11AM Day 1 Convective Outlook for Friday, August 22. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE NORTHERN/CENTRAL HIGH PLAINS TO THE UPPER MIDWEST/GREAT LAKES AND PORTIONS OF SOUTHERN/CENTRAL ARIZONA
SUMMARY
Strong to locally severe thunderstorms may occur today from parts of the northern and central High Plains to the Upper Midwest/Great Lakes, and separately across parts of Arizona. Damaging winds and occasional hail should be the main threats.
Synopsis
A mid/upper-level trough/low centered over Manitoba late this morning will move east-southeastward across Ontario and the Upper Midwest/Great Lakes through the period. At the surface, a weak cold front is forecast to develop southeastward across the northern/central High Plains and Upper Midwest this afternoon and evening. This boundary should serve as a focus for additional thunderstorm development later today, some of which could become strong to severe. A mid/upper-level anticyclone will remain anchored over the Four Corners region today, with modest easterly mid-level flow persisting over parts of AZ.
Northern/Central High Plains to the Upper Midwest/Great Lakes
A broad zone of isolated strong to severe thunderstorm potential remains evident today, generally along/near a convectively reinforced cold front extending from the central High Plains east-northeastward to the Upper Midwest/Great Lakes. Within this broad zone, three somewhat separate regimes of severe potential are apparent.
Thunderstorms have recently developed across parts of central/eastern MN and far northwest WI ahead of a remnant MCV/subtle mid-level shortwave trough. The airmass downstream from this activity across northern WI into the U.P. of MI is only weakly unstable, with scattered to broken mid/upper-level clouds present. Stronger mid-level flow is also forecast to generally lag behind the surface cold front across this region. Still, a small cluster or two may eventually evolve from the ongoing convection in MN, and spread eastward through the afternoon/early evening. Occasional damaging winds should be the main threat, but some hail may also occur with the stronger cores given cool mid-level temperatures and around 25-35 kt of deep-layer shear aiding updraft organization. Considered adding a Slight Risk for severe/damaging winds across parts of northern WI and vicinity, but confidence in more robust destabilization occurring remains low. Trends will be monitored for a possible upgrade at 20Z, if needed.
Outflow from earlier thunderstorms continues to advance southward across parts of eastern NE and northwest IA per recent surface observations and visible satellite imagery. The effect of this outflow surge on additional thunderstorm development this afternoon remains unclear, as the primary cold front lags the outflow by a considerable extent. Some guidance suggests that a couple of supercells or perhaps a small cluster may develop over far southeast SD into eastern NE and western IA later today as the airmass across these areas gradually destabilizes. At least an isolated risk for severe hail and damaging winds will exist if this occurs. But, the overall severe threat remains too uncertain for a categorical upgrade with this outlook update.
Farther west, a subtle mid-level ridge-peripheral disturbance noted in water vapor imagery across southern WY and northwest CO, along with strong daytime heating, should aid thunderstorm development initially across the higher terrain of the central Rockies later this afternoon. This activity should propagate east-southeastward toward lower elevations in the presence of moist, post-frontal low-level upslope flow. Isolated severe hail and winds are possible with this activity, especially within a corridor across interior/southeast WY into northeast/east-central CO.
Arizona
A fair amount of convective cloud debris is present today across parts of AZ per recent visible satellite imagery. This may tend to delay diurnal heating and related steepening of low-level lapse rates to some extent. But, eventual airmass recovery is still expected by late afternoon/early evening, with seasonably moist low-levels still noted on the 12Z observed PHX sounding. Thunderstorms should once again form initially along the higher terrain of the Mogollon Rim and southeastern AZ before spreading west-southwestward across the lower elevations of central/southern AZ through the evening. Sufficient instability, a very well-mixed boundary layer, and modest easterly mid-level flow should all support an isolated threat for severe/damaging winds with this activity.