Subj : DAY1SVR: Day 1 Convective To : All From : Mike Powell Date : Thu Aug 22 2024 08:40:00 ACUS01 KWNS 221255 SWODY1 SPC AC 221254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Thu Aug 22 2024 Valid 221300Z - 231200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE CENTRAL HIGH PLAINS... ....SUMMARY... The greatest threat for severe gusts and hail appears to be over parts of the central High Plains this afternoon and evening. ....Synopsis... A highly amplified pattern in mid/upper levels will persist through the period, but with some weakening noted for both mean troughing near the East Coast, and a central CONUS ridge extending northward from a high over west TX. Southwest flow over the Intermountain West will strengthen and back somewhat in response to slow southeastward movement of an intense cyclone -- now centered west of the northern OR Coast --- while a strong shortwave trough pivots through its southern semicircle and onshore the northern CA coast by the end of the period. Farther east, on the periphery of that southwest flow, a shortwave trough (with some incorporation of a Sonoran MCV from 24 hours earlier) was apparent in moisture-channel imagery over northern AZ and southern UT. This perturbation should eject northeastward over western CO by 00Z, then into the central Plains overnight, perhaps with some further convective-vorticity reinforcement then. To the north, closely spaced shortwave troughs now over south-central SK and parts of ND/southeastern SK should pivot eastward across the rest of ND, northern MN and southern/central MB through the period. At the surface, a quasistationary frontal zone from northern FL across the northern Gulf Coast, east/north TX and the TX Panhandle should remain near that position through the period, with the richest low-level moisture remaining to its south. However, gradual moisture return of more incompletely modified (yet still convectively sufficient) character is apparent up the High Plains from the Panhandles across western NE to the central Dakotas. This is east of a lee trough over eastern parts of WY/CO, and a weak cold front analyzed from north-central ND to western SD. The latter front should proceed eastward across the Dakotas and into northwestern MN through this evening. ....Central High Plains... Highly variable thunderstorm coverage is expected over this region, with the greatest density in the 15%/"slight" area, where upscale evolution of increasingly outflow-driven thunderstorm clusters should support severe-gust potential this afternoon into evening. The primary foci for formation of eventually severe activity should be the northern Raton Mesa region, Front Range/Sangre de Cristo, and/or Palmer Divide, with some contribution from the lee trough as well. Though far south and east of the stronger flow aloft, sufficient veering of wind with height is expected to result in effective-shear magnitude in the 30-40-kt range, indicating potential for some organized convection. The boundary layer will remain well-heated/ mixed and deep, with very steep lapse rates, even as moisture/ buoyancy increase eastward. With surface dewpoints in the mid 50s to mid 60s F, peak/preconvective MLCAPE should reach 1500-2000 J/kg (locally higher) in a corridor near and just east of the longitude of the CO/KS line. This will foster pulses of strong to isolated severe gusts in initially discrete to semi-discrete activity atop a drier boundary layer, becoming better-organized with potential cold-pool aggregation eastward toward east-central/northeastern CO and adjoining portions of KS/NE into the higher CAPE. While specific shape, size, and track(s) of this clustering still depend on mesoscale details, enough consensus/confidence is now apparent in the general area of greatest severe-wind potential to ratchet up unconditional probabilities a level. With MLCINH increasing eastward from about the KS/CO border longitude and nocturnal near-surface cooling, the severe threat should weaken eastward from northwestern KS and southwestern NE tonight. ....Eastern Great Basin and vicinity... Widely scattered to scattered thunderstorms are possible this afternoon into early evening, offering strong to isolated severe gusts and marginally severe hail. After the shortwave trough and related precip now over parts of this region shifts northeastward, most of the day should remain for diabatic heating/destabilization of the higher terrain across the region, with airmass recovery furthered by warm/theta-e advection from the south. In concert with available monsoonal moisture, a roughly meridional plume of essentially uninhibited MLCAPE in the 500-1500 J/kg range should develop. This will be superimposed under the southeastern rim of stronger upper flow related to the West Coast troughing/cyclone. The result should be strengthening cloud-layer shear and some potential for storm organization, as activity moves generally northeastward over well-mixed boundary layers. ....Upper Midwest... A large area of clouds and precip -- with widely scattered embedded thunderstorms -- should proceed eastward today across the remainder of eastern ND and parts of northern MN near the international border. Though the associated lead perturbation now over ND should be past the area before this afternoon, cyclonic flow aloft should continue in response to the stronger perturbation farther north, moving into MB. As such, little if any hindrance to convective potential is expected from large-scale subsidence, while the low-level airmass destabilizes diabatically from surface heating behind the morning activity. Isolated to widely scattered thunderstorms may develop along/ahead of the cold front, within a narrow plume of 1000-2000 J/kg MLCAPE supported by 60s F surface dewpoints. Modest low/middle-level flow is expected, through enough veering with height should develop to support effective-shear magnitudes locally near 35 kt. The strongest cells may produce marginally severe hail or gusts, before convection weakens this evening. ...Edwards/Mosier.. 08/22/2024 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .