Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 231254
SPC AC 231252

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Wed May 23 2018

Valid 231300Z - 241200Z


Isolated severe thunderstorms are possible this afternoon and
evening across the High Plains and eastern portions of
Virginia/North Carolina.

...Northern Plains...
An upper low over the Great Basin/central Rockies will continue to
move northeastward toward the northern Rockies and adjacent High
Plains through the afternoon and tonight. Lee cyclogenesis across
the north-central High Plains will influence a northward increase in
low-level moisture (50s to some 60s F surface dewpoints) in the warm
sector to the south of a northward-shifting warm front and to the
east of the lee trough. Beneath a northeastward-advecting elevated
mixed layer (EML), this moisture will support moderate
destabilization with as much as 1000-2500 J/kg MLCAPE from western
Nebraska and far eastern portions of Wyoming and Colorado northward
into far eastern Montana and portions of the Dakotas.

An eroding cap/orographic influences will result in isolated to
scattered surface-based thunderstorm development initially along the
front range vicinities potentially spanning northern Colorado,
eastern Wyoming into southeast Montana.  On the periphery of the
previously described upper low, it also appears that a subtle
peripheral disturbance/speed max, located near the Four Corners area
this morning, may be favorably timed to arrive into the region
around peak heating, with further influences on forcing for ascent,
increasing vertical shear etc.

The potential for organized storms appears greatest over parts of
the north-central High Plains where vertical shear is forecast to be
strongest. Various models imply a higher storm coverage and severe
potential from far eastern Wyoming northeastward into western
Nebraska and the Black Hills vicinity, with the possibility that
storms will grow upscale and spread northeastward across South
Dakota/northern Nebraska this evening with the aid of a
strengthening low-level jet. Bouts of large hail and damaging winds
will be the primary risks.

...Eastern New Mexico/southern High Plains...
Another day of widely scattered thunderstorm development is expected
across the region later today as moist low-level upslope flow
persists across the region. 12Z regional RAOBS from Amarillo,
Midland, and Del Rio, Texas all sampled Precipitable Water values
that are in the upper 20% of daily climatological norms. Mid-level
winds will continue to weaken today as heights also rise, but
aforementioned upslope and orographic lift/differential heating will
lead to widely scattered strong to severe thunderstorms this
afternoon in the presence of steep lapse rates and ample buoyancy in
excess of 2000+ J/kg MLCAPE. Scenario favors some multicells or at
least pulse-type storms capable of large hail across eastern New
Mexico and far west/southwest Texas. Congealing outflows and a
modestly increasing southerly low-level jet could yield one or more
small-scale complexes that move eastward into west/northwest Texas
during the evening and overnight, which would include the lingering
possibility of severe-caliber wind gusts on a localized basis.

...Virginia/North Carolina...
A cold front will progress southeastward across the region through
the afternoon and evening. Temperatures ahead of the front will
likely reach the mid-80s coincident with surface dewpoints in the
upper 60s F, which is supportive of air mass destabilization and
moderate buoyancy with as much as 1500-2000 J/kg MLCAPE this
afternoon. Deep-layer shear is forecast to favor multicells with
some upscale growth into southeastward-moving segments expected.
Isolated damaging wind gusts should be the most likely hazard,
although some hail could occur as well.

A weak cold front will continue to spread southeastward across Maine
today before clearing the coast by late afternoon/early evening. It
appears that a pocket of relatively steep mid-level lapse rates, as
partially sampled by the Maniwaki Quebec 12Z observed sounding, will
accompany an amplifying/southeastward digging shortwave trough and
speed max located over northern Quebec this morning. Although the
boundary layer will not be overly moist near and ahead of the front,
steepening low/mid-level lapse rates in the presence of increasing
dynamic forcing for ascent will be favorable for diurnally
increasing thunderstorms. Relatively well-mixed boundary layer
profiles coupled with strengthening westerly mid-level winds could
yield some thunderstorms that are capable of locally damaging wind
gusts along with small hail.

..Guyer/Goss.. 05/23/2018

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