SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0742 AM CDT Wed Sep 11 2024
Valid 111300Z - 121200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN THE CENTRAL
GULF COAST VICINITY...
...SUMMARY...
Hurricane Francine poses a tornado threat today and tonight in the
central Gulf Coast vicinity.
...Synopsis...
The midlatitude westerly belt will amplify somewhat in mid/upper
levels, related somewhat to general height rises over the mid/upper
Mississippi Valley, Great Lakes and Northeast. However, the main
influence will be troughing related to a cyclone now closing off,
just offshore from the Pacific Northwest Coast. The trough will
move inland today and cross the interior Northwest, reaching an axis
from southern BC across eastern parts of WA/OR to the Sierra south
of RNO by 00Z. By 12Z, the trough should extend from the
southernmost Canadian Rockies, across westernmost MT, through a
500-mb low over eastern ID, to near LAS, with associated cyclonic
flow covering most of the CONUS west of 100W and north of 35N.
Farther southeast, moisture-channel imagery indicates that a weak,
cut-off, 500-mb low analyzed last night at 00Z over north-central TX
has moved little since. However, the low is expected to drift/
redevelop northeastward through the period, reaching southeastern OK
by 12Z tomorrow. In response, and as forecast by NHC, Hurricane
Francine should move inland tonight across southeastern LA, and into
southwestern MS before daybreak tomorrow.
At the surface, 11Z analysis showed a quasistationary front from
northern FL across the shelf waters of the FL Panhandle/AL/MS,
becoming a diffuse warm front over the Mississippi River mouth and
southern Terrebonne Parish, then just offshore again around the
northern/northwestern rim of Francine's circulation. This boundary
should move slowly northward/inland over the north-central Gulf
Coast through the period, with the decaying center of Francine
likely catching the front inland tonight. Elsewhere, a lee trough,
with several weak lows attached, was drawn over eastern portions of
MT/WY/CO, and should strengthen today amidst peripheral mass
response to the major mid/upper trough moving into the northwestern
CONUS.
...Gulf Coast: Hurricane Francine...
Increasing southwesterly deep shear on the cyclone as a whole is
expected, contributing to greater asymmetry in the overall precip/
convective pattern, with increasing supercell/tornado potential
today northeast through east of center. The threat is expected to
ramp up through the remainder of the morning, from south to north
and west to east along the coast, with the greatest potential still
appearing to be this afternoon into early evening, near and south of
the I-10/I-12 corridor from southeastern LA eastward toward MOB,
then overnight, spreading eastward into some of the western FL
Panhandle.
This should occur as:
1. Destabilization spreads somewhat inland -- both from theta-e
advection along and south of the warm front, and weak diurnal
heating;
2. Relatively large low-level hodographs spread over the region
along and south of the warm front -- in the outer parts of the
closed TC circulation; and
3. Both the greater instability and shear spread into a regime of
developing, discrete to semi-discrete convection in bands that can
organize to supercells.
Overland penetration of the most favorable parameter space will
occur overnight, but should be restricted in extent and magnitude by
abundant inland precip and associated stability, as well as
nocturnal/diabatically driven stability inland.
See SPC Mesoscale Discussion 2051 for near-term assessments