LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Sun Aug 13 2023
Valid 131300Z - 141200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER THE OZARK
PLATEAU REGION...
...SUMMARY...
Severe thunderstorm gusts, large hail and a tornado threat are
expected today in and near the Ozarks.
...Synopsis...
Though strong mid/upper-level ridging will persist over the Gulf
Coast States into central/west TX, northern-stream amplification
will intensify gradient flow aloft to its north. Much of that
process will be related to a strong northern-stream shortwave trough
-- apparent in moisture-channel imagery from a vorticity max over
northwestern ND southwestward to northwestern WY, and northeastward
across southeasternmost portions of SK. This feature will
strengthen further and dig southeastward across the northern
Plains/Upper Midwest regions through the period, reaching an axis
near TVF-PIR-CYS by 00Z, and DLH-OMA-GLD by 12Z tomorrow. Well
eastward in the downstream westerlies, a weaker shortwave trough now
over southeasternmost ON and central NY should reach from
northwestern ME to NH by 00Z, exiting the CONUS to Atlantic Canada
within about 6 hours thereafter.
The 11Z surface analysis showed a low over southern QC related to
the lead shortwave trough, with the CONUS part of its trailing cold
front extending across central NY, western PA, and extreme southern
OH, becoming quasistationary over southern parts of IN/IL, and a
slow-moving warm front over northeastern MO to northwestern IA. The
front should move eastward across all of New England before 06Z.
Meanwhile the Plains low should move/redevelop to near FSD by 00Z,
With a secondary low possible over northwestern MO or northeastern
KS (specific position strongly influenced by intervening convective
processes). From there, the front should extend across south-
central KS, northwestern OK, the TX Panhandle, and northeastern NM.
By 12Z, either an eastward-shifted version of the secondary 00Z low,
or a new frontal-wave low, should lie along the boundary over the
western/southern IL region. A warm to stationary front should
extend from there toward WV. The cold front should extend from
there across northern AR, southeastern OK, north-central/northwest
TX, and south-central NM. The warm to stationary front still should
extend from the low roughly eastward into WV.
...Ozarks to south-central Plains, lower Missouri and Tennessee
Valleys...
A complex convective regime still is apparent across this region,
with multiple fronts and warm-sector convective boundaries rendering
considerable uncertainty -- especially around the outlook's fringes.
The highest-confidence area still appears to be over and near the
Ozark Plateau, where the "enhanced" area is maintained based on the
most-favorable expected total-severe parameter space (hail, wind and
tornado potential) late this afternoon into evening.
In that corridor, and perhaps somewhat westward through KS and
eastward into IL, the greatest potential exists for both warm
advection and diabatic heating to favorably destabilize a richly
moist air mass behind the morning activity now over parts of
northern AR and southern MO. This should occur as winds aloft
strengthen in the mass response to the approaching shortwave trough,
ahead of the surface cold front and southwest of the warm front.
Forecast soundings suggest considerable veering of winds with
height, leading to enlarged, supercell-favoring hodographs and low-
level wind profiles (e.g., effective SRH potentially into the 250-
400 J/kg range). Effective-shear magnitudes will be more marginal
(30-40 kt), owing to the late approach of the shortwave trough.
This suggests that convective mode could get clustered and messy
rather quickly after initiation, but with embedded supercells and
LEWP/bowing mesocirculations contributing to all-hazards severe
threats.
The warm-sector parameter space away from that core corridor will be
characterized by
1. More-veered prefrontal surface winds westward toward the front;
2. Weaker instability northward, getting into greater influence
from a mass of precip and convection initially over much of the
lower Missouri Valley;
3. Cloud cover and weaker deep-layer lapse rates eastward toward
the synoptic warm/stationary front and the lower Ohio Valley region.
Still, discrete to clustered convection with wind and hail potential
may develop over some of those areas, especially along fronts and
outflow/differential/heating boundaries.
...Central/Eastern New England...
Isolated to widely scattered thunderstorms are expected to form
during the midday hours over western/central New England, along/
ahead of the surface cold front. The front, and slightly precursory
field of large-scale ascent preceding the shortwave trough, should
impinge on a destabilizing boundary layer with minimal MLCINH, and
enough moisture to contribute to peak/preconvective MLCAPE in the
1500-2000 J/kg range. Deep-layer flow will be nearly unidirectional
in/above the warm sector, with weak near-surface winds yielding
small hodographs. Still, mid/upper-level flow will be strong enough
ahead of the trough to yield effective-shear magnitudes in the 30-
35-kt range, supporting some organized multicells, and sporadic
isolated pulse-severe potential in the form of marginal hail and/or
gusts.
..Edwards/Dean.. 08/13/2023
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