Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 241728
SPC AC 241727

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1227 PM CDT Thu May 24 2018

Valid 251200Z - 261200Z


Isolated to scattered severe thunderstorms are possible on Friday in
parts of the lower Missouri Valley and upper Mississippi Valley. A
few marginally severe storms are also possible in the southern and
central Plains, in northern Maine and over the interior Northwest.

...Lower Missouri Valley/Upper Mississippi Valley...
An upper-level trough will move into the lower Missouri Valley and
upper Mississippi Valley on Friday. At the surface, a cold front
will advance southeastward into the upper Mississippi Valley
extending southwestward into the central Plains. Ahead of the front,
surface dewpoints should generally be in the mid to upper 60s F with
moderate instability developing by afternoon across much of the
moist sector. As surface temperatures warm during the day and as
low-level convergence increases along the front, isolated to
scattered thunderstorms are expected to initiate. Large-scale ascent
associated with the upper-level trough will aid convective
development. Several thunderstorm clusters should develop in the
afternoon and evening, moving southeastward across the region.

NAM and GFS forecast soundings suggest the strongest instability
will be located from central Wisconsin southwestward to southern
Iowa Friday afternoon. MLCAPE values along this corridor are
forecast to reach the 2500 to 3500 J/kg range with low to mid-level
lapse rates approaching 7.5 C/km.  In addition, forecast soundings
ahead of the front show south-southwest winds at the surface and
veering winds with height to the northwest at the mid-levels. This
is forecast to create 0-6 km shear in the 25 to 35 kt range which
would be sufficient for severe multicells capable of producing wind
damage and hail. A supercell with isolated large hail and wind
damage would also be possible in areas that destabilize the most.

...Southern and Central Plains...
An upper-level ridge will move slowly eastward across the Rockies on
Friday as an upper-level trough moves into the Ozarks. Northwest
mid-level flow will be in place across the southern and central
Plains as a cold front advances southeastward into central Kansas
and the Texas Panhandle. Ahead of the front, a moist airmass will be
in place with surface dewpoints in the mid to upper 60s F. This
should result in a corridor of moderate instability by afternoon
from parts of western Oklahoma into southern and eastern Kansas.
Although large-scale ascent is forecast to remain weak, low-level
convergence along the front will aid convective development during
the mid to late afternoon. Although most of the convection should
remain isolated, a cluster of storms could organize and persist into
the evening in areas where mesoscale conditions are favorable.

NAM and GFS forecast soundings ahead of the front from Clinton, OK
northeastward to Topeka, KS for 00Z/Sat show MLCAPE values reaching
the 2500 to 3500 J/kg range with steep low to mid-level lapse rates.
Winds are forecast to veer with height from the southeast to the
northwest which could create enough deep-layer shear for a severe
threat. Hail and strong gusty winds will be possible with cells that
can develop and persist. However, model forecasts are considerably
different on how much instability and shear will be available for
the developing storms on Friday. Also, convective coverage should
remain isolated due to the limited large-scale ascent. For this
reason, any severe threat is expected to remain marginal.

A shortwave trough and an associated cold front is forecast to
approach the Northeast during the day on Friday. Ahead of the front,
surface dewpoints should reach the mid 50s to near 60 F. As surface
temperatures warm, some instability should develop in central and
northern Maine by afternoon. In addition, strong deep-layer shear
will be in place due to a mid-level jet oriented from east to west
across Quebec. A few thunderstorms may develop in north-central
Maine on the southern edge of the mid-level jet during the
afternoon. The strong deep-layer shear may be enough for hail and
isolated marginally severe wind gusts.

...Interior Northwest...
An upper-level trough will approach the Pacific Northwest Coast on
Friday. Ahead of the trough, a corridor of instability is forecast
to develop by afternoon from eastern Oregon northeastward into
western Montana. Low to mid-level lapse rates will become steep
along this corridor by late afternoon with isolated to scattered
thunderstorms developing. The steep lapse rates and relatively cold
air aloft may be enough for hail and strong gusty winds. Deep-layer
shear is expected to be too weak for an organized severe threat.

Tornado:   2%     - Marginal
Wind:     15%     - Slight
Hail:     15%     - Slight

..Broyles.. 05/24/2018

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