SPC AC 171619

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1119 AM CDT Mon Apr 17 2017

   Valid 171630Z - 181200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO EARLY TONIGHT FOR PARTS OF THE CENTRAL/NORTHERN HIGH PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ALONG THE LOWER RIO GRANDE VALLEY...

   ...SUMMARY...
   Very isolated severe thunderstorms with hail and strong wind gusts
   are expected across parts of the High Plains and the lower Rio
   Grande Valley this afternoon and evening.

   ...Western SD area late this afternoon into early tonight...
   A compact shortwave trough over ID this morning will reach the
   northern High Plains by this evening, and continue eastward over the
   Dakotas/MN overnight.  Associated lee cyclogenesis will help draw
   low-level moisture northward across the High Plains, while surface
   heating in cloud breaks and advection of steeper midlevel lapse
   rates from the west contribute to near-surface destabilization as
   far north as southwestern SD.  Thunderstorms that develop by late
   afternoon could be rooted at the surface near and south of the Black
   Hills, and convection will likely be elevated farther
   north-northeast.  The focused forcing for ascent, weak buoyancy, and
   strong cloud-layer vertical shear will support at least a low-end
   risk for hail/damaging winds from late afternoon through early
   tonight.

   ...Western NE to southeastern CO this afternoon/evening...
   Low-level moisture (dewpoints in the low-mid 50s) is beginning to
   return northward across the High Plains from southeastern CO to
   western NE, with evidence of weak terrain-driven cyclonic
   circulations in CO near La Junta and northeast of Denver.  Some
   cloud breaks by afternoon along the west edge of the richer moisture
   will allow surface heating and weakening of the cap from below, and
   the development of MLCAPE ranging from 1500 J/kg in southeastern CO
   to 500 J/kg across western NE.  Isolated thunderstorm development is
   expected by late afternoon, possibly in conjunction with the
   lingering terrain circulations.  Though mid-upper flow is weak this
   morning, some increase is expected by afternoon/evening as far south
   as eastern CO, where some increase in low-level shear will also
   occur.  The net result will be an environment at least marginally
   favorable for supercells capable of producing isolated large hail,
   damaging gusts, and perhaps a tornado.  

   ...South TX this afternoon...
   An MCV is drifting eastward over the middle/lower TX coast as of mid
   morning, and much of south TX has been somewhat stabilized by the
   overnight convection (see the 12z CRP sounding).  Outflow from the
   convection may retreat to just north of the international border
   along the lower Rio Grande River, and provide a focus for additional
   storm development this afternoon.  The mesoscale storm environment
   will be at least marginally favorable for supercells, with an
   attendant risk for isolated large hail/damaging winds near the
   international border.  Farther north, other than some elevated
   convection associated with warm advection atop the remnant cold
   pool, it is not clear that destabilization will be sufficient for
   additional strong/severe storm development this afternoon. 

   ...North TX to western TN this afternoon...
   Convection will likely develop and/or persist along an outflow
   boundary from western TN across southern AR to north TX.  An
   isolated pulse-type downburst and/or small hail cannot be ruled out
   (especially north TX), but weak vertical shear and at best moderate
   buoyancy suggest that the severe-storm risk is too marginal to
   warrant the introduction of an outlook area. 

   ...Northern NC/southeastern VA this afternoon...
   Strong surface heating will boost afternoon surface temperatures to
   the lower 80s, though poor midlevel lapse rates and modest low-level
   moisture will limit MLCAPE to at or below 500 J/kg.  There will be
   some potential for gusty outflow winds with convection along the
   front, where low-level lapse rates will be steep along the southern
   fringe of 25-30 kt midlevel flow.  Still, the marginal buoyancy and
   modest flow suggest the damaging wind risk is too marginal to add an
   outlook area.

   ..Thompson/Gleason.. 04/17/2017

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