SPC AC 181924
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0224 PM CDT Fri Aug 18 2017
Valid 182000Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MID
ATLANTIC INTO SOUTHERN NEW YORK...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHWEST
MINNESOTA AND VICINITY...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE LOWER
TO MIDDLE MISSOURI VALLEY INTO NORTHERN OKLAHOMA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
THE MID ATLANTIC INTO SOUTHERN NEW ENGLAND...
Scattered strong to localized severe gusts capable of wind damage
are forecast from the Chesapeake Bay vicinity northward into
southern New York. Isolated severe thunderstorms are possible
across portions southwestern Minnesota and northern Missouri.
No changes were necessary to the existing outlook.
.PREV DISCUSSION... /ISSUED 1130 AM CDT Fri Aug 18 2017/
Daytime heating of a very moist boundary layer in place east of the
mountains is ongoing, within a relatively cloud-free zone from
southeast Pennsylvania south-southwestward east of the Appalachian
crest. As moderate CAPE develops through early to mid afternoon, an
increase in storms -- particularly within a weak zone of convergence
observed in lee of the mountains -- is expected. Strongest flow
aloft will remain largely north of the area of greatest anticipated
destabilization, but amply strong west-southwest flow extending
southward across the mid-Atlantic region will likely support a few
stronger storms/bands of storms. As such, 15% wind probability/SLGT
risk remains evident across this region. Convection will move
eastward with time, weakening into the early evening hours as storms
approach/move off the coast.
A compact center of circulation remains evident near the southeast
North Dakota/northeast South Dakota border, moving slowly
southeastward with time. As clouds/lingering precipitation continue
to move east across Minnesota, diurnal heating will permit ample
destabilization -- supportive of convective redevelopment near/ahead
of the center of circulation. With amply strong deep-layer
northwesterly flow aloft, an organized, southeastward-moving cluster
of storms may evolve, with associated risk for locally damaging
winds and hail. Convection should weaken into the evening as it
shifts south-southeast out of Minnesota and across Iowa with time.
...Parts of northern Missouri...
As weak large-scale ascent spreads southeast in conjunction with
mid-level short-wave troughing over the north-central states, a weak
surface low and convergence zone now over the western IA vicinity
will likewise drift southeast. Agreement amongst some CAM members,
and morning NAM and GFS output, suggest that storms will develop
over the northern Missouri vicinity this afternoon/evening near the
low, as destabilization peaks. With amply strong northwest flow
aloft, storms may organize locally, possibly growing upscale into an
organized band with time. Risk for hail and locally damaging winds
may evolve with any stronger convection, warranting continuation of
SLGT risk for this afternoon and evening.
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