ACUS01 KWNS 121250
SWODY1
SPC AC 121248

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Sat Aug 12 2023

Valid 121300Z - 131200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
OHIO ACROSS WESTERN/NORTHERN PENNSYLVANIA AND PORTIONS OF
WESTERN/SOUTHERN NEW YORK...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED AREA...AND ALSO OVER PARTS OF THE TENNESSEE VALLEY AND
SOUTH-CENTRAL GREAT PLAINS...

...SUMMARY...
The most concentrated area of severe-thunderstorm threats today
extends from northeast Ohio across parts of Pennsylvania and into
central New York, with damaging to severe wind expected, along with
large hail and a tornado or two possible.

...Synopsis...
In mid/upper levels, a zonally elongated anticyclone will remain
over much of TX and the western Gulf Coast States, with ridging
eastward offshore from JAX. A cut-off cyclone -- initially centered
just offshore from southern CA -- will meander near its present
position through most of the remainder of the period. In between
those features, a series of low-amplitude shortwaves with embedded
cyclonic vorticity maxima will traverse the southerly/southwesterly
flow aloft, from northwestern MX to the central High Plains, where
they will join the southern part of the prevailing westerlies.

In the CONUS subset of the northern-stream belt, the western portion
will amplify through tomorrow morning. A shortwave trough will dig southeastward from western Canada to parts of SK/MT by 00Z, then
strengthen and move to eastern MT and WY by 12Z tomorrow.
Downstream, another strong shortwave trough was apparent in
moisture-channel imagery over Lake Superior, adjoining parts of
northwestern ON, and south-southeastward over parts of Lower MI.
This perturbation should shift eastward by 00Z, reaching eastern ON,
the Lower Great Lakes, and parts of western NY to PA.

Preceding the Great Lakes shortwave trough, a related, occluded
surface cyclone was analyzed at 11Z over eastern Lake Superior, with
cold front extending from a triple point over Lake Huron
southwestward through southern Lower MI, northern IL, northern MO,
east-central KS, and northeastern NM. By 00Z, the main surface
cyclone should elongate west/east as the occlusion process persists.
The cold front should reach northern OH, south-central IL, and
southwestern MO, becoming quasistationary or perhaps retreating
slightly northward over the area from northern/western OK across the
northern TX Panhandle and Raton Mesa. Outflow boundaries will
precede the front over portions of OK, MO, and the lower Ohio
Valley.

...Northeast...
Scattered thunderstorms should develop later this morning into
afternoon, over the mid/upper Ohio Valley and Lower Great Lakes
regions. This activity should pose an increasing threat for hail,
damaging to severe gusts, and a few tornadoes as it moves eastward
across the outlook area, with the greatest threats in and near the
"enhanced" corridor.

This will occur as large-scale ascent, deep-layer wind profiles and
vertical shear all strengthen ahead of the Great Lakes shortwave
trough, and perhaps ahead of an eastward-moving MCV or convectively
enhanced vorticity lobe from morning activity over IL. These
processes, occurring along and ahead of the surface cold front, may
support multiple rounds of convection. Activity will impinge on a
diurnally destabilizing and favorably moist boundary layer, commonly characterized by surface dewpoints in the upper 50s to upper 60s F.
In addition to heating, moist advection ahead of the convection
should help to:
1. Strengthen buoyancy over areas of PA/NY now containing more
modest theta-e than in the Ohio Valley, and
2. Offset vertical boundary-layer mixing enough to maintain or even
increase preconvective dewpoints farther west.

MLCAPE in the 1500-2000 J/kg range should become common, in a supercell-supporting kinematic parameter space of 40-55-kt
effective-shear magnitudes. Though near-surface flow may be weak,
veering with height and hodograph curvature should be greatest near
the NY/PA border and into western NY, with effective SRH in the
150-300 J/kg range. Analog soundings and a 2-D hail model applied
to forecast soundings each suggest significant-severe hail may
occur, with the risk of a few such reports now appearing large
enough to draw an unconditional area. Otherwise, the wind and
tornado threats largely appear to be similar to those described in
the previous outlook, with minor adjustments made for more-recent
progs of convective trends.

