SPC AC 170600

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Fri May 17 2019

   Valid 171200Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF NEBRASKA AND SOUTHEAST SOUTH DAKOTA...AS WELL AS PARTS OF WESTERN
   TEXAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   HIGH PLAINS NORTHEASTWARD INTO THE MIDDLE MISSOURI VALLEY...OVER
   PARTS OF WESTERN TEXAS...AND FROM OHIO EAST-SOUTHEASTWARD ACROSS
   VIRGINIA...

   ...SUMMARY...
   Severe storms with very large hail, damaging winds, and a few
   tornadoes (a couple of which could be strong) are expected, mainly
   from the central High Plains northeastward to the mid Missouri
   Valley.  Several severe storms -- capable of producing very large
   hail and damaging winds, and a couple of tornadoes -- will also be
   possible across portions of the western half of Texas.  Finally,
   scattered afternoon storms -- capable of producing locally
   gusty/damaging winds will be possible from Ohio through the Middle
   Atlantic region.

   ...Synopsis...
   Weak ridging will prevail across the southeastern U.S. and
   northwestward across central portions of the country today, ahead of
   a large trough traversing the West.  As the trough impinges on the
   Plains during the afternoon and evening, a central High Plains
   surface low is progged to shift northeastward out of Colorado across
   Nebraska -- focusing an area of showers and storms -- and
   accompanying severe risk.  Other storms/severe weather risk will
   evolve across parts of western Texas, and across portions of the
   Ohio Valley into the Mid-Atlantic region, within a zone of
   northwesterly flow aloft.

   ...The central Plains vicinity, centered over Nebraska...
   As the upper trough advances across the Intermountain West,
   southwesterly flow aloft will continue advecting an elevated mixed
   layer across the central Plains, allowing the airmass to remain
   capped across a large portion of the central Plains.  However, a lee
   low over eastern Colorado is forecast to begin shifting
   northeastward across northwestern Kansas/southwestern Nebraska by
   late afternoon, and then northeastward across Nebraska along a
   surface baroclinic zone through the evening.  Ascent near -- and
   just to the cool side -- of this boundary will permit eventual
   initiation of storms to occur, from southeast Wyoming/northeast
   Colorado and the Nebraska Panhandle during the afternoon, and then
   spreading northeastward across Nebraska and South Dakota through the
   evening.  A couple of storms may also initiate farther south along a
   dryline across western Kansas, though stronger capping across this
   region should limit storm coverage.  Still, very large hail and
   locally damaging winds, along with some tornado risk, will accompany
   any dryline storm.

   Greater storm coverage -- and thus severe risk -- will remain
   farther north however, near the surface front and ahead of the
   advancing surface low.  To the north -- into South Dakota, large
   hail will be the primary severe risk.  Meanwhile, near the boundary
   -- expected to bisect west-central Nebraska from southwest to
   northeast during the afternoon and evening -- damaging winds and a
   few tornadoes will accompany the stronger storms, along with the
   risk for very large hail.  Some potential for a strong tornado or
   two is evident -- especially if a discrete storm or two can move
   northeastward near the baroclinic zone.  

   As a low-level jet increases through the evening, storms will likely
   grow upscale, with some increase in wind risk possible, but
   eventually most convection will evolve north of the surface front --
   where hail risk with elevated convection will become the main threat
   overnight.

   ...Portions of western Texas...
   Daytime heating of a moist airmass over western portions of Texas
   will result in moderate afternoon destabilization, ahead of the
   advancing upper trough.  With a weak perturbation progged to shift
   across eastern New Mexico and far West Texas by afternoon, isolated
   storms are expected to develop -- from the Transpecos region
   northeastward into western North Texas -- during the late afternoon
   as capping is locally breached.  With moderate/veering flow with
   height, shear supportive of supercells suggests risk for very large
   hail, and locally damaging wind gusts.  During the evening, a
   south-southeasterly low-level jet is forecast to develop -- which
   may initially enhance low-level shear and result in a briefly
   greater tornado risk with a couple of the strongest storms. 
   Eventually however, upscale growth into more linear -- and possibly
   bowing -- clusters may occur, possibly accompanied by risk for
   damaging winds as storms move eastward/northeastward toward central
   and northern Texas into the overnight hours.

   ...Parts of southern Ohio eastward/southeastward to the Chesapeake
   Bay...
   Ongoing storms are forecast to be moving quickly southeastward
   across parts of the region at the start of the period, before
   vacating/dissipating through midday.  In the wake of this early
   convection, heating/destabilization of the upstream airmass is
   expected, which should support new storm development during the
   afternoon.  With moderate/roughly unidirectional west-northwesterly
   flow across the region, quick-moving storms/small storm clusters
   will be capable of producing gusty winds and local tree damage
   across the region, before convection weakens during the evening.

   ..Goss/Wendt.. 05/17/2019

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