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    Day 2 Outlook >
Jul 21, 2018 1300 UTC Day 1 Convective Outlook
Updated: Sat Jul 21 12:59:02 UTC 2018 (Print Version | 20180721 1300Z Day 1 shapefile | 20180721 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 211259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Sat Jul 21 2018

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF BOTH
   THE SOUTHEAST AND THE NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms are expected across parts of the southeastern
   U.S. and the northern High Plains.

   ...Synopsis...
   A seasonally high-amplitude pattern characterizes the middle/upper
   troposphere over the CONUS.  A longstanding, strong anticyclone now
   centered over TX Panhandle is forecast to retrograde slowly across
   northeastern NM through the period.  This will occur as a large,
   closed, occluded, essentially stacked cyclone -- now centered over
   the Michiana area -- drifts southeastward obliquely across the IN/OH
   border.  Associated cyclonic flow will cover most of the nation
   between the Mississippi River and Atlantic coast.  Within that
   cyclonic flow, a negatively tilted shortwave trough is noted over
   the Carolinas and adjoining Atlantic.  This feature should eject
   northward over the Tidewater/Delmarva region through 00Z, then wrap
   north-northwestward over the inland Mid-Atlantic region overnight. 
   A series of convectively induced/enhanced, lower-amplitude vorticity
   lobes also will orbit upstream areas in the southern semicircle of
   the synoptic cyclone.

   Elsewhere, a strong northern-stream shortwave trough is apparent in
   moisture-channel imagery extending southwestward from a compact
   cyclone over western AB, across Vancouver Island to the Pacific west
   of WA/OR.  The cyclone is progged to devolve to an open wave through
   the period.  Nonetheless, a strong shortwave trough should reach
   southwestern SK and north-central/southwestern MT by 12Z.

   At the surface, an occluded low was analyzed at 11Z near SBN, with
   occluded front to northern KY, and cold front across western TN into
   some outflow from overnight convection over AR.  The frontal zone
   was quasistationary over OK but should sag southward slowly there
   today, while moving southeastward across TN/AL.  A cold front
   associated with the northern-stream perturbation should cross much
   of MT through the period, with a frontal-wave low developing tonight
   over eastern MT.  Warm frontogenesis already is evident across
   southeastern MT into western NE, and this should shift northward
   across the northern High Plains through the period.

   ...Southeast...
   Two primary/ongoing convection/precip areas related to prior
   overnight MCS action will impact the severe potential today.  A
   decayed western MCS that had produced severe/damaging gusts over
   portions of AR/MS has left an area of precip, embedded/subsevere
   thunderstorms and outflow that continues to move southeastward over
   portions of MS into extreme western AL.  Another, still well-defined
   arc of thunderstorms is moving southeastward across northern/central
   GA and extreme eastern AL, with a trailing  outflow boundary still
   moving southwestward over parts of northern/central AL (but likely
   to decelerate/stall with time).  See SPC watch 197 and related
   mesoscale discussions for near-term guidance on the eastern MCS. 
   Should this complex survive an area of relatively strong CINH this
   morning over GA, it may re-strengthen over southern GA and/or parts
   of SC this afternoon, while impinging on a diurnally heated, very
   moist, destabilizing air mass.  Damaging gusts and isolated hail
   would be the main concerns.

   Otherwise, scattered thunderstorms are expected to develop again
   this afternoon along and near the boundary left by the eastern
   complex, offering a risk of damaging gusts and large hail, and
   perhaps a tornado or two.  This boundary, whose specific location
   several hours from now still is uncertain, but generally
   northwest/southeast across parts of GA/AL, will demarcate the
   diabatically destabilizing eastern rim of some very richly
   moist/high-buoyancy warm-sector air.  The rate/intensity of heating
   may be hampered somewhat by cloud debris left over from the western
   complex, particularly with northward extent and closer to the
   synoptic front.  Furthermore, the forecast westerly surface
   component west of that outflow boundary appears likely to advect
   convectively processed boundary-layer air eastward into areas now
   covered by a more favorable air mass ahead of the front.  As such,
   convective development/coverage is more uncertain over parts of
   northern AL and MS, and the unmodified warm-sector plume should
   narrow from west to east with time.

   In that more-unperturbed warm-sector slot nearer to the boundary,
   and southward to the Gulf Coast, expect low/mid-70s F surface dew
   points, 16-19 g/kg mean mixing ratio and PW 1.75-2 inches (locally
   higher).  Forecast soundings suggest a corridor of 2500-3500 J/kg
   MLCAPE near the boundary -- increasing to around 4000 J/kg near the
   FL Panhandle coast, and decreasing gradually eastward/northeastward
   around the eastern outflow pool to the Carolinas where lapse rates
   aloft will be weaker.  Strongest vertical shear also should be near
   the boundary where relatively backed flow and west to northwest
   winds aloft contribute to 35-45 kt effective-shear magnitudes. 
   Meanwhile shear will lessen westward into the westerly surface
   winds, and northward into a zone of weaker, difluent mid/upper winds
   over the central/southern Appalachians, to the left of the jet
   stream aloft.

   ...Northern High Plains...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon in a meridional corridor from northeastern WY and
   near the Black Hills northward, ahead of the surface cold front,
   near a surface trough/confluence line.  Activity will pose a risk of
   damaging gusts and sporadic large hail before moving into
   lower-theta-e air over the Dakotas this evening and weakening.  A
   narrow but well-defined corridor of rich low-level moisture over the
   central High Plains -- relatively unperturbed by assorted MCS
   activity the last few days farther southeast -- was evident in
   overnight surface analyses and sampled well by 12Z LBF RAOB.  This
   moist plume, somewhat diluted by lateral and vertical mixing, should
   advect northwestward in support of convective potential.  Steep
   low/middle-level lapse rates and well-mixed subcloud layers are
   forecast, with preconvective MLCAPE reaching 1500-2500 J/kg in a
   very narrow corridor, juxtaposed with 40-50 kt effective-shear
   magnitudes.

   ...Coastal Mid-Atlantic/New England late...
   As the mid/upper shortwave trough ejects northward, closer to this
   region, the surface low should deepen and move generally up the
   coast to NJ by late evening, then inland while weakening overnight. 
   As this occurs, both low-level moisture/theta-e and low-level
   vertical shear will increase over the outlook area, resulting in
   weak buoyancy tonight amidst lapse rates only slightly greater than
   moist adiabatic in low/middle levels.  The latter factor will limit
   storm intensity/coverage, despite favorable low-level hodographs.  A
   brief tornado cannot be ruled out, though the potential appears very
   marginal and conditional at this time.

   ..Edwards/Peters.. 07/21/2018

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

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

        
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