Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 271249
SWODY1
SPC AC 271248

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Thu Jul 27 2017

Valid 271300Z - 281200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
THE SOUTHERN PLAINS ACROSS THE SOUTHERN/CENTRAL APPALACHIANS TO
PARTS OF THE NORTHEAST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER SOME OF THE
NORTHERN/CENTRAL HIGH PLAINS AND ROCKIES FOOTHILLS...

...SUMMARY...
Marginally severe wind gusts will be possible with a few of the
stronger thunderstorms from parts of the southern Plains east-
northeastward into parts of the Northeast.  Hail and isolated strong
wind gusts also may occur across parts of the central and northern
High Plains.

...Synopsis...
The upper-air pattern will be characterized on the synoptic scale by
slowly westward-shifting ridging from the southern Plains to the
Southwest, and height falls across much of the eastern U.S.  A
compact shortwave trough -- now apparent in moisture-channel imagery
over southern MB -- is forecast to amplify as it moves
southeastward.  By 12Z, this feature should reach parts of Lower MI
and Lake Michigan, with an embedded 500-mb low developing.  Ahead of
that, a larger trough is moving eastward across QC and extends
southwestward over Lake Huron.  The trough will move off the New
England and Mid Atlantic Coasts overnight.

At the surface, a cold front preceding the leading trough was
analyzed from southeastern ON southwestward across northwestern IN,
west-central MO, south-central KS, and the eastern CO/NM border
area.  By 00Z, this front is forecast to reach northern NY, central
OH, southern IL, northern/western OK, and the TX Panhandle.  By the
end of the period, the front should reach the Delmarva Peninsula
area, southern WV, the lower Ohio Valley, southern OK, and east-
central NM.  The front will be preceded by numerous aggregated
outflow and differential-heating boundaries through tonight.

...Mid Mississippi Valley to Northeast...
Isolated to scattered thunderstorms, both discrete and in clusters
of locally denser coverage, are expected to form along and ahead of
the front through this evening.  This activity will offer the
potential for sporadic wind damage and isolated severe gusts, mainly
in the most intensely water-loaded downdrafts.  Surface dew points
ranging from the low/mid 60s F in the Northeast to the low/mid 70s F
near the Mississippi River will offset modest mid/upper-level lapse
rates enough to yield MLCAPE ranging from around 500 J/kg near the
eastern end of the outlook where deep shear is stronger to 1500-2000
J/kg over parts of the Mid-South, amidst weak flow aloft.  More
concentrated areas of clustered/cold-pool development will depend
strongly on storm-scale affects and localized boundary interactions.
Within this broad-brushed 5%/marginal area, one or two relatively
concentrated areas of strong/damaging convective wind may occur,
especially in the south-central Appalachians/Piedmont region.
However, in this regime of generally weak low-level flow and weak
deep shear, predictability is too low to outline more specific
threat corridors at this time.

...Southern Plains to Ozarks...
Widely scattered thunderstorms are expected to develop this
afternoon into early evening near the front and any prefrontal
outflow/differential-heating boundaries left by ongoing clouds/
precip.  A few of these may produce severe gusts.  Convective
potential may be augmented on the mesoscale by weak lift preceding
an eastward-moving MCV now apparent in composite-reflectivity loops
over southwestern KS.  With pockets of very strong diabatic surface
heating and deep boundary-layer mixing, low-level lapse rates will
become suitably steep, driving DCAPE values above 1000 J/kg in many
areas.  However, despite favorable low-level moisture, actual
buoyancy will be substantially limited by weak mid/upper-level lapse
rates, with 500-mb temps generally in the -3 to -5 deg C range.
Though some threat exists for localized concentrations of
severe-wind, the smaller-scale/boundary-driven dependency of that,
in conjunction with weak flow/shear, and poor lapse rates aloft,
preclude more than a marginal-severe outline at this time.

...Central/northern High Plains and vicinity...
Widely scattered thunderstorms are expected to develop this
afternoon and move generally eastward to southeastward across the
outlook area, offering the risk of isolated severe gusts and hail.
Forecast soundings suggest effective-shear magnitudes will peak
around 40-45 kt across eastern WY, the western NE Panhandle and
southwestern SD, amidst strong deep-layer directional shear.  While
weak midlevel winds will limit hodograph sizes and storm-relative
wind vectors near and a couple km above the top of the boundary
layer, the profile will support supercells.  However, lack of
stronger midlevel lapse rates should limit overall buoyancy and keep
the hail risk below well-organized/significant levels.  Well-mixed
subcloud layers will support isolated severe-gust potential,
especially with any aggregating clusters/cold pools.  The risk
should diminish markedly after nightfall, given near-surface
diabatic cooling and the lack of substantial upper support.

..Edwards/Smith.. 07/27/2017

$$



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