12AM Day 1 Convective Outlook for Friday, August 22. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL/NORTHERN HIGH PLAINS AND UPPER MIDWEST AND OVER PARTS OF THE SOUTHWESTERN UNITED STATES
SUMMARY
Strong to locally severe thunderstorms are possible from parts of the central and northern High Plains into the Upper Midwest, and across parts of the southwestern United States.
Central/Northern High Plains-Upper Midwest
Central Canadian upper trough is forecast to advance into western ON/MN by 23/00z as 500mb speed max settles into the base of the trough over the upper Red River region of eastern ND/MN. High-level diffluent flow will be noted across much of the Upper Midwest ahead of this feature which will encourage deep convection along the frontal zone from the upper Great Lakes into portions of the Mid MO Valley. Early in the period, remnant MCS activity may be ongoing across southern MN/southeast SD. However, left-over convective debris may also inhibit surface heating, but may enhance an afternoon boundary for potential thunderstorm development. Latest model guidance suggests the front may be multi faceted with some of the stronger boundary-layer heating expected behind the initial wind shift across SD into northern MN - well northwest of high PW air mass. As a result, forecast soundings suggest surface parcels may struggle to reach convective temperature along the leading wind shift, with only modest lapse rates expected. Even so, deep-layer shear will be more than adequate for sustaining organized updrafts where frontal convergence aids convective development.
Higher confidence in robust convection is expected across portions of CO into southeast WY. Strong surface heating should aid thunderstorm development across the higher terrain, along the northern periphery of the Four Corners upper high. This activity should propagate southeast toward lower elevations where it will encounter favorably moist, but weak upslope flow. Hail and wind are possible with this activity.
Arizona
Upper high is forecast to shift slowly west across southern UT during the day1 period. This will ensure modest easterly 500mb flow across the southern half of AZ. Strong boundary-layer heating will once again prove instrumental in convective development over the higher terrain. This activity will spread toward the lower deserts during the evening with an attendant risk for at least locally severe wind gusts.