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Jul 20, 2018 0600 UTC Day 2 Convective Outlook
Updated: Fri Jul 20 05:39:02 UTC 2018 (Print Version | 20180720 0600Z Day 2 shapefile | 20180720 0600Z Day 2 KML)
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 Forecast Discussion

   SPC AC 200539

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1239 AM CDT Fri Jul 20 2018

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY AFTERNOON
   AND EVENING ACROSS PARTS OF THE TENNESSEE VALLEY THROUGH SOUTHERN
   APPALACHIANS AND ADJACENT PIEDMONT...

   ...SUMMARY...
   Severe thunderstorms are possible Saturday, mainly during the
   afternoon and evening, across parts of the Tennessee Valley through
   the southern Appalachians and adjacent portions of the Piedmont.

   ...Synopsis...
   Generally south of the stronger mid-latitude westerlies, and to the
   east of a prominent subtropical high centered over the southern high
   Plains, models indicate that large-scale troughing will continue to
   evolve across areas east of the Mississippi Valley through the
   Atlantic Seaboard.  A closed low becoming increasingly detached from
   the westerlies appears likely to remain the most prominent embedded
   feature, gradually digging with an associated surface low south of
   the Great Lakes into the Ohio Valley by 12Z Sunday.  A more modest
   impulse may progress northward through the eastern periphery of the
   larger-scale troughing, accompanied by another more compact surface
   cyclone.  While it appears that this latter feature may migrate
   mostly offshore to the east of the mid Atlantic Coast, the spread
   among model output remains sizable.

   Upstream, to the north of the subtropical high, another significant
   short wave trough and embedded closed low are forecast to accelerate
   east/northeast of the Canadian/northern U.S. Rockies through the
   Canadian Prairies.  The main surface low associated with this
   feature appears likely to develop well to the north of the
   international border, but a weak low may develop within surface
   troughing ahead of the associated southeastward advancing surface
   front, across the northern high Plains.

   While perhaps highest moisture content will remain focused near/east
   of the wave off the Mid Atlantic coast, seasonably high moisture
   content appears likely to persist at least one more day across the
   eastern Gulf into south Atlantic Coast states, before at least some
   drying associated with the upper trough commences.  Otherwise, at
   least a pocket of relatively moist air may persist in closer
   proximity to the digging closed low, while more modest moisture
   lingers or develops northward through the northern Plains and across
   much of the Southwest.  This is all expected to contribute to
   seasonable diurnal destabilization and areas of scattered
   thunderstorm activity.  Some of this may pose a risk for severe
   weather.

   ...Southeast...
   Considerable uncertainty lingers, particularly concerning
   sub-synoptic details which could impact convective potential for
   Saturday.  There is a signal within at least some model output that
   remnant convection and/or convective outflow from Friday night may
   be in the process of spreading across parts of the central/eastern
   Gulf states at 12Z Saturday.  It is possible that this activity
   could re-intensify and pose a severe wind risk as it advances toward
   Alabama/Florida Panhandle coastal areas, where CAPE may become
   moderate to large by midday.  

   If this initial development does indeed occur, it remains unclear
   how this may impact the environment to the north.  However, guidance
   is generally suggestive that the lingering moisture, daytime surface
   heating, and weak to modest mid-level cooling probably will
   contribute to moderately large CAPE across at least parts of the
   Tennessee Valley, southern Appalachians and adjacent Piedmont by
   late afternoon, if not before.  It appears that this may be
   favorably timed with forcing for ascent accompanying a 30-50 kt
   mid-level jet streak, which would provide sufficient vertical shear
   for organizing storm clusters.  Isolated supercell structures might
   be possible initially, perhaps accompanied by a risk for hail and a
   tornado or two, but the risk for potentially damaging wind gusts
   appears the primary potential hazard at the present time.

   ...Northern Plains...
   With stronger forcing for ascent expected to generally pass well to
   the north of the international border Saturday, late afternoon and
   evening thunderstorm development near the pre-frontal surface trough
   may remain fairly isolated in nature.  However, given the
   development of moderate CAPE in the presence of steep lapse rates
   and shear associated with the westerlies, a couple of supercells
   appear possible.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:  <2%     - None
   Wind:     15%     - Slight
   Hail:     15%     - Slight

   ..Kerr.. 07/20/2018

   CLICK TO GET WUUS02 PTSDY2 PRODUCT

   NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z

        
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