...South-central Plains...
Widely scattered to scattered thunderstorms should develop this
afternoon over higher terrain, from the eastern slopes of the Sangre
de Cristos into the Raton Mesa region, and perhaps over intensely
heated Plains areas of the northwestern TX Panhandle and
northeastern NM. Activity then should move eastward and coalesce
into one or more complexes, with severe wind the main concern, and
isolated large hail possible. Development and initial growth should
occur as these processes overlap:
1. Moisture/theta-e advection from lower elevations, amidst the
easterly to east-northeasterly flow north of the front;
2. Upslope mechanical lift, augmented by...
3. Strong heating, which will combine with moistening to remove
remaining MLCINH preferentially on the higher elevations;
4. Shots of large-scale ascent aloft related to aforementioned
southwest-flow perturbations.

Activity will move over a well-mixed, deep boundary layer, with
progressively hotter surface temperatures southward until CAPE
becomes negligible south of the frontal zone due to boundary-layer
dryness. The optimal corridor for convective maintenance and
upscale growth appears to be from southeastern CO and the OK/
northern TX Panhandles across southern KS and northern OK, where
just enough flow aloft exists to encourage eastward-directed, forward-propagational cold-pool surges into a very unstable boundary
layer, with preconvective MLCAPE in the 1500-3000 J/kg range.
Larger CAPE may exist farther north across central/western KS, but
MLCINH will increase with northward extent under an EML. One
exception may be across the northeastern CO/northwestern
KS/southwestern NE region, where forecast soundings and convective
progs suggest a short window of opportunity for high-based
convection with isolated severe potential. In KS/OK, severe-gust
potential should decrease with eastward extent tonight as activity
encounters a gradually stabilizing boundary layer, but uncertainty
exists over cold-pool strength vs. CINH and how fast the threat will
diminish.

...Southeast...
Scattered to locally numerous thunderstorms may develop this
afternoon over parts of the Tennessee and lower Ohio Valley regions,
and perhaps the Mid-South. Damaging to locally severe gusts will be
the main concern.

Outflow and differential-heating boundaries -- both from residue of ongoing/morning convection farther north, and over this region --
should provide foci for afternoon initiation as strong heating and
rich low-level moisture minimize MLCINH. Given the strong mesoscale
dependence of these features through the remainder of the morning
into early afternoon, enough uncertainty remains in their position
and strength that outlook lines over this region may need
substantive adjustment again before the convective event starts.
The air mass over the region will be located largely well south of
the strongest mid/upper-level flow, but with rich low-level moisture
(surface dewpoints commonly upper 60s to mid 70s F). Based on a
mildly modified 12Z BMX RAOB and model soundings, this will support
widespread 3000-4000 J/kg MLCAPE with negligible MLCINH, southeast
of both the front and any areas of persistent cloud cover lingering
from morning convection. The deeply buoyant thermodynamic profile
and lack of inhibition will led to potential for clusters of
strongly water-loaded downdrafts and mesobeta-scale cold pools to
yield relative concentrations of damaging gusts.

...North-central Plains...
Isolated to scattered thunderstorms should develop this afternoon
over higher terrain in and near the Black Hills, then move generally southeastward across the Plains. Isolated severe gusts and hail
will be possible.

With southeasterly surface winds underlying strengthening mid/
upper-level westerlies, vertical shear will be favorable for some
supercell evolution, with effective-shear magnitudes in the 40-50-kt
range. While not as intensely heated nor moist as areas farther
south over the south-central Plains, a corridor of maximized low-
level moisture will extend northward through western NE to western
SD. This will act in concert with diabatic heating of the elevated
terrain to remove MLCINH and support development. MLCAPE in the
500-1500 J/kg range (locally higher) should develop atop a deep,
well-mixed boundary layer supporting maintenance of both strong-
severe downdrafts and hailstones to the surface. Severe potential
should diminish overnight and with southeastward extent, as the
inflow layer stabilizes and becomes more elevated.

..Edwards/Dean.. 08/12/2023

$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

ACUS01 KWNS 131254
SWODY1
SPC AC 131253

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

$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)