SPC AC 131957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Tue Jun 13 2017

   Valid 132000Z - 141200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EASTERN SD/ND...EASTERN NE...WESTERN MN...AND NORTHWEST IA...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF THE EASTERN TX PANHANDLE AND WESTERN OK...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER MUCH OF THE
   PLAINS STATES...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER SOUTHERN NEW
   ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   SOUTHERN GREAT LAKES TO SOUTHERN NEW ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER MUCH OF THE
   PLAINS STATES...

   ...SUMMARY...
   Severe thunderstorms, capable of large hail, damaging winds, and a
   few tornadoes, will be possible across the northern Plains and upper
   Mississippi Valley this afternoon and evening. Farther south across
   the central/southern Plains, scattered severe storms, primarily
   capable of large hail and damaging winds, are expected through the
   evening hours. Isolated severe storms will also be possible across
   southern New England.

   ...Central US...
   Minor changes have been made to the severe risk areas in the central
   US, mainly to adjust the higher threat area slightly westward into
   central parts of South Dakota and Nebraska where visible imagery
   indicates cumulus clouds deepening along the surface convergence
   boundary.  Otherwise the current outlook addresses the threat areas
   well and no other changes have been made.  See Mesoscale Discussion
   1013 for more details regarding near-term severe storm threat in the
   northern plains, and Mesoscale Discussion 1011 for details over the
   southern high plains.

   ...Southern New England...
   The Slight Risk area has been adjusted westward toward the Hudson
   River where strong/severe storms have recently developed.  There
   remains a possibility of isolated severe storms developing over the
   next few hours before activity diminishes this evening.

   ...Southern Great Lakes eastward toward southern New England....
   The Marginal Risk area has been extended westward and refined based
   on visible satellite imagery showing locations of primary outflow
   boundaries, lake breeze convergence zones, and boundary
   intersections that are providing foci for additional strong
   thunderstorm development.  The winds aloft and associated vertical
   shear above this corridor is modest at best resulting in loosely
   organized small thunderstorm clusters with updrafts tending to
   exhibit pulse characteristics.  Brief hail and locally strong wind
   gusts may accompany the stronger storms into the evening hours.

   ..Weiss.. 06/13/2017

   .PREV DISCUSSION... /ISSUED 1122 AM CDT Tue Jun 13 2017/

   ...Northern Plains...
   An upper cyclone continues to affect the central/northern Rockies
   today, while a series of shortwave troughs rotate through the base
   of the cyclone and into the central/northern Plains.  Strong daytime
   heating and rich low-level moisture will result in high CAPE values
   this afternoon along and east of a surface front/dryline extending
   from the central Dakotas into KS.  Models are consistent in the
   development of thunderstorms along the boundary by late afternoon,
   tracking eastward across the ENH/SLGT risk areas.  Supercells
   capable of large hail and a tornado or two are initial threats, but
   most 12z model solutions suggest a fairly rapid transition to bowing
   structures, with damaging winds becoming the dominant concern as
   storms move into MN.

   Have maintained a small 10% tornado risk area over parts of
   MN/SD/ND.  Rapid recovery behind the morning MCS should result in a
   corridor of favorable environment later today.  Forecast soundings
   in this region show backed low-level winds and strong CAPE. 
   However, confidence in discrete storm initiation along the warm
   front has decreased from previous outlooks, and will be re-addressed
   at 20z.

   ...Southern Plains...
   Full sunshine along the surface dryline/front over west TX/OK and
   parts of KS will result in strong CAPE and scattered thunderstorm
   development by late afternoon.  Deep-layer vertical shear will
   promote rotating storms, with large hail /possibly very large/ and
   damaging wind gusts possible. Given the consistency in 12z model
   guidance regarding the timing/placement of convective development,
   have added a narrow ENH risk zone to parts of western OK and the TX
   Panhandle.

   ...CT/MA...
   Visible satellite imagery shows strong heating occurring over
   southern New England today, where surface dewpoints are in the mid
   60s.  Scattered thunderstorms are likely to develop this afternoon
   as a mid-level trough approaches from the northwest.  Forecast
   soundings are not particularly impressive, with only moderate lapse
   rates and rather weak low-level winds.  However, sufficient CAPE and
   deep layer vertical shear suggest the risk of a few intense cells
   capable of hail and damaging wind gusts.

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