ACUS01 KWNS 051250
SWODY1
SPC AC 051249

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Mon Aug 05 2024

Valid 051300Z - 061200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
OHIO VALLEY INTO NEW ENGLAND...ACROSS PORTIONS OF THE UPPER
MIDWEST...AND FROM NORTHERN FLORIDA INTO SOUTHERN GEORGIA AND
SOUTHEAST SOUTH CAROLINA...

...SUMMARY...
Severe thunderstorms are possible today and tonight from the Upper
Ohio Valley into New England, across portions of the Upper Midwest,
and from northern Florida into southern Georgia and southeast South
Carolina.

...Synopsis...
With height falls well underway on its northern rim -- over the
northern Rockies -- the Four Corners regional anticyclone has become
ore zonally elongated, with a shift/reformation of the associated
500-mb high over the San Juan Mountains or adjoining San Luis
Valley. Through the period, associated ridging will extend
east-southeastward across southern KS to AR, west-southwestward over
the LAS area to southern CA, and will rebuild northwestward over the
northern Rockies.  The latter will occur as a series of closely
spaced shortwave perturbations -- apparent in moisture-channel
imagery from the western Dakotas to north-central MT -- shift
eastward across the Dakotas to MN by 00Z.  These features then
should cross WI and Lake Michigan tonight.  Just west of the ridge,
a weak shortwave trough now over the central/northern Sierra should
move north-northeastward toward the northern Rockies through the
day.

Farther east, a strong shortwave trough was evident over northern ON
north of Lake Superior, in the southwest quadrant of a vast cyclone
covering most of eastern Canada.  This trough should move
east-southeastward across southernmost QC to ME by 06Z, then over
the Canadian Maritimes.

At the surface, 11Z analysis showed a wavy, slow-moving (mostly
stationary) frontal zone from central New England to a weak low over
southern Lake Huron, across southern Lake Michigan and northern IA,
to another low over south-central SD between PIR-VTN, then across
central WY.  As the mid/upper shortwave trough approaches and grazes
the area, the eastern part of the front should move slowly
southeastward across the Lower Great Lakes, Adirondacks, White
Mountains and much of ME by 00Z.  The trailing part should drift
southward into northern parts of OH/IN/IL, then decelerate/stall
overnight.  The SD low should ripple east-southeastward along the
front and across the Siouxland region by 00Z, then over northern IA
tonight.  The trailing cold front from there should extend across
eastern NE, northwestern KS and southeastern CO by 00Z, stalling
overnight over southwestern KS and southeastern CO in response to
cyclogenesis near LAA.

...FL/GA/SC -- Hurricane Debby...
Hurricane Debby has made landfall in the FL "Big Bend" coastal area,
and the center is forecast by NHC to proceed slowly northeastward
over the FL/GA line through the period.  See SPC Tornado Watch 596
and related mesoscale discussions for near-term potential.  Refer to
latest NHC advisories for forecast track/intensity of Debby, and
tropical watches/warnings.

The existing tornado potential southeast of center largely is
expected to translate along with Debby, while expanding
northeastward to the Atlantic Coast of northeastern FL and GA, as
favorable low-level theta-e advection from the Atlantic destabilizes
the nearshore airmass there.  An extensive area of precip northeast
through north of center should be maintained, substantially
restricting instability over northwestern FL and southwestern/
south-central GA to the left of the forecast center track, and
reinforcing the baroclinic gradient there.  That should act as a
buoyancy-limiting northwestern bound to the supercell/tornado
threat.  Meanwhile, as low-level flow veers further on the FL West
Coast through the remainder of this morning, and weaker lift with a
more skeletal convective structure moves across the area south of
center, supercell/tornado potential will diminish there.  The
optimal CAPE/shear parameter space should shift through northern FL
into southeastern GA today, while expanding along the GA and
eventually SC Coast later today through tonight.

...Northeast and Great Lakes to upper Mississippi Valley...
Through tonight, multiple rounds of strong to severe convection are
possible across this lengthy but relatively narrow corridor near the
front, through tonight.  Damaging winds will be the main concern in
the Northeast, though enough low-level shear/hodograph enlargement
exists in forecast soundings across parts of northern New England to
support some supercell potential.  Meanwhile, supercells with large
hail and a tornado threat may develop before dark or in early
evening between southern MN and southern Lake Michigan.  Activity
overall should evolve upscale to a more clustered, wind-driven
threat with time, lasting later in the evening over western areas.

Surface-based development is likely to occur earlier in the eastern
"slight risk" area -- from midday through this afternoon, as
peripheral mass response to the Canadian perturbation increases
low-level convergence and tightens the height gradient aloft.  The
latter will increase deep shear, though nearly unidirectional wind
profiles should persist over the corridor, except in parts of
north-central New England and northern IA/southern MN, each to the
east of frontal-wave lows.  Strong diurnal warming will occur in
conjunction with favorable low-level moisture, with surface
dewpoints commonly in the mid/upper 60s from OH eastward and low 70s
from northern IN across the upper Mississippi Valley.  This should
yield peak/preconvective MLCAPE ranging from around 500-1500 J/kg
over the Northeast -- where midlevel lapse rates will be weaker --
to 2000-3000 J/kg in a narrow corridor near the front from northern
OH to the IA/MN border vicinity.  However, capping will be stronger
over the IA/MN/WI part of the front, and diurnal development/
coverage accessing the greatest surface-based buoyancy is more
uncertain than evening/overnight severe hail/gust potential near and
north of the front.

...Northern Rockies and vicinity...
Isolated to scattered thunderstorms are expected to develop this
afternoon and evening over much of ID into western MT, as modest
large-scale ascent preceding the shortwave trough spreads across
diurnally heated higher elevations.  Development will be supported
further by at least marginally favorable moisture, represented by
lower-elevation surface dewpoints now in the upper 40s to low 60s F,
and not expected to dry very much via mixing through early/mid
afternoon.  Still, heating/mixing should be strong enough to build
steep low/middle-level lapse rates and "inverted-v" thermodynamic
profiles supporting strong/isolated severe gusts,  Some of this
activity also may take the form of dry thunderstorms, especially in
eastern areas -- see SPC Fire Weather products for more on that
concern.

..Edwards/Grams.. 08/05/2024

$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)