Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 211253
SPC AC 211251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Mon May 21 2018

Valid 211300Z - 221200Z


Isolated to widely scattered strong to severe thunderstorms are
possible across the Ohio Valley as well as portions of New Mexico
and the central and southern High Plains. Damaging wind and large
hail will likely be the main severe threats.

...Ohio Valley...
Scattered showers and thunderstorms remain prevalent early this
morning from near the confluence of the Mississippi and Ohio Rivers
northward across Illinois toward the southern Lake Michigan
vicinity. Part of this convection continues to muddle a warm sector
to the east of a weak northeastward-transitioning surface wave and
near/south of an eastward-extending warm front that will continue to
develop slowly northward across Illinois, Indiana, and Ohio.

The ongoing precipitation and related cloud cover casts some
uncertainty on the exact degree/locations of more appreciable
destabilization later today, although cloud breaks are noted in
early-morning visible satellite imagery from Kentucky into southern
portions of Indiana and Ohio. While the overall magnitude/extent of
the severe risk remains uncertain, at least some severe
thunderstorms can be expected this afternoon into evening. The most
supercell-favorable deep-layer and low-level shear later today will
exist just ahead of the aforementioned weak surface wave and the
eastward extending warm front, mostly across central portions of
Indiana into Ohio in areas along/north of I-70. Severe hail and
possibly a tornado could occur within this general corridor, while
isolated bouts of damaging winds will be the main hazard across a
broader part of the region.

...New Mexico and west/southwest Texas...
Moist post-frontal low-level upslope flow will continue to yield an
increase in moisture with west-northwestward extent across New
Mexico during the day. The region will not be overtly influenced by
the slow-moving/digging closed low over the southwest states, but
relatively plentiful moisture and orographic ascent will lead to
scattered thunderstorms this afternoon. Initial thunderstorm
development is expected this afternoon across interior New Mexico
and by late afternoon/early evening farther east across eastern New
Mexico and far west Texas. 30-35 kt of effective shear, steep lapse
rates, and ample moisture for the region will support multicells and
possibly a few supercells capable of severe-caliber hail, along with
gusty winds. Areas such as far west Texas and southeast/
south-central New Mexico will continue to be reevaluated for a
possible categorical Slight Risk upgrade given a somewhat more
severe-favorable environment.

...Central High Plains...
Gradual lee-side cyclogenesis will influence weak upslope
trajectories with a modest influx of low-level moisture through the
day. While upper heights will generally rise, weak low-level upslope
flow and differential heating along the mountains/higher terrain
should yield isolated east/southeastward-drifting thunderstorms this
afternoon. Sufficient low-level moisture and effective shear (30-40
kt) coincident with very steep lapse rates could support some strong
multicells/brief supercells capable of severe hail/gusty winds on a
very isolated basis.

..Guyer/Goss.. 05/21/2018

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