SPC AC 101953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Fri Jul 10 2020

   Valid 102000Z - 111200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION
   OF THE NORTHERN AND CENTRAL PLAINS...

   ...SUMMARY...
   Multiple clusters of severe storms are expected across parts of the
   northern and central Great Plains this afternoon into tonight, with
   the potential to produce very large hail, damaging winds, and a few
   tornadoes.

   ...Northern through central Plains region...

   Primary change to previous outlook has been to expand SLGT risk
   farther southeast into northeast KS and northwest MO for the
   likelihood of whatever MCS that develops tonight continuing into
   this region. Otherwise, forecast appears mostly on track with storms
   in the process of developing along a progressive cold front over the
   northern Plains, and additional activity is likely farther south
   near the dryline/outflow boundary intersection across southwest SD
   or northwest NE as well as the higher terrain of northeast WY.

   ...Mid Atlantic through southern New England Coastal areas...

   Primary change has been to downgrade to MRGL in this region.
   Substantial low-level destabilization has been limited due to
   widespread clouds, though a clear slot is spreading northward
   suggesting some heating may occur later this afternoon over the
   southern New England coastal area. However, very warm temperatures
   aloft with -4 C at 500 mb will continue to limit instability. Though
   a brief tornado or two will remain possible, overall threat appears
   marginal.

   ..Dial.. 07/10/2020

   .PREV DISCUSSION... /ISSUED 1116 AM CDT Fri Jul 10 2020/

   ...Synopsis...
   A fairly significant short wave trough within the mid-latitude
   westerlies, now crossing the northern U.S. Rockies, appears likely
   to turn eastward across the remainder of Montana through late this
   afternoon.  It is forecast to then dig southeastward across the
   Dakotas tonight, as upstream ridging builds to the east of the
   Canadian Rockies, ahead of a vigorous, progressive short wave trough
   and embedded mid-level low approaching the British Columbia coast. 
   At the same time, models indicate that subtropical ridging, centered
   along a major axis from southern California into the southern plains
   Red River Valley, will become increasing prominent.  

   To the east of this ridging, the axis of weak large-scale troughing
   is forecast to continue shifting east of the Mississippi Valley
   through the vicinity of the Appalachians.  In association with a
   northward progressing impulse on the eastern periphery of this
   cyclonic flow field, the center of Tropical Storm Fay is forecast to
   migrate northward along the New Jersey coast into the Hudson Valley
   before weakening later tonight into Saturday.

   ...Great Plains...
   Convection allowing guidance has been quite varied concerning the
   potential evolution of convection for this period, and overall
   predictability appears relatively low.  

   Very warm and potentially capping air in the 700-500 mb layer
   (including 500 mb temps to around -4 C) is forecast to continue
   advecting east of the Rockies, particularly across Nebraska, Kansas
   and Oklahoma today into tonight.  As this occurs the most probable
   focus for thunderstorm initiation seems to be deepening surface
   troughing across parts of the western Dakotas into eastern Montana,
   with the approach of the mid-level short wave impulse by mid to late
   afternoon.

   Otherwise, the thermal gradient around 700 mb, on the leading edge
   of the warmer and more strongly capping elevated mixed-layer air may
   remain a focus for continuing convective development in association
   with warm advection.  This activity could eventually become rooted
   in a destabilizing boundary layer, as the mid-level gradient shifts
   eastward across the mid Missouri Valley today through tonight.  

   Regardless, the development of sizable CAPE, beneath strengthening
   west/northwesterly mid-level flow is expected to contribute to an
   environment potentially conducive to isolated supercells, and
   upscale growing, organizing clusters late this afternoon through
   early evening.  Some of this activity may pose a risk for large hail
   and isolated tornadoes initially, before strong wind gusts become
   the more prominent hazard this evening.

   Farther south, warm advection above a significant convectively
   generated surface cold pool has been maintaining the southward
   development of an ongoing cluster of storms into central Oklahoma
   (refer to SPC Mesoscale Discussion 1169).  However, general trends
   in intensity have been downward, and activity is expected to
   continue diminishing in the presence of weak mid/upper support and
   increasing inhibition through early afternoon.

   ...Lower Great Lakes/middle Ohio Valley vicinity...
   Forcing for ascent associated with a short wave trough overspreading
   the region likely will continue to support increasing thunderstorm
   development through this afternoon in the presence of moderately
   large mixed-layer CAPE on the order of 2000+ J/kg.  Deep-layer wind
   fields and shear around the developing convection likely will remain
   weak, but merging storms and some upscale growth may be accompanied
   by localized potentially damaging wind gusts, associated with heavy
   precipitation loading in downdrafts and along evolving surface cold
   pools.

   ...Northern Mid Atlantic/southern New England...
   The landfall of Tropical Storm Fay will pose at least a conditional
   risk for a couple of tornadoes or damaging wind gusts with
   convection spreading inland ahead of the center, mainly this
   afternoon into this evening.

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