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Thursday, June 24, 2021

SPC Jun 24, 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 Thu Jun 24 2021 Valid 241300Z - 251200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF NORTHERN KANSAS...SOUTHERN NEBRASKA AND NORTHERN MISSOURI.... ...SUMMARY... A corridor of severe weather (including destructive hail, severe wind and a few tornadoes) is possible today into tonight near the Kansas/Nebraska line, eastward over northern Missouri. ...Synopsis... In mid/upper levels, a series of shortwave troughs from the northern Rockies to the Great Lakes -- including a few that are convectively generated/enhanced -- will contribute to the formation of a positively tilted synoptic trough. By 12Z, the broader 500-mb trough should extend from the northern High Plains across the Great Basin, connecting to a low near (or just offshore) from the SFO area. To its east, a somewhat confluent, nearly zonal pattern will prevail across the central/northern Plains, Upper Midwest, and upper Great Lakes. An MCV over IA, from an ongoing MCS over IA and northern MO, is expected to move eastward across eastern IA and northern IL today. The associated perturbation should weaken as it crosses southern Lower MI, northern IN and northwest OH tonight. West of the synoptic trough, a northern-stream shortwave trough -- apparent in moisture-channel imagery over the BC/AB Rockies -- will move east-southeastward to the MT/AB/SK border region by 00Z, then weaken and move more slowly southeastward over eastern MT tonight. At the surface, the 11Z analysis showed a quasistationary frontal zone just offshore from the Carolinas, arching across southern GA, then becoming a weakening warm front over northern MS, northern AR and central MO. The western limb of this boundary will become more diffuse today as it moves northeastward over the mid Mississippi Valley. A cold front was drawn from northwestern ON to near the MN North Shore, through a weak low near MSP, across southeastern SD and central/southwestern NE to eastern CO. By 00Z, that cold front should extend from WI southwestward across IA to a low over central KS, then to southeastern CO. By 12Z, the low should reach central/eastern Upper MI and southeastern IA, becoming slow-moving to stationary southwestward over northwestern MO, central KS and the OK Panhandle. A dryline initially near the KS/CO border, and over eastern NM, will mix eastward today to west-central/southwestern KS, the eastern TX Panhandle, and the lower Pecos Valley area of west TX. Cyclogenesis and frontogenesis also are expected this afternoon and evening over eastern MT ahead of the northern-stream perturbation. ...Central Plains to mid Mississippi Valley... The ongoing MCS over southern IA and northern MO is moving into lower boundary-layer theta-e on the cool side of the diffuse warm front, but may produce isolated gusts near severe limits over the remainder of the morning as it encounters a marginally favorable but destabilizing boundary layer. An outflow boundary -- trailing southwestward and westward from the complex across northern portions of KS and MO -- will decelerate this morning, then retreat northward this afternoon. Scattered thunderstorms are expected to form this afternoon near the boundary, with isolated to scattered convection also occurring near the dryline and perhaps part of the nearly cold front. Large to giant hail is possible in early-stage convective evolution, when supercells are most favored, with severe gusts and a few tornadoes also possible (especially near the boundary). Clustering and upscale organization to one or more complexes should occur through the evening, concentrating the wind threat better. Possible foci for initiation this afternoon and evening include the boundary, cold front and dryline/lee trough. The outflow boundary will be the main focus for severe potential today. In the absence of substantial cloud cover, the air mass on both side of the western and perhaps eastern portions of the boundary (KS and MO respectively) will destabilize substantially today amidst strong insolation and rich low-level moisture, with surface dew points commonly in the upper 60s and 70s F, under favorable midlevel lapse rates. Modified model soundings for along and just north of the boundary indicate MLCAPE may reach 2500-4000 J/kg, while slightly weaker (but still moderate-strong) buoyancy occurs to its south, amidst stronger heating and deeper warm-sector mixing. Meanwhile, with some reinforcement from the front, the surface flow on the north side of the boundary will remain relatively backed, leading to enlarged shear and storm-relative flow vectors in the boundary layer. Modest mid/upper winds will impede venting and likely contribute to messy/heavy-precip modes in fairly short order, but the backed winds north of the boundary still may yield around 40-50-kt effective-shear magnitudes. The extent of outflow-boundary return northward remains uncertain, and further mesoscale adjustments to the outlook area (including tornado probabilities tied largely to the vorticity/hodograph enhancement along and to its north) are possible. ...Northern High Plains... Widely scattered thunderstorms are expected to develop this afternoon across portions of western through north-central/ northeastern MT, offering the potential for isolated, marginally severe hail and gusts. This activity should form as a plume of large-scale DCVA-related lift ahead of the shortwave trough superimposes with a diurnally destabilizing boundary layer with weakening MLCINH. Also contributing will be increasing low-level convergence near the area of cyclogenesis. Additional convection should develop over parts of eastern WY, western SD and northwestern NE, near the Black Hills and along a lee trough, with potential for isolated severe gusts for a few hours. Across the northern Plains corridor, residual moisture (surface dew points 40s to 50s F) and steep low/middle-level lapse rates will contribute to peak MLCAPE in the 500-1000 J/kg range, atop deep/well-mixed subcloud layers. Activity should diminish markedly this evening with near-surface cooling. ..Edwards/Broyles.. 06/24/2021 Read more LIVE: spc.noaa.gov