SPC AC 271423

   Day 1 Convective Outlook RESENT 1
   NWS Storm Prediction Center Norman OK
   0923 AM CDT Thu Aug 27 2020

   Valid 271300Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SEPARATELY OVER
   PARTS OF THE NORTHEASTERN U.S. AND NORTHERN PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   LOWER MISSISSIPPI VALLEY REGION AND MID-SOUTH FOR HURRICANE LAURA'S
   TORNADOES...AND ALSO...ACROSS HE GREAT LAKES CONNECTING THE TWO
   ENHANCED RISKS...

   ...SUMMARY...
   Damaging winds and large hail are expected across parts of the
   Northeast, Great Lakes and northern Plains, with a few tornadoes
   possible.  The tornado threat continues today and tonight with
   Hurricane Laura, mainly in its eastern half.

   ...Synopsis...
   In mid/upper levels, a belt of enhanced westerlies will be
   maintained astride the Canadian border from the Pacific Northwest to
   the upper Great Lakes.  Several embedded, mainly low-amplitude
   perturbations are evident in moisture-channel imagery from eastern
   MT to the NV/OR line; these will eject across the north-central
   Rockies and northern Plains through the period.  The main flow belt
   will become northwesterly across the northeastern CONUS, in response
   to a persistent, deep cyclone forecast to pivot eastward across
   extreme eastern QC, the Gulf of St. Lawrence, and Newfoundland/
   Labrador.  An embedded shortwave trough -- initially over southern
   ON -- will move southeastward over portions of NY and New England
   today. 

   Elsewhere, a mid/upper trough with weak/embedded low, that has
   influenced the recurvature of Hurricane Laura, will persist across
   the southern plains for much of the period, before its primary
   vorticity lobe weakens and pivots southward around the midlevel
   circulation of Laura.

   At the surface, 11Z analysis showed a wavy warm front from offshore
   NJ across eastern PA to western NY and a low near Georgian Bay.  The
   front then extended westward as a cold to stationary front, to
   another low over northwestern WI, then a weak cold front
   southwestward across southwestern NE.  The Plains segment of this
   front should weaken, with an area of low pressure consolidating over
   west-central/southwestern SD.  The Lake Huron-area low should
   move/redevelop east-southeastward across NY and along the
   slow-moving to quasistationary surface baroclinic zone today.

   ...Northeastern CONUS...
   Widely scattered to scattered thunderstorms are forecast to develop
   episodically near the low and frontal zone from midday through
   afternoon, where low-level lift will be relatively maximized amidst
   weak CINH.  Activity should move east-southeastward to southeastward
   across the outlook area as the inflow/boundary layer to the south
   continues to destabilize diurnally.  Some supercells are possible,
   and convection should pose a threat for all severe types.

   Diurnal heating and 60s F surface dew points along and south of the
   boundary will offset modest midlevel lapse rates enough to yield
   MLCAPE in the 1000-1500 J/kg range -- locally/briefly above 2000
   J/kg.  Elongated, somewhat curved low-level hodographs are expected
   in the warm sector before and as the low passes a given longitude,
   before the surface flow substantially veers.  Effective-shear
   magnitudes of 50-60 kt may be common, in support of supercells and
   small bows.  Some aggregation of early convection into fast-moving
   clusters with enhanced wind-damage potential is possible.  

   ...Northern Plains...
   Widely scattered thunderstorms are ongoing across portions of
   western SD, in an elevated low-level warm-advection regime with
   steep midlevel lapse rates.  This activity may pose a
   marginal/isolated severe-hail threat while moving eastward this
   morning, and should exit the area in time for sufficient diurnal
   heating to support the expected afternoon convection in and near the
   "Enhanced" outlook.  

   Widely scattered to scattered thunderstorms are forecast to develop
   by late afternoon over the northern High Plains, mainly near the
   frontal zone and north of the low along an inverted trough.  Progs
   reasonably favor the area north of the Black Hills toward eastern MT
   and southwestern ND for initial strong-severe development (including
   supercell potential).  Damaging gusts and large hail -- some
   potentially significant/2+ inches in diameter -- will be possible
   while convection is relatively discrete.  The thermodynamic profile
   will be characterized by deeply mixed subcloud layers supporting
   maintenance of severe hail/downdrafts to the surface, amidst
   strongly veering winds with height.  Forecast soundings depict
   generally 45-55-kt effective-shear magnitudes, with 1500-2500 J/kg
   peak preconvective MLCAPE. 

   Afternoon/early-evening convection should coalesce upscale into one
   or two predominant, forward-propagating, wind-dominant clusters/MCSs
   across central/northern SD and perhaps southern ND.  The southern
   area eventually should become the most favored for persistence due
   to greater access to favorably high-theta-e inflow, and the
   resulting complex may reach the southern WI area overnight. 

   ...Hurricane Laura...
   For near-term tornado-threat coverage, see tornado watch 454 and
   related mesoscale discussions.  The area of favorable low-level
   shear will spread northward in step with the northeastern and
   southeastern quadrants of the hurricane, which also will be largely
   downshear with respect to the ambient deep-tropospheric (e.g.,
   850-200-mb) shear vectors influencing the precip/buoyancy
   distribution and asymmetries in the favorable kinematic fields
   around the system.  Coverage of supercells is uncertain in the area
   of strongest low-level (e.g., 0-1-km) shear from northern LA across
   central AR and into parts of the mid/upper Delta/Mid-South regions,
   given the extensive cloud/precip canopy and related impediments to
   substantial diurnal/diabatic destabilization in and near the eastern
   core region.  As such, the field of unconditional probabilities is
   maintained at 5% level for now, though any mesoscale areas of
   sustained heating/destabilization may focus a greater threat with
   any convection involved.  Slight eastward expansion has been done
   over much of MS to accommodate the outer rim of that favorable
   overlap.

   Very peripheral convective bands and cells are expected to develop
   this afternoon into tonight across parts of southern LA/MS, in a
   broad confluence/convergence zone extending outward/southeastward
   from Laura's core convective/precip region over AR.  The up-ramp of
   buoyancy throughout the day on those southeastern fringes will
   correspond with shrinking with time of both low-level hodographs and
   boundary-layer shear vectors, as the cyclone pulls away and
   gradually weakens.  Still, there may be a few hours of temporal
   overlap when low-level shear still is large enough to support
   supercells and an associated tornado threat, before the near-surface
   winds veer/lessen too much, and nocturnal/diabatic cooling sets in. 
   Meanwhile, convective mode, low-level shear and instability each
   will become less favorable with westward extent from roughly a
   north-south line ahead of the center, as well as south and southwest
   of the eye, and probabilities have been adjusted accordingly.

   ..Edwards/Leitman.. 08/27/2020

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