SPC May 11, 2017 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0256 PM CDT Thu May 11 2017

Valid 112000Z - 121200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
NORTHEAST OKLAHOMA...FAR SOUTHEAST KANSAS...SOUTHWEST MISSOURI...AND
NORTHWEST ARKANSAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENH RISK AREA...AND EXTENDING EASTWARD TO THE MID-ATLANTIC REGION
AND SOUTHWARD INTO EAST TEXAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF EASTERN WASHINGTON...NORTHEAST OREGON...NORTHERN IDAHO...AND
NORTHWEST MONTANA...

...SUMMARY...
Severe thunderstorms -- including risk for very large hail, locally
damaging winds, and a couple of tornadoes will continue across parts
of the southern Plains, Ozarks, and Ark-La-Tex region. Other strong
to severe thunderstorms will be possible as far east as the
mid-Atlantic States/North Carolina, while strong storms will also be
possible across parts of the northern Rockies.

...Discussion...
Aside from minor adjustments to the ENH risk area across the
northeast Oklahoma vicinity -- where multiple severe/supercell
storms are ongoing, few changes are required to current outlook
areas.  Forecast reasoning from prior outlooks remains valid with
respect to the evolving convective scenario.

..Goss.. 05/11/2017

.PREV DISCUSSION... /ISSUED 1137 AM CDT Thu May 11 2017/

...Parts of OK/AR/KS/MO this afternoon and evening...
A closed trough centered over the south-central High Plains this
morning will continue generally east-southeastward as it gradually
opens. While a somewhat diffuse pattern will exist at the surface
given the evolution of the parent cyclone, a semi-focused corridor
of severe thunderstorm occurrence is likely this afternoon
especially in areas north/east of a weak surface wave and
outflow-modulated effective frontal corridor. Coincident with
relatively cool mid-level temperatures (around -15C at 500 mb into
central OK per 12Z upper-air data) atop mid 60s F surface dewpoints,
very steep mid-level lapse rates, moderate buoyancy, and 40+ kt
effective shear will support initial semi-discrete supercells
capable of large hail. Although low-level shear/SRH will not be
strong, at least a brief tornado or two could occur owing to ambient
vorticity related to the upper low and outflow. Multiple clusters
may eventually merge by late afternoon/early evening with some
concern for damaging winds as well.  

Farther south, a somewhat separate corridor of storms is anticipated
today across east/northeast TX as a diffuse warm front and
boundary-layer dewpoints in the 70s shift northeastward. Supercells
will be possible within a broader corridor of cells/clusters, with
some tendency for upscale growth into larger clusters/line segments
expected by this evening. Large hail and damaging winds will be the
main risks through tonight as convection spreads eastward toward the
MS River. Like areas farther north, relatively weak low-level shear
will tend to limit the tornado risk to favorable storm interactions
(if any), while hail may not be as large as areas farther north into
OK as a result of warmer temperature profiles aloft and somewhat
lesser mid-level lapse rates.

...KY/WV/VA/NC this afternoon into early tonight...
An MCV over southern IL late this morning will continue eastward
along the slow-moving front and lingering outflow boundaries across
KY this afternoon, providing a focus for additional storm
development this afternoon. Potentially moderate buoyancy (MLCAPE 
near 1500 J/kg) and largely unidirectional wind profiles/straight
hodographs will favor organized clusters capable of producing
damaging winds and large hail. There is some potential for a
cluster to persist while crossing the Appalachians this evening and
turning more to the southeast across VA/NC, where other more
isolated storms may also form along a stalled front this afternoon.

...Northern Rockies this afternoon/evening...
A pronounced midlevel trough and low/mid-level baroclinic zone will
move slowly eastward over the Pacific Northwest. Along and
immediately east of the front, moisture and lapse rates will be
sufficient for scattered thunderstorms. Given the steep lapse rates
and strong deep-layer shear, isolated damaging outflow gusts and
marginally severe hail will be possible with the strongest storms.

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