SPC AC 210615

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0115 AM CDT Wed Jun 21 2017

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
   OKLAHOMA/TEXAS PANHANDLES NORTHWARD INTO PARTS OF THE
   MID-MISSISSIPPI VALLEY AND RED RIVER VALLEY REGION OF THE DAKOTAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ALONG THE GULF
   COAST REGION OF LOUISIANA...MISSISSIPPI...AND ALABAMA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN NEW
   MEXICO NORTH-NORTHEASTWARD TO THE WESTERN GREAT LAKES AND NORTHERN
   PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM FAR
   SOUTHEAST TEXAS EASTWARD TO SOUTHWESTERN ALABAMA AND THE FLORIDA
   PANHANDLE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
   OHIO EASTWARD TO SOUTHERN NEW ENGLAND...

   ...SUMMARY...
   Severe thunderstorms are possible from parts of the southern High
   Plains to the north-central states for Wednesday into Wednesday
   night, with the greatest severe thunderstorm potential from parts of
   the Texas and Oklahoma Panhandles northeastward to Minnesota and
   Wisconsin.  Also, a tornado risk will exist in portions of the
   north-central Gulf Coast region.

   ...Synopsis...
   A quasi-zonal flow pattern aloft will evolve across the northern
   tier of the Lower 48 today as a mid-level trough migrates eastward
   out of the New England area.  Upstream, a strong mid/upper jet will
   develop in the northern Plains on the southern periphery of a
   vigorous system centered over Saskatchewan.  A persistent ridge over
   the Desert Southwest will finally weaken some throughout the day. 
   At the surface, a trough will extend from the Dakotas
   south-southwestward to the Texas Panhandle and eastern New Mexico. 
   The Dakotas portion of this trough will move eastward during the
   day, reaching Minnesota tonight.  Farther east, a synoptic boundary
   will extend from the mid-Mississippi Valley eastward across the Ohio
   Valley and into southern New England.  Eastern portions of this
   boundary will migrate southward during the day (across the
   northeast) while western portions of the boundary (across the
   mid-Mississippi Valley) will interact with a strengthening low-level
   jet and lift northward into Iowa by mid-afternoon.  Farther south,
   Tropical Storm Cindy will approach the Texas/Louisiana coastlines
   throughout the day.

   ...Eastern North Dakota/western Minnesota southward to northern
   Nebraska and eastward to Wisconsin...
   Scattered elevated storms are expected to be ongoing early in the
   forecast period, with a marginal risk of large hail in stronger
   thunderstorm cores.  This activity will generally occur along and
   north of a surface warm front, which will retreat northward over the
   course of the afternoon in response to strengthening southwesterly
   low-level flow across the region.  Models suggest that some hail
   threat will persist throughout the day and into the evening hours as
   convergence and lift along the warm front foster redevelopment of
   elevated convection in a region of steep mid-level lapse rates.  A
   slight risk has been maintained for this threat, accompanied by 15%
   large hail probabilities.

   Farther west, convection should redevelop along the surface trough
   from central North Dakota southward to northeastern Nebraska.  Deep
   shear vectors perpendicular to the initiating boundary will foster
   isolated to scattered supercell development during the afternoon and
   early evening hours, with an attendant threat for damaging winds and
   large (perhaps significant hail), especially in northeastern
   Nebraska and vicinity where surface-based instability will likely be
   the greatest.  The tornado threat in northern portions of this
   region is non-zero, but will likely be mitigated by relatively weak
   low-level wind shear and only modest low-level moisture (low 60s F
   dewpoints).  Farther south, there is a some chance of storms
   interacting favorably with a warm front/outflow boundary from
   northeastern Nebraska into northern Iowa that, when combined with
   stronger low-level flow/shear and stronger surface-based
   destabilization, may result in a localized area of higher potential
   for a tornado or two, in addition to significant hail and damaging
   wind gusts.

   ...Portions of the Great Plains from the eastern New Mexico
   northward to southern Nebraska...
   Moderate to strong destabilization will occur during the afternoon 
   resulting from steepening lapse rates, 60s surface dewpoints, and
   convergence along the aforementioned surface trough across the
   region.  A narrow corridor of minimal convective inhibition will
   develop by mid-afternoon along the surface trough, allowing for
   isolated to scattered thunderstorm development to occur.  Low-level
   shear will be a bit weak in much of the region, but will be slightly
   stronger in Nebraska closer to an 850mb jet axis.  Storms should
   organize into clusters and southward-propagating linear segments
   with an attendant large-hail and damaging-wind threat through the
   afternoon and early evening.

   ...Southern Louisiana eastward to southwestern Alabama and the
   western Florida Panhandle...
   Strong low-level shear profiles will exist for much of the day
   across the region in response to an approaching Tropical Storm
   Cindy.  Recent high-resolution model guidance suggests that any
   precipitation shield may not be continuous across inland areas,
   which may allow for pockets of insolation/destabilization in areas
   away from the immediate coast.  As a result, marginal and slight
   probabilities have been maintained and expanded - primarily driven
   by a tornado risk with scattered supercells that will traverse the
   region throughout the forecast period.

   ...Southern Ohio eastward to southern New England...
   Models indicate potential for several bands of thunderstorms to
   develop and move southeastward across the region during the early
   afternoon.  Strong deep shear and steep low-level lapse rates may
   allow for isolated instances of wind/tree damage, especially in and
   near linear segments that can organize.  This threat will be
   primarily diurnally driven, and should wane after dusk.

   ...South Dakota overnight...
   Surface cyclogenesis will occur late in the period across northern
   Nebraska.  Steep lapse rates aloft and subtle ascent associated with
   the mid-level low over Canada will allow for a few elevated
   thunderstorms to develop overnight, with some potential for large
   hail in stronger cores.

   ..Cook/Goss.. 06/21/2017

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