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Outlook for Friday, August 7

Outlook Summary

A cluster of thunderstorms may evolve across the Upper Midwest this evening, accompanied by a risk for severe wind, hail, and perhaps a tornado or two.

Outlook Images

overview

tornado 2%

wind 15%

hail 15%

Detailed Outlook

SPC AC 071934

Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0234 PM CDT Fri Aug 07 2020

Valid 072000Z - 081200Z

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL INTO NORTHERN MINNESOTA INTO NORTHWEST WISCONSIN

### SUMMARY

A cluster of thunderstorms may evolve across the Upper Midwest this evening, accompanied by a risk for severe wind, hail, and perhaps a tornado or two.

Discussion

Only two minor changes were made this outlook update. 1) Reduced hail/wind probabilities from 15 to 5 percent across parts of eastern ND and northwest MN. The MCV/vorticity center is forecast to track to the southeast of this area with lower storm coverage and potential for severe located to the north and northwest of the mid-level vort. 2) Added a small 5 percent hail probability in parts of the Mid-Atlantic.

..Smith.. 08/07/2020

.PREV DISCUSSION… /ISSUED 1129 AM CDT Fri Aug 07 2020/

Synopsis

Troughing within a weak belt of westerlies lingers west through south-southwest of the Appalachians. However, the models indicate that the primary embedded perturbation will shift east-northeast of the Ohio Valley through the northern and mid Atlantic coast by late tonight, as subtropical ridging becomes a bit more prominent across much of the United States.

A belt of stronger mid-latitude westerlies, now nosing inland near the western Canadian/U.S. border, is forecast to tend to shift north of the international border through this period, and undergo some amplification, with broad ridging building across the northeastern Pacific into the British Columbia and Pacific Northwest coast. Downstream, a vigorous short wave impulse on the leading edge of this regime is forecast to migrate east-northeast of the Canadian/northern U.S. Rockies through the Canadian Prairies.

In response to these developments, a broad plume of very warm elevated mixed-layer air appears likely to continue advecting eastward across much of the Great Plains, while also nosing northeastward into portions of the Upper Midwest. Some southward suppression of this air mass is possible across eastern Montana into the Dakotas by late tonight, with the surface cold front advancing more rapidly southward through northwestern Minnesota and much of the Dakotas by 12Z Saturday.

Northern Great Plains into Upper Midwest

Seasonably moist boundary layer conditions are returning to the region, due to both moisture advection and evapotranspiration, beneath the warming and capping elevated mixed-layer air. It appears that this will contribute to large mixed-layer CAPE (2000-3000+ J/kg) with daytime heating, ahead of the cold front, across much of the northern Great Plains.

However, to the southeast of the short wave migrating into the Canadian Prairies, and in the wake of a much more subtle preceding perturbation (now progressing into the Red River Valley), mid-level height rises may contribute to suppression of deep convective development across much of the Dakotas. While the eastward advancing surface cold front could provide a focus for at least attempts at sustained thunderstorm development across parts of the western into central Dakotas late this afternoon and early evening, it is becoming increasingly unclear that storms will be able to develop. Even if they do, with stronger mid-level cooling and wind fields lagging to the northwest of the front, severe weather potential may remain limited.

Lift associated with the weaker lead perturbation seems likely to provide the primary focus for sustained thunderstorm development later today through tonight. This may initiate near the edge of the stronger capping elevated mixed-layer air, perhaps in the immediate wake of the convectively generated vorticity center currently over southeastern North Dakota, which is forecast to turn eastward across northern Minnesota today through tonight.

Near and to the southwest of the mid-level cyclonic circulation, a developing zone of stronger differential surface heating may provide the low-level focus for the strongest convective development. This probably will include one or two supercells initially, then perhaps an organizing upscale growing convective system by this evening. An associated risk for severe hail, and perhaps a tornado or two, probably will transition to primarily a severe wind threat, which could spread into northwestern Wisconsin late tonight before diminishing.

Central Appalachians into northern Mid Atlantic coast

Although southwesterly deep-layer mean flow remains modest to weak at around 20 kt or less, some enhancement at mid-levels along and to the lee of the Appalachians could perhaps enhance convective development this afternoon.

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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z

National Risk Overview

Friday, August 7
TORNADO: 2%
HAIL: 15%
WIND: 15%
Saturday, August 8
TORNADO: 2%
HAIL: 15%
WIND: 15%
Sunday, August 9
ANY SEVERE: 15%
Monday, August 10
ANY SEVERE: low / uncertain
Tuesday, August 11
ANY SEVERE: low / uncertain
Wednesday, August 12
ANY SEVERE: low / uncertain
Thursday, August 13
ANY SEVERE: low / uncertain
Friday, August 14
ANY SEVERE: low / uncertain

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