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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.


Thursday, August 25, 2022

SPC Aug 25, 2022 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0742 AM CDT Thu Aug 25 2022 Valid 251300Z - 261200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF WESTERN MONTANA AND THE NORTH-CENTRAL HIGH PLAINS... ...SUMMARY... Isolated severe hail and thunderstorm gusts are possible across portions of western Montana and the north-central High Plains today. ...Synopsis... In mid/upper levels, mean troughing will remain over the eastern CONUS through the period, anchored by a large cyclone centered over southern Baffin Island. Meanwhile, the western mean ridge will deamplify somewhat, as progressive troughing shifts eastward and southeastward from the Gulf of Alaska and southern AK. This will contribute to eastward movement of a long-lived, initially cut-off cyclone over the ID Panhandle and vicinity. By 00Z, the associated 500-mb low should reach northwestern MT, then move slowly east- northeastward to southern AB overnight. To its southeast, a small but prominent shortwave trough is evident in moisture-channel imagery over western WY. This feature should move east- northeastward to eastern WY and extreme southeastern MT by 00Z, amidst strongly difluent mid/upper flow. By 12Z, the trough should become more negatively tilted and reach the western Dakotas. The 11Z surface analysis showed a cold front across northern New England to eastern Lake Ontario, becoming wavy/quasistationary westward to a low over west-central WI. The front became cold again southwestward over northwestern NO, then nearly stationary over southwestern NE, northeastern CO and south-central WY. The marginal severe-thunderstorm threat areas discussed below should remain north of the frontal zone through the period. The front will remain well-displaced from the very rich/tropical moisture plume over the Southeast, where scattered disorganized thunderstorms are expected today amidst weak deep shear. ...Western MT... Damaging to locally severe thunderstorm winds and isolated large hail are possible, mainly this afternoon into early evening. Clouds/precip now apparent in satellite and radar imagery over much of this area should break up from west-east through the day, as minor midlevel drying apparent in moisture-channel imagery spreads over the area. Scattered thunderstorms are forecast to develop predominantly over the mountains this afternoon, where diabatic surface heating of elevated terrain and large-scale lift/cooling in midlevels will combine to destabilize the troposphere soonest. With favorable low-level moisture remaining in place (e.g., surface dewpoints upper 40s to mid 50s F in lower elevations), despite boundary-layer mixing, MLCAPE around 1000-1500 J/kg should be common, with values near 2000 J/kg on the adjoining High Plains. Vertical shear will be modest overall, because of a lack of stronger low/middle-level flow. However, the wind profile should be characterized by robust veering with height, contributing to pockets of 30-40-kt effective-shear magnitudes. This should support a few organized multicell clusters, and marginal/short-lived supercell structures cannot be ruled out. ...North-central High Plains... Widely scattered to scattered thunderstorms should develop this afternoon over the Bighorns and northwestern parts of the Black Hills today, moving northeastward across adjoining portions of northeastern WY, extreme southeastern MT and western SD. Isolated severe hail/gusts are possible. Activity should commence as the vorticity max with the western WY perturbation moves across the Bighorns and toward the area between SHR-GCC this afternoon. Its preceding field of large-scale lift/ cooling in midlevels should time well to optimize deep-layer destabilization, while overlapping afternoon diabatic surface heating. Associated steep low/middle-level lapse rates (around 7-8 deg C/km) will overlap the western part of a moist axis extending east-southeastward to eastern NE. Accompanying surface dewpoints in the 50s to near 60 F will support 1000-1500 J/kg MLCAPE. Veering low-level winds with height, beneath a channel of 30-35-kt southwesterlies in the 650-700-mb layer, will yield curved hodographs in the first few km above ground level, indicating some potential for storm-scale rotation. However, that will be tempered by weaker mid/upper-level winds near the trough itself, with the dominant mode being multicellular and clustered. Convection should diminish late this evening over SD as it encounters a stabilizing boundary layer. ..Edwards/Jewell.. 08/25/2022 Read more CHECK UPDATE ZOOM GRAPHIC
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