SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Mon Jun 26 2023 Valid 261300Z - 271200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PART OF THE MID-ATLANTIC AND TIDEWATER REGIONS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK...AS WELL AS OVER PARTS OF THE ARKLATEX TO LOWER MISSISSIPPI VALLEY...AND PORTIONS OF THE NORTHERN HIGH PLAINS... ...SUMMARY... Scattered severe thunderstorms, with damaging to severe gusts and occasional large hail, are likely across the Mid-Atlantic States this afternoon and evening. ...Synopsis... In mid/upper levels, the pattern will remain broadly characterized by mean troughing in the West and East, and a ridge extending northward from a high over west TX to the northern High Plains and beyond. The eastern trough now is anchored by a low initially centered over Lower MI, and forecast to drift eastward over Lake Huron by the end of the period. Accompanying cyclonic flow will cover most of the CONUS east of the Mississippi River. Embedded small shortwaves and vorticity lobes will traverse the southern semicircle of the gyre, leading to amplification of the synoptic trough across the Carolinas and GA. The western trough is defined along a series of closed to nearly closed smaller circulations from southern AB across southeastern BC, the interior Pacific Northwest, northern CA, and Pacific waters well west of southern CA. A broad area of downstream southwest flow will spread over the Rockies and adjacent High Plains as the ridge shifts slowly eastward through tonight. At the surface, 11Z analysis showed an occluded low over northern Lower MI, and a cold front arching across eastern parts of OH/KY/TN, becoming quasistationary over northern MS, southern AR, to a low between SPS-ABI. As the occluded low continues to fill and move eastward under the parent vortex aloft, the cold front should move slowly eastward across the inland Mid-Atlantic and Carolinas through the period. The front will be preceded by a prefrontal surface trough in the lee of the Appalachians. That portion of this front from GA westward should stall for much of the day, with some southward drift possible overnight from GA-OK. A dryline will mix eastward across the southern/central High Plains today, reaching about the longitude of TCC-AKO by late afternoon. ...Mid-Atlantic/Northeast to Carolinas... Several lines and clusters of thunderstorms are expected to develop throughout the day -- mainly over higher terrain to the west, near the surface trough, and around any boundaries left by ongoing cloud cover and precip. Resulting thunderstorms (including a few supercells) should become strong to severe as they move across the outlook area -- generally eastward over the Carolinas to north- northeastward across the NY/New England area. Activity will move into a diurnally destabilizing, favorably moist air mass with little or no MLCINH, leading to considerable overall convective coverage. Scattered damaging gusts and sporadic severe/50+ kt thunderstorm winds are possible, with the densest concentration expected in and near the "enhanced" area. Sporadic large hail also possible, especially from the Carolinas to eastern PA/NJ, with significant/ 2-inch-diameter hail a concern over the western/central Carolinas into parts of southern VA. A tornado or two also may develop, especially where storm/boundary interactions yield locally enhanced low-level shear. Given the geometry and positioning of the mid/upper-level cyclone, the strongest flow aloft, as well as the favorable/proximate left-exit region of the upper-level jet, should most affect southwestern parts of the outlook across the Carolinas/western VA region this afternoon. Relatively maximized midlevel lapse rates, deep shear and large-scale lift should result, overlapping the greatest low-level moisture and heating. This is somewhat unusual for an Eastern Seaboard severe event, where more typically, shear increases northward and instability is greater southward. The result should be a threat for supercells with very large hail in this area. Supercells also may occur, either as early stages of upscale growth or embedded in lines, farther northeastward, but with somewhat weaker overall lapse rates. Bow and LEWP features within QLCS modes should contribute to locally enhanced severe-wind potential as well. Peak/preconvective MLCAPE should range from around 2000-3000 J/kg over the Carolinas, to around 1000-2000 J/kg over outlooked areas of NY and New England, where lapse rates and deep shear also will be least. ...Northern High Plains/Black Hills and vicinity... Multiple episodes of thunderstorms may move eastward to east- southeastward across this corridor, offering sporadic severe gusts and large hail through at least this evening. A plume of showers and widely scattered embedded thunderstorms is ongoing over parts of the region from northeastern WY east-southeastward across the western NE/SD border region, in a zone of low-level warm advection and moisture transport, underlying steep midlevel lapse rates. Isolated large hail and strong-severe gusts are the main concern this morning. See SPC Mesoscale Discussion 1278 for details. Additional convection may develop on the resulting outflow/ differential-heating boundary this afternoon, as well as in a zone behind the boundary and west of a lee trough, where enough easterly near-surface wind component will persist to support lift with orographic forcing (aided by diurnal heating). Mid 40s to mid-50s surface dewpoints away from the higher terrain will support around 1000-1500 J/kg MLCAPE. Long hodographs and 30-40 kt effective-shear magnitudes are expected over much of the area, as light boundary- layer winds veer strongly with height and increase to around 20-30 kt in midlevels. This will support a mix of multicell and supercell modes, with supercells offering hail potentially exceeding 2 inches in diameter, and some upscale clustering and forward propagation possible before activity weakens tonight. ...Arklatex to Mississippi Delta region... A southward-sagging, northwest/southeast-aligned belt of convection is ongoing across the outlook area, with large hail and sporadic severe gusts continuing as the main concerns. See Severe Thunderstorm Watch 390 and related mesoscale discussions (including 1279) for near-term concerns. This convection may persist at or near severe levels through much of the remainder of the morning. A second, similarly oriented round of strong-severe thunderstorms is possible tonight from southern AR across parts of northern LA and west-central/southwestern MS, also posing a threat for severe hail and isolated damaging gusts. This activity would arise from a similar regime of low-level warm advection and moisture transport, this time over the outflow boundary left by morning convection. The boundary should last all day, given the organization and likely persistent cloud cover associated with the ongoing convection. Westerly midlevel flow above the regime should advect more elevated mixed-layer air, maintaining steep lapse rates aloft, in support of MUCAPE in the 2500-4000 J/kg range. Given the organization of ongoing convection and potential for another round in nearly the same area tonight, the aggregate threat is being upgraded in this outlook cycle. ...Southern High Plains... Isolated to widely scattered, high-based thunderstorms may develop near the dryline during mid/late afternoon and move a short distance eastward, with a threat for isolated severe gusts/hail. Though a strong EML has built up across the region, intense surface heating should be enough to reach convective temperature over parts of the region. A low-level moisture lobe will extend westward from OK north of the front, with easterly moist advection from areas of 50s to 60s F surface dewpoints helping to offset mixing, and maintain a moist axis across the TX Panhandle. Where convection can overcome the cap and reach maturity, 1500-2500 J/kg MLCAPE will be possible. Low-level flow will be modest in strength, but backed over most of the area, with deep shear increasing to supercell-favorable levels northward under stronger flow aloft. As such, large hail will be possible, as well as strong-severe gusts favored by the deep/well-mixed sub cloud layer. ..Edwards/Mosier.. 06/26/2023
