SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Wed Aug 07 2024
Valid 071300Z - 081200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CENTRAL GREAT PLAINS AND COASTAL CAROLINAS...
...SUMMARY...
Marginal severe-weather potential is apparent over parts of the
central Great Plains and coastal Carolinas.
...Synopsis...
The persistent western CONUS anticyclone will contract through the
period and shift westward across the southern Great Basin, in
response to both infusions of positive vorticity in its eastern
semicircle, and more strongly, amplification/progression of a
northern-stream synoptic trough over western/central Canada. That
trough -- evident in moisture-channel imagery from a low over
north-central MB west-southwestward across the Canadian Rockies --
will move southward through the period. By 12Z tomorrow, the 500-mb
low should cross the remainder of central/southern MB to near the
eastern ND/MB border. Associated height falls and cyclonic flow
aloft will spread southward over the northern Plains today, to the
central Plains late tonight.
Downstream, the northern-stream flow will flatten to a more-zonal
configuration through the period, across the Upper Great Lakes, the
St. Lawrence Valley and northern New England. This temporarily
maintains a dearth of steering flow near T.S. Debby, with weak
poleward/inland motion forecast by NHC through 12Z tomorrow.
See latest NHC advisories for track/intensity forecasts for Debby,
as well as tropical watches/warnings.
At the surface, 11Z analysis showed a wavy, quasistationary to
slowly moving cold front from southern New England coastal waters
westward over northeastern PA, then southwestward through a low near
HTS, becoming a cold front over central KY to northeastern OK,
through a low near LTS, then quasistationary again northwestward to
a weak low over southeastern CO. The boundary should continue
southward across parts of KY, TN, northern MS and AR, amidst broader
northerlies extending westward from the circulation of T.S. Debby.
A separate cold front -- related to the northern-stream perturbation
over Canada -- was drawn from southeastern MB to near a BIS-RAP-RWL
line. By 00Z, this front should extend from western parts of
northwestern ON southwestward over northern MN, eastern SD, central
NE, and northeastern CO. By 12Z tomorrow, the front should reach
western parts of Lake Superior and WI, then southwestward over
western IA, southeastern NE, southwestern KS, and northeastern NM.
...Central Great Plains...
Widely scattered to scattered thunderstorms are forecast to form
this afternoon and evening -- mainly on the cold front over NE and
southeastern SD, and over higher elevations of southeastern WY and
north-central/northeastern CO behind the front. Activity will pose
a threat for isolated large hail and severe gusts before weakening
overnight.
Once an area of clouds and precip now over southern SD and northern
NE exits the area, afternoon destabilization should reduce
EML-related CINH ahead of the front through the afternoon, with
frontal lift expected to be strong enough to break the cap in a few
spots by late afternoon. Surface dewpoints in the upper 50s to low
60s F (locally higher) will contribute to a prefrontal plume of
1000-1500 J/kg MLCAPE that will narrow with northeastward extent
into lower antecedent theta-e. The southern fringe of falls and a
tightening gradient of heights aloft will extend across parts of
this area from late afternoon onward as well, contributing to
favorable deep shear for some storm organization and isolated
supercell potential -- with effective-shear magnitudes generally
35-45 kt. Stronger veering with height -- but somewhat weaker
mid/upper flow -- will characterize the post-frontal upslope regime,
with elongated, nearly straight hodographs and similar bulk-shear
values. Well-mixed subcloud layers will support gust potential,
which may become concentrated on the mesobeta scale where any
upscale clustering and cold-pool development can occur (most
probable with activity initiated in the higher elevations).
...T.S. Debby -- Coastal Carolinas...
As Debby gradually turns and moves northward to northwestward, only
a slow corresponding shift of the tornado potential will occur.
Inland destabilization is expected to be minimal, owing to extensive
cloud cover and areas of precip north of center, and acting as a
northern/northwestern limiter to the threat. Meanwhile the skeletal
to almost nonexistent core convective structure and weaker shear
will minimize potential closer to center. This leaves a segment of
eastern NC near the sounds and Outer Banks today and tonight, as
well as perhaps a small part of the adjoining SC coastal areas
today, with potential for isolated, mainly short-lived supercells in
the middle/outer bands. While hodographs are not forecast to be
especially large, compared to many TC-tornado events, 150-250 J/kg
effective SRH for northwestward-moving cells has been present in VWP
hodographs and 6-hourly RAOBs, and should continue. This will
overlap favorable buoyancy within roughly 75-100 nm inland of the
beaches in NC. If a relatively concentrated area of sustained,
multiple supercell development or potential appears for a particular
band, a smaller area of greater probabilities may be added in a
subsequent outlook.
..Edwards/Dean.. 08/07/2024
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