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Friday, June 25, 2021

SPC Jun 25, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0736 AM CDT Fri Jun 25 2021 Valid 251300Z - 261200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF THE SOUTHERN/CENTRAL HIGH PLAINS TO THE MID MISSISSIPPI VALLEY AND MIDWEST... ...SUMMARY... Scattered strong to severe storms are expected today from portions of the southern/central High Plains to the mid Mississippi Valley and Midwest. Damaging wind gusts, large hail and a couple of tornadoes will be possible. ...Synopsis... A high-amplitude mid/upper-level pattern will take shape, featuring a mean trough from Hudson Bay across the central Plains to the southwestern CONUS. Meanwhile, a weak mid/upper low near the central CA coastline will move southeastward and devolve to a shortwave trough across the lower Colorado Valley by the end of the period, leaving behind a pronounced omega block over the West Coast, with anchoring anticyclone over southern BC. To the east of the positively tilted mean trough, a series of mainly small shortwaves (some with convectively generated/enhanced vorticity fields) will traverse the southwesterly flow from the central/southern Rockies across the central Plains, mid Mississippi Valley and Great Lakes. One of these -- currently evident as a pronounced MCV over northern IL with trough trailing toward the STL area -- will move east- northeastward to southeastern Lower MI by 00Z. At the surface, 11Z analysis depicted a quasistationary front from western Upper MI southwestward across western IA, southeastern NE, and southwestern KS. This frontal zone, lying nearly parallel to the flow aloft, will remain nearly stationary, but for minor ripples in response to shortwaves embedded in that flow. Through the day-1 period, the more-important surface baroclinic zone mostly will remain an aggregate of outflow boundaries -- both antecedent and newly generated -- south of the synoptic front. ...Central/southern High Plains to Midwest... Widely scattered to locally numerous thunderstorms in clusters are expected to develop near the composite outflow boundary from central KS across the mid Mississippi Valley, and on both sides of the effective frontal zone westward to the central/southern High Plains. Additional convection may develop near the dryline -- which will extend southward from the front across the western TX Panhandle/ South Plains region by 00Z. Damaging gusts, large hail, and a tornado or two are possible in this corridor. Boundary intersections and storm/boundary interactions may better focus supercellular hail/tornado potential locally. Very rich moisture will continue to characterize the warm sector south of the front/outflows, as well as within modifying outflow air just north of the boundaries as they retreat northward to varying extents today. Surface dew points in the upper 60s to low 70s will be common -- along with PW of 1.75-2.25 inches -- from central KS at least into IN. Lesser but still favorable moisture will be available behind the front/outflows on the High Plains as well, in support of afternoon initiation there. That moisture, in tandem with favorable midlevel lapse rates, will combine with diurnal heating to render little or no MLCINH over much of the outlook corridor by mid-late afternoon. 2000-3000 J/kg MLCAPE should be common from southwestern/central KS to western IN away from areas of ongoing convection and precip. These values should decrease westward from there into weaker moisture, eastward into smaller lapse rates, and northward into lower theta-e north of the synoptic front. Vertical wind profiles mostly will favor multicells, especially south of the antecedent boundaries, with weak near-surface winds and less than 35 kt effective-shear magnitudes. However, stronger deep shear with some supercell potential may occur in three regimes: 1. Over the central High Plains and TX Panhandle region, where a channel of faster midlevel flow will develop, 2. Along and north of the outflow boundaries from eastern KS to central IL where backed low-level flow enlarges hodographs and elongates the deep-shear vectors, and 3. Perhaps under a channel of enhanced midlevel winds related to (and southeast of) the MCV. Though the "slight"/15% wind/hail and marginal tornado probabilities extend over a large area, actual severe threat will be far from uniform within it, as mesobeta- and smaller-scale features will tend to regulate upscale-growth/ clustering potential and concentrate severe threat accordingly. ..Edwards/Broyles.. 06/25/2021 Read more LIVE: spc.noaa.gov
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