Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 201723
SWODY2
SPC AC 201722

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1222 PM CDT Sun Aug 20 2017

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE MID MISSOURI VALLEY AND MID TO UPPER MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL PLAINS...LOWER TO MID MISSOURI VALLEY...MID TO UPPER
MISSISSIPPI VALLEY AND WESTERN GREAT LAKES...

...SUMMARY...
Severe thunderstorms associated with wind damage and hail are
forecast to impact parts of the mid Missouri Valley
east-northeastward into the mid to upper Mississippi Valley on
Monday afternoon and evening.

...Mid Missouri Valley/Mid to Upper Mississippi Valley/Central
Plains/Western Great Lakes...
A low amplitude cyclonic flow pattern will be in place across the
north-central United States on Monday as an upper-level trough moves
into the central Plains. At low-levels, a jet maxima is forecast
across Iowa during the morning. A cluster of elevated storms may be
ongoing near the exit region of the low-level jet in north-central
Iowa and southern Minnesota at the start of the period. This
convection is forecast to move eastward into the western Great Lakes
region by midday and could be associated with a marginal severe
threat. An outflow boundary may be left behind across parts of
northeast Nebraska and Iowa which could be a focus for surface-based
convective development during the mid to late afternoon. A cold
front is also forecast to move southeastward into the mid Missouri
Valley by late afternoon which will also be a focus for convection.
Depending upon where the morning outflow boundary sets up, a moist
and unstable airmass should be in place by afternoon. Model
forecasts suggest that MLCAPE values may reach 2000 J/kg across
parts of the slight risk area by late afternoon. Forecast soundings
along this corridor at 21Z/Monday show moderate deep-layer shear and
steep low-level lapse rates. Surface winds may be locally backed
just to the south of the cold front helping to enhance shear and
making conditions more favorable for severe storms. The wind profile
may support the development of isolated supercells in areas where
moderate instability is realized. Large hail and wind damage will be
possible with supercells. However, multicells should be the more
common storm mode. Wind damage and hail will be possible with the
multicell line segments. The severe threat should gradually develop
east-southeastward into parts of southern Wisconsin, northwest
Illinois, southern Iowa and far northern Missouri during the early
to mid evening.

Further to the west in the central Plains, an upper-level trough is
forecast to move across the region during the mid to late afternoon.
Thunderstorms that form along the front to the west of the trough,
mainly in central and northern Nebraska, should have a less
favorable shear environment due to veered low-level winds. In spite
of this factor, steep low-level lapse rates and moderate instability
may be enough for a marginal wind damage threat.

..Broyles.. 08/20/2017

$$


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