SPC AC 231248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0748 AM CDT Thu Jul 23 2020

   Valid 231300Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   NORTHEAST AND THE NORTHERN GREAT PLAINS...

   ...SUMMARY...
   Severe-thunderstorm potential appears most concentrated today into
   this evening over parts of the Northeast and the northern Great
   Plains.  Damaging wind and at least isolated large hail will be
   possible in both areas.

   ...Synopsis...
   The upper-air pattern over the CONUS will be defined by troughing in
   the West, a progressive ridge crossing the northern Plains and
   reaching the upper Mississippi Valley late in the period, and
   troughing across the Northeast.  The western lobe of a positively
   tilted shortwave trough -- now evident in moisture-channel imagery
   from southern QC to the upper Great lakes -- will reach southeastern
   ON and Lake Ontario by 00Z, then cross New England overnight. 
   Meanwhile, to its south, several convectively aided/induced
   vorticity maxima -- embedded in weaker westerlies over the
   lower/middle Ohio Valley region -- will move eastward over the
   central Appalachians to the Mid-Atlantic and Tidewater regions
   through the period.

   Farther west, a series of vorticity maxima and shortwave troughs
   will eject northeastward out of the mean trough.  The first -- a
   strongly negatively tilted feature extending from southwestern MT
   southeastward across central/southern WY -- will move northeastward
   to eastern MT and western/central SD by 00Z.  The trough then should
   move over southeastern SK and parts of southern MB overnight.  An
   upstream perturbation -- currently exhibiting a small/closed cyclone
   over north-central CA -- will weaken as it moves slowly east-
   southeastward to the southern Sierra overnight. A still-weaker
   vorticity lobe over southeastern OR will eject across the northern
   Rockies.

   At the surface, the 11Z analysis showed a wavy warm to stationary
   front, interspersed with several weak lows, across southern ME,
   northern VT, southernmost ON, northern IN, central IL, and
   southwestern IA, becoming a well-defined warm front northwestward to
   a low between GGW-GDV.  The boundary should pivot northward across
   northern New England today as a warm front, and southward over
   central/western NY.  The northern Plains/Upper Midwest warm front
   should reach the eastern Dakotas by 00Z, then drift east-
   northeastward to western MN before the end of the period.  A
   well-defined lee trough will continue to extend southward from the
   MT low, which should drift northeastward into southeastern SK
   tonight.

   ...Northeastern CONUS...
   Widely scattered to scattered thunderstorms are expected to develop
   and move generally eastward across portions of eastern NY/PA and New
   England today, along and south of the front, with a threat for
   damaging gusts and isolated severe hail.

   The environment will be aided subtly by large-scale lift related
   both to the approaching positively tilted trough and the right-rear
   quadrant of a cyclonically curved upper jet streak.  Rich low-level
   moisture is forecast over most of the outlook area, with surface dew
   points in the 60s to low 70s F.  This, along with patches/episodes
   of stronger diurnal heating in between areas of transiently thicker
   cloud cover, should contribute to weak MLCINH and offset modest
   midlevel lapse rates enough to yield around 1000-1500 J/kg MLCAPE
   over most of the 15%/slight-risk wind area.  25-35 kt effective-
   shear magnitudes are progged, supporting largely multicellular
   organization, with small clusters or lines offering the most
   damaging-wind potential on a local basis in water-loaded downdrafts.

   ...Northern Great Plains...
   Scattered thunderstorms are expected to develop this afternoon over
   eastern MT and portions of the western Dakotas, as strengthening
   large-scale ascent/DCVA impinge upon a favorable low-level mass
   response, with lift related to diurnal heating and convergence near
   the surface trough.  Hail and strong-severe gusts are possible with
   relatively early/discrete cells.  Damaging gusts will become the
   dominant severe threat with time as upscale aggregation occurs and
   resulting cluster(s) move northeastward this evening into tonight.

   Ahead of this activity, backed surface winds will maintain favorable
   storm-relative flow, as well as support moisture/theta-e advection
   beneath steep midlevel lapse rates.  Meanwhile, surface heating
   today will increase low-level lapse rates and yield a well-mixed
   subcloud layer in support of convective-wind potential.  Forecast
   soundings reasonably suggest 2500-3500 J/kg MLCAPE, locally/briefly
   grater, will be attained.  Bulk shear will be modest, with a deep
   layer of 15-20-kt southerly midlevel winds helping to limit
   effective-shear magnitudes to less than 35 kt over much of the area.
   As such, multicellular modes should predominate throughout the
   convective evolution.

   ...Northern Great Basin to western MT...
   Widely scattered to scattered thunderstorms in episodic clusters are
   expected to move northeastward across this region throughout the
   day, offering isolated severe gusts/hail.  This includes early
   development this morning through midday across portions of central
   ID into southwestern MT, ahead of the ejecting vorticity lobe
   mentioned above.  Associated clouds/precip will limit diurnal
   heating of the higher terrain across much of the northern Rockies
   part of the outlook and downshear into central MT, where abundant
   multilevel cloudiness already is apparent in satellite imagery. 
   Still, available low-level moisture and muted diurnal heating will
   support areas of 300-800 J/kg MLCAPE, locally near 1000 J/kg, atop
   sufficiently well-mixed subcloud layers to foster strong/isolated
   severe downdrafts.  Stronger heating but weaker large-scale forcing
   is expected farther southwest into the northern Great Basin, in
   support of isolated/marginal severe potential with afternoon
   convection there.  Lack of substantial vertical shear will be an
   organizational limiting factor area-wide.

   ...Western Gulf Coast...
   Peripheral convection in the circulation envelope of what now is
   T.D. 8 may begin to affect the immediate LA and/or extreme southeast
   TX coastline, near the end of the period (around 12Z tomorrow). 
   This especially would be true if more-aggressive scenarios verify
   with regard to TC intensification and size (wind radii).  No
   unconditional tornado probabilities will be introduced yet to the
   coast for this period, given:
   1.  Current NHC forecasts for both those aspects and translational
   speed, along with
   2.  Morphological uncertainty inherent to such systems this early in
   their evolutionary cycle.
   Refer to NHC advisories for the latest tropical watch/warning and
   track/intensity guidance on this system.

   ..Edwards/Dean.. 07/23/2020

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