SPC AC 011254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Sun Aug 01 2021

   Valid 011300Z - 021200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER EASTERN PARTS
   OF VIRGINIA AND THE CAROLINAS...AND OVER PARTS OF CENTRAL/WESTERN
   NEW YORK AND CENTRAL PENNSYLVANIA...

   ...SUMMARY...
   Strong to severe storms with damaging wind and perhaps a tornado may
   impact southern portions of the Mid Atlantic today. A few strong to
   severe storms are also possible over the Northeast and the Gulf
   Coast States.

   ...Synopsis...
   In mid/upper levels, the large-scale pattern will continue to be
   dominated by:
   1. High-amplitude ridging over western North America, near which 
   weak shortwave troughs now over western ID and southern NV will move
   slowly northward through the period;
   2. Cyclonic flow over most of the eastern CONUS, northeastward to a
   large, complex cyclone covering much of northern QC/Labrador and
   adjoining waters.  A strong shortwave trough -- now apparent in
   moisture-channel imagery over Lake Huron, eastern Lower MI and
   northwestern OH -- is forecast to pivot eastward to the lower Great
   Lakes and upper Ohio Valley by 00Z, then to western New England and
   southeastern NY/NJ by 12Z tomorrow.

   The surface cold front associated with the Great Lakes perturbation
   was drawn at 11Z from a low over Lake Ontario through extreme
   southern ON, the Michiana area, to north-central IL and southern IA.
   The low is expected to move generally east-northeastward across the
   Adirondacks today and northern New England overnight.  The front
   should reach eastern PA, central WV, and the lower Ohio Valley by
   00Z.  Thereafter, the front will become more diffuse in a broader
   field of northwesterly low-level winds and cold advection occurring
   behind a deepening cyclone that will be moving offshore from the
   Mid-Atlantic.  The associated southern frontal zone was drawn across
   northern/western NC, northern AL/MS, southeastern OK, northwest TX,
   and southeastern NM.  This boundary -- its baroclinicity reinforced
   in places by outflow -- should move slowly southward across the
   Carolinas, GA, AL, MS, AR/northern LA, southern OK, and north-
   central, central and southwest TX through the period.

   ...Carolinas/Southeast VA region...
   Ongoing convection across parts of northern NC and extreme south-
   central/southeastern VA may pose a local, marginal threat across all
   severe modes this morning, limited by lack of greater low-level
   lapse rates/buoyancy.  Additional convection may develop southward
   into more of NC as well, before the more-substantial boundary-layer
   destabilization occurs from midday into afternoon.  By then,
   scattered thunderstorms are expected near the front, a prefrontal
   surface trough, and perhaps sea-breeze boundaries.  Other convection
   may develop behind the morning activity near the Blue Ridge of VA
   and move southeastward into higher theta-e as that air mass recovers
   behind the morning activity.  Mostly multicells are possible, though
   a few supercells may also occur.  Damaging gusts, a slight tornado
   threat, and isolated/marginally severe hail are expected.

   Rich low-level moisture, with surface dew points in the upper 60s to
   mid 70s F, and diurnal heating will offset modest mid/upper-level
   lapse rates enough to yield a corridor of MLCAPE in the 1500-2500
   J/kg range roughly corresponding to the 15%-wind/"slight risk" area.
    Vertical shear (both low-level and deep-tropospheric) should be
   relatively maximized near the front, where surface flow is backed,
   and weaken with southward extent under progressively less mid/upper
   flow.  Effective-shear magnitudes of 35-45 kt are possible. 
   Consolidations/merger into a band or clusters of convection should
   occur with time as activity nears the coast, with the most intense
   embedded cells still capable of strong-severe gusts and perhaps a
   tornado.

   ...Northeast...
   Scattered to locally numerous thunderstorms are expected to develop
   from late morning through the afternoon along/ahead of the cold
   front -- first in northern/western portions of the region, then
   spreading/expanding eastward.  Sporadic damaging winds and isolated
   severe gusts/hail are possible.  While magnitude of damaging gusts
   should be mostly subsevere to marginally severe, given the lack of
   both greater moisture/buoyancy and low-level flow, the expected
   dense coverage of convection may result in enough events to justify
   a 15%/categorical upgrade to wind probabilities at this time.

   Convection will be supported by a combination of weak MLCINH,
   diurnal heating, modest but adequate low-level moisture, boundary-
   layer convergence near the front and low, and large-scale lift aloft
   ahead of the shortwave trough.  Moist advection should increase
   surface dew points over most of this area to the upper 50s and low
   60s F.  This will contribute to MLCAPE mainly in the 300-800 J/kg
   range.  Strong mid/upper (anvil-level) winds will aid in
   organization, though low-level flow will remain too weak to enlarge
   hodographs appreciably.  Well-mixed sub cloud layers will aid in
   localized strong-severe gust potential, until evening stabilization
   of the boundary layer removes enough low-level CAPE/lapse rates to
   weaken convection substantially with proximity to the Mid-Atlantic
   Coast and western New England.

   ...Gulf Coast States to GA...
   Isolated to scattered thunderstorms - sometimes in clusters --
   should develop near the surface front and move roughly southward to
   southeastward across the outlook area.  Damaging to isolated severe
   gusts are possible.  A combination of very rich boundary-layer
   moisture (surface dew points commonly in the 70s F, strong diurnal
   heating, and a deep troposphere will offset modest lapse rates aloft
   enough to generate favorable buoyancy, with peak afternoon MLCAPE in
   the 2000-3500 J/kg range across this corridor.  Deep-layer flow and
   vertical shear will be weak, however, with multicell organization
   and strong-severe pulse downdrafts as the main concerns.  Some
   aggregation and upscale growth of cold pools may occur, further
   concentrating potential for strong-severe gusts, but such processes
   will be strongly dependent on meso-alpha to storm-scale processes
   not reliably predictable at this time.

   ..Edwards/Leitman.. 08/01/2021

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