NWS Storm Prediction Center - Day 1 Convective Outlook

Jul 20, 2017 2000 UTC Day 1 Convective Outlook
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 Forecast Discussion - Day 1 Convective Outlook


   SPC AC 202006

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0306 PM CDT Thu Jul 20 2017

   Valid 202000Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE NORTHERN
   HIGH PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
   THE NORTHEAST STATES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   NORTHERN HIGH PLAINS TO THE NORTHEAST STATES...

   ...SUMMARY...
   Severe storms are expected to develop over the northern High Plains
   this afternoon into the early evening. Very large hail and damaging
   wind will be the main threats. Otherwise Widely scattered severe
   thunderstorms will persist this afternoon and early evening from a
   portion of the northeast States into a portion of the upper Midwest
   with a few damaging wind gusts and hail the main threats.

   ...NORTHERN High Plains...

   Primary change to this portion of the outlook has been to introduce
   a sig area for large hail given the favorable thermodynamic
   environment with steep lapse rates and moderate instability in the
   presence of strong vertical shear supportive of supercells.
   Initially discrete storm modes will further promote the large hail
   threat.

   ...Northeast States through Upper Midwest...

   Have trimmed back on the Ohio Valley portion of the slight risk
   mainly due to storm coverage concerns with capping issues associated
   with base of warm elevated mixed layer, mid-level subsidence and
   height rises in wake of MCV. Nevertheless, isolated storms might
   still develop near stationary boundary from IA into northern IL
   downstream from a shortwave trough moving through NE. These storms
   could pose a risk for downburst winds and hail.  Additional storms
   will likely develop over the upper MS valley region tonight posing
   at least a marginal risk for hail.

   ..Dial.. 07/20/2017

   .PREV DISCUSSION... /ISSUED 1126 AM CDT Thu Jul 20 2017/

   ...Portions of the Midwest to the Ohio Valley region to the
   Northeast...
   The remnants of a persistent convective cluster are crossing central
   Indiana around mid-day, and have shown some signs of
   intensification. Further development of this activity could occur as
   it spreads eastward/southeastward into Ohio and vicinity, amid a
   moderately unstable air mass with 25-40 kt of effective shear. A
   separate cluster of convection will likely spread across parts of
   southern NY, northern PA, and vicinity amid stronger deep shear and
   in a regime of warm/moist advection. Additional convective
   development will be likely along a cold front trailing southwestward
   into Ohio, with activity subsequently spreading eastward with time.
   Widely scattered damaging wind gusts, and perhaps marginally severe
   hail, may accompany all of the aforementioned activity. However, the
   convective evolution is overall uncertain, as ascent is primarily
   being driven by convectively-influenced mass fields at the meso-beta
   scale.

   To the west of the Indiana/Ohio area of convection across parts of
   the Midwest, isolated to widely scattered thunderstorm development
   is anticipated along a trailing outflow boundary and surface 
   front -- enhanced at the intersection of these boundaries (IA
   vicinity). Strong buoyancy may support isolated severe hail/wind,
   though weak deep shear will tend to limit convective organization.

   Also, some isolated severe hail/wind risk may affect parts of New
   England -- aided by long mid/high-level hodographs, though weak
   buoyancy will mitigate the overall severe risk.

   ...Portions of the northern High Plains...
   Severe thunderstorm development is anticipated this afternoon along
   a sharpening dryline in eastern MT vicinity with activity spreading
   into portions of the Dakotas. This will occur as a mid-level speed
   maximum crests an antecedent ridge aloft, aiding in its
   de-amplification. Lee cyclogenesis will contribute to a vertically
   veering wind profile east of the dryline, with 40-50 kt of effective
   shear supporting supercell structures. Severe hail/wind may occur,
   and -- though uncertain at this time -- some significant severe hail
   risk could evolve. While convective cloud bases will tend to be
   quite high, a tornado cannot be ruled out given sufficiently curved
   low-level hodographs -- particularly in proximity to a developing
   warm front from northeast MT into northwest and west-central ND.
   Convection may continue into the overnight hours with some hail risk
   as it moves across parts of the Dakotas and vicinity, though the
   overall severe risk will be lessening.

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Data courtesy the Storm Prediction Center
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