SPC AC 221616
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1116 AM CDT Fri Sep 22 2017
Valid 221630Z - 231200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
UPPER PENINSULA OF MICHIGAN...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
NORTHWEST MINNESOTA AND NORTHEAST NORTH DAKOTA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE PLAINS STATES AND UPPER MIDWEST...
Isolated severe hail and wind storms are possible from the Upper
Midwest to the southern High Plains, mainly during the late
afternoon and evening. The greatest threat today is over parts of
the Upper Peninsula of Michigan. This afternoon and evening, the
threat will focus over northwest Minnesota.
An ongoing cluster of strong/severe storms has been affecting
northern WI for several hours. This cluster is in an environment
where continued weak low level warm/moist advection and daytime
heating will help destabilize the boundary layer. Given the
persistent nature of the storms, and the slowly increasing mesoscale
organization of the activity, have added a SLGT risk for wind damage
across parts of the MI Upper Peninsula.
The overall forecast for this region remains unchanged. A very
moist/unstable air mass will develop this afternoon in the warm
sector of a low over northwest MN and eastern ND. Vertical shear
profiles will also be quite strong across this region with forecast
soundings suggesting a conditional risk of supercells and tornadoes
in warm sector. However, virtually no 12z CAM guidance shows robust
convective initiation in the warm sector. Instead, all storms form
along and northwest of the warm front, affecting northwest MN and
northeast ND this afternoon and evening. Supercell storm structures
are still expected in the elevated-convective regime, with stable
near-surface lapse-rates likely limiting the tornado and wind damage
potential. Large hail is expected to be the main threat.
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