A warm to quasistationary front was drawn across the Red River region of TX/ OK, to the lower/middle Ohio Valley, becoming a cold front over parts of PA/upstate NY. This boundary will move eastward through the northern Appalachians, New England and much of the Mid-Atlantic through the period, while diffusely shifting northward amid areas of convection over the Mid-South and southern Plains. A dryline -- initially analyzed from the Big Bend region of TX across extreme eastern NM -- will develop northward over western parts of KS/NE today while the airmass to the east moistens. This boundary also will shift eastward into the eastern TX Panhandle near the OK border, as well as northwest/west-central TX through the afternoon. ...Central/northern Plains... Scattered thunderstorms are expected this afternoon and evening along parts of the warm and cold front near the surface low, with supercells possible in the first 2-3 hours. Damaging gusts, large to very large hail and a few tornadoes are possible. With relatively strong deep-layer and frontal forcing superimposed ahead of the progressive Rockies shortwave trough, an organized band of strong-severe thunderstorms should evolve along/ahead of the cold front and move southeastward across parts of the Dakotas and NE, eventually backbuilding into northern KS tonight as the cold front overtakes the dryline. This will cause the severe threat to transition more toward wind, with a swath of significant wind (65+ kt) potentially accompanying the QLCS. A few tornadoes also are possible with embedded mesocirculations near its edge. Though not optimally moist for the time of year, a corridor of 50s to low 60s surface dewpoints should become common this afternoon between the warm and cold fronts, combining with diurnal heating and steep midlevel lapse rates to support 1500-3000 J/kg MLCAPE. 45-55 kt effective-shear magnitudes should be common, with larger values possible near the warm front and low (but also, weaker instability). This will support both early supercells and persistence of the upscale MCS into parts of the central Plains and lower Missouri Valley regions. The southern end of well-organized, severe-wind potential over KS may be limited by residual outflow air advecting into the area from the south, while the eastern/northern ends are more uncertain. Although no change was made to the area of greatest wind probabilities this cycle, some reshaping of the "enhanced" area may be needed through the day as informed by 12Z and later guidance, as well as mesoscale analytic trends. However, with longer persistence of the MCS possible across IA/MO tonight, parts of the "slight" and "marginal" equivalent probabilities have been expanded eastward. ...Southern Plains... The width of the convectively undisturbed warm sector ahead of the southern Plains dryline is still uncertain, as considerable variability exists in progs for upscale growth of convection now manifest as a small but growing cluster over the Edwards Plateau/Hill Country/Concho Valley regions. This activity may produce sporadic large hail and damaging wind as well, through the remainder of the morning, as it proceeds across portions of central TX, with wind potential conditionally increasing where aggregation of cold pools can drive strongest forward propagation. Outflow and related theta-e deficits from any resulting complex will be advected northward into north TX and OK, with ambient theta-e decreasing northward into KS. Still, a corridor of favorable moisture should advect around the west (back) side of any such cold pool today and support isolated to widely scattered dryline development from northwest TX into parts of western OK/KS. Any sustained convection arising from that process may become supercellular, with all severe modes possible, and very large/ damaging hail a strong possibility. The least-modified air by earlier activity will have surface dewpoints in the upper 60s to low 70s F beneath EML-related steep lapse rates, fostering 3000-4000 J/kg MLCAPE. Long low-level hodographs will favor splitting storms, especially in the first couple hours after initiation, with some LLJ-aided enlargement possible in the 00-03Z time frame before parcels begin to stabilize unfavorably near the surface. ...Mid-South to Mid-Atlantic and New England... Isolated to scattered thunderstorms -- sometimes in small bands or clusters -- will persist through this afternoon along/ahead of the front from the Northeast to the lower Mississippi Valley. Areas of relative concentration within this belt will depend strongly on mesoscale boundary interactions ahead of the front (outflow and differential heating, as well as a prefrontal surface trough in the Northeast). Midlevel lapse rates, intensity of diurnal heating, and boundary-layer moisture each should generally decrease with northeastward extent. This should contribute to peak/preconvective MLCAPE in the 2000-3000 J/kg range around the lower/middle Mississippi and lower Ohio Valleys to 500-1500 J/kg over parts of the Mid-Atlantic and Northeast. Somewhat favorable deep shear amidst nearly unidirectional wind profiles will support potential for occasional damaging to isolated severe gusts throughout the Atlantic Coast States part of the corridor. A patchy distribution of marginal severe (hail and wind), with some mesobeta-scale concentrations possible, is expected across the remainder of the "marginal risk" swath. ..Edwards/Goss.. 05/23/2024 $$