NWS Storm Prediction Center - Day 1 Convective Outlook

Sep 23, 2019 1300 UTC Day 1 Convective Outlook
Updated: Mon Sep 23 12:49:13 UTC 2019
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Overview Overview

 Forecast Discussion - Day 1 Convective Outlook


   SPC AC 231249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Mon Sep 23 2019

   Valid 231300Z - 241200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THROUGH LATE
   EVENING ACROSS A SMALL PART OF SOUTHWEST AZ...

   ...SUMMARY...
   Thunderstorms with very large hail and damaging winds will be
   possible today across parts of southwest and central Arizona. Storms
   with a few strong wind gusts will be possible in parts of the
   Northeast.

   ...AZ through tonight...
   A transition season severe weather episode is expected today into
   tonight across AZ.  A midlevel shortwave trough over northwest NV
   this morning will dig south-southeastward toward the lower CO River
   valley and evolve into a closed low by Tuesday morning.  Low-level
   mass response to the approaching midlevel trough will result in
   northward transport of moisture from southwest/south central AZ to
   the rim through the day.  Some slightly elevated convection is
   ongoing along the north edge of the returning moisture, and this
   convection may persist through the morning and pose a marginal hail
   threat.  The background environment will become favorable for
   additional thunderstorms through the day as ascent and midlevel
   cooling develop southeastward into AZ.  Daytime heating across
   southwest AZ will support surface-based thunderstorm development by
   late morning (17-18z) along a pseudo-dryline near the CO River, and
   storms will spread eastward near the rim through the afternoon and
   into this evening.

   Surface dewpoints have increased to 70-76 F across southwest AZ with
   a moisture surge from the Gulf of CA.  After accounting for some
   daytime heating/mixing, boundary-layer dewpoints of 65-70 F  beneath
   midlevel lapse rates near 7 C/km will result in MLCAPE of 1500-2500
   J/kg and minimal convective inhibition.  Effective bulk shear of
   40-50 kt and relatively long/straight hodographs will favor
   splitting supercells capable of producing isolated very large hail
   near 2 inches in diameter, and there is some potential for
   clustering of hail events west of Phoenix this afternoon.  The
   stronger supercells, as well as eventual upscale growth into small
   clusters, will pose a threat for damaging winds.  Otherwise, an
   isolated tornado cannot be ruled out, but this threat will be
   secondary given relatively weak low-level flow/shear. Convection
   should persist into the overnight hours, with at least a low-end
   threat for hail/wind continuing.  

   ...NY and vicinity this afternoon/evening...
   A midlevel shortwave trough and associated surface cold front will
   move eastward across NY by late afternoon into early tonight. 
   Clouds and poor lapse rates in the pre-frontal warm sector will
   limit buoyancy, with MLCAPE likely to remain near 500 J/kg.  Still,
   an increase in midlevel flow with the approach of the shortwave
   trough could support isolated strong gusts with downward momentum
   transfer in convection near the front this afternoon/evening.

   ..Thompson/Leitman.. 09/23/2019

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z



Data courtesy the Storm Prediction Center
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