Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
SPC AC 171621
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1121 AM CDT Fri Jul 17 2020
Valid 171630Z - 181200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
THROUGH TONIGHT ACROSS PARTS OF THE NORTHERN PLAINS AND UPPER
MIDWEST...
...SUMMARY...
A significant severe weather event is possible this evening and
tonight over parts of North Dakota, northern Minnesota, and
northwest Wisconsin. Widespread damaging winds are possible.
...ND to WI/Upper MI...
Morning water vapor loop shows fast westerly flow across the
northern tier of states, with an embedded low-amplitude shortwave
trough over eastern MT. Southerly low-level winds are present
across the Dakotas, with local VADs showing 20-30 knots at 1-2km.
These winds are helping to transport 70s surface dewpoints
northward, yielding an axis of MLCAPE exceeding 4000 J/kg later
today from central ND into western MN. Thunderstorms are expected
to form early this afternoon as forcing overspreads the returning
moisture, with a few supercells capable of large hail, damaging
winds, or perhaps a tornado over western/central ND. All guidance
lends confidence that these storms will congeal into one or more
bowing structures by this evening over eastern ND. Storms will
track rapidly eastward at 40-50 knots across parts of MN and into
northern WI and upper MI overnight.
Given the very favorable thermodynamic profiles in place, the strong
westerly flow aloft, and the consistent model guidance, it appears
possible that a derecho will occur this evening and tonight from ND
to WI - with the threat of a corridor of widespread and significant
damaging winds. Have expanded the MDT/ENH risks eastward into parts
of WI/Upper MI where the bowing complex appears most likely to track
late tonight. Have also expanded the ENH southward into more of
central MN due to uncertainty of how far south the MCS might
develop.
...VA/NC...
Warm/humid surface conditions are present today over parts of VA/NC,
with moderate CAPE values and a weak front moving into the area.
Scattered afternoon thunderstorms are expected to develop
along/ahead of the front from central VA into northern NC, with
activity sagging southward through the afternoon and early evening.
Winds aloft are weak, limiting convective organization. However,
disorganized multicell and pulse convection may pose a risk of
gusty/damaging winds in the strongest downdrafts.
..Hart/Moore.. 07/17/2020
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