SPC AC 240555
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1255 AM CDT Mon Jul 24 2017
Valid 251200Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
EASTERN DAKOTAS EAST INTO NORTHWEST WISCONSIN AND WESTERN UPPER
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA AND EXTENDING AS FAR SOUTH AS NEBRASKA AND IOWA...
Strong/locally severe thunderstorms will be possible across portions
of the north-central U.S. Tuesday.
As an upper trough across the Northeast moves east and eventually
offshore, and a second trough lingers near the West Coast, ridging
will prevail across much of the country. During the second half of
the period, this ridging will become suppressed over the
north-central U.S., as weak short-wave troughing moving
east-northeast out of the Rockies and into the Plains phases loosely
with a low/trough crossing the Canadian Prairie.
At the surface, a cold front -- associated with the aforementioned
Prairie/Plains upper troughing -- is forecast to move southeast
across the northern Plains, eventually extending from the upper
Mississippi Valley area southwestward across the central Plains by
late in the period. Elsewhere, a weak/lingering front extending
from VA/NC and westward into the mid South will drift very slowly
southward, as surface high pressure shifts slowly east across the
Great Lakes into the Northeast.
Daytime heating of a moistening warm sector will support substantial
diurnal destabilization, with afternoon mixed-layer CAPE values
averaging 1500 to 2500 J/kg expected to evolve near/ahead of the
advancing cold front. Thus will likely prove sufficient, in
combination with ascent focused near the front, to allow isolated to
scattered storms to develop from the Minnesota vicinity
west-southwest along the front to the northern High Plains area.
The strongest mid-level westerlies are forecast to remain across the
North Dakota/northern Minnesota area, though a weaker/secondary
speed max shifting out of the Rockies suggests that ample flow for
isolated/organized cells will extend as far southwest as eastern
Wyoming and into Nebraska. As such, will maintain a broad area of
5%/MRGL risk across the north-central U.S., with greatest (SLGT
risk) probabilities focused over the eastern Dakotas/Minnesota
through the afternoon and early evening. With time, upscale growth
of storms into one or two MCSs is possible, with risk possibly
spreading east of the upper Mississippi Valley later in the evening.
Convection is expected to diminish overnight, due to a decrease in
instability -- both diurnally, and with eastward extent into the
upper Great Lakes area.
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NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z