SPC AC 051937

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0237 PM CDT Tue Jun 05 2018

   Valid 052000Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHERN PLAINS...

   ...SUMMARY...
   A few severe thunderstorms with large hail and wind damage are
   forecast across the northern Plains late this afternoon into
   tonight.  Isolated strong to severe storms are also possible this
   afternoon from east Texas into Louisiana, over the southern High
   Plains late this afternoon and evening, and from the upper Ohio
   Valley into the Mid-Atlantic region this afternoon.

   ...Northern Plains Update...
   East/northeasterly flow is maintaining 50s F dewpoints into eastern
   MT with an expanding CU field toward the Wind River range. Upslope
   flow and continued heating may result in a few severe storms, with
   areal coverage limited by a minor capping inversion to the east.
   However, long hodographs and substantial deep-layer shear will
   support storm longevity, perhaps as far as western SD. As such, have
   expanded the Slight Risk westward into MT and far northeast WY for a
   hail and wind threat.

   ..Jewell.. 06/05/2018

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Tue Jun 05 2018/

   ...Northern Plains Region...
   Water vapor imagery shows a well-defined low amplitude short wave
   trough currently approaching the northern Rockies.  This feature is
   expected to move across the far northern Plains tonight.  In
   response to this short wave trough, a surface low is expected to
   persist over western South Dakota into the evening before moving
   toward southeast North Dakota tonight, with a northeast-southwest
   oriented cold front progressing eastward and reaching a northwest
   Minnesota-south central South Dakota line by the end of the period. 

   Visible imagery indicates limited clouds over the area with strong
   diabatic heating likely to occur into the afternoon.  However, 12Z
   regional soundings exhibit a strong cap that will inhibit
   development of surface-based convection during peak heating over the
   Dakotas east of the higher terrain.  A modest increase in low-level
   moisture is possible this afternoon and evening with surface dew
   points in the upper 50s to lower 60s expected, and with steep
   mid-level lapse rates in place, will contribute to moderate
   instability with MLCAPE of 1500-2000 J/kg. 

   Storms are more likely to develop initially over the higher terrain
   of northern Wyoming/southern Montana and the Black Hills vicinity by
   late afternoon and early evening as orographic forcing combines with
   weak large scale ascent in advance of the aforementioned short wave
   trough.  The convection will spread eastward across parts of the
   Dakotas and eventually move into parts of northern and central
   Minnesota as a nocturnally enhanced southwest low-level jet
   strengthens during the evening and shifts eastward with time. 
   Deep-layer shear associated with 35-45 kt westerly mid level winds
   will be sufficient to enhance storm organization with a few
   supercells and possible bow echo segments developing.  Occasional
   large hail and severe wind gusts will be the primary threats as
   activity spreads eastward through much of the overnight hours.

   ...East Texas into southern Louisiana...
   Storms clusters continue to develop along the southwest periphery of
   a short wave trough that is moving southeastward across the Arklatex
   region.  The widespread clouds associated with the convection have
   generated a differential heating boundary from east central Texas
   into central Louisiana, with visible imagery showing cumulus cloud
   streets spreading into the boundary across southeast Texas and
   southwest Louisiana.  12Z soundings showed moderate-to-strong
   mid-level lapse rates over the area, and with surface dew points in
   the low-to-mid 70s, the air mass is unstable with MLCAPE of
   1500-2000 J/kg.  Northwest winds aloft around 30 kt contributing to
   sufficient shear to support primarily organized multi-cell clusters
   and short line segments this afternoon.  

   Storms along the leading edge of the current convection will
   continue to develop southeastward with potential for stronger cells
   to produce strong wind gusts.  Current radar imagery shows the
   convective outflow moving ahead of the storm cores in west-central
   Louisiana, which may limit potential for a more widespread severe
   wind threat.  In addition, the favorable lapse rates suggest
   potential for marginally severe hail.  Activity will continue
   southeastward through the afternoon before weakening begins by early
   evening. 

   ...Southern High Plains...
   Strong heating is occurring over this region leading to vertical
   profiles with steep low-to-mid level lapse rates this afternoon and
   evening.  Boundary layer mixing will result in a dry line shifting
   eastward this afternoon which will provide a focus for convective
   initiation.  Veering winds with height immediately in advance of the
   upper ridge will support organized multi-cells or an isolated
   supercell during the afternoon and evening, but weak large scale
   ascent may limit the extent of convective coverage.  Large hail and
   strong wind gusts will be possible with stronger cells.

   ...Upper Ohio Valley to Delmarva Region...
   A strong short wave trough will progress southeastward this
   afternoon from the lower Great Lakes into the Mid-Atlantic region
   accompanied by a cold front moving across the area.  Despite
   marginal moisture and instability, strong dynamic forcing for ascent
   and convergence along the front may focus development of low-topped
   convection into the evening hours.  12Z CAMs provide consistent
   guidance for this scenario, and given very strong westerly winds
   aloft above the surface layer, a threat for a few damaging wind
   gusts will exist.

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