SPC AC 111956

   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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