SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

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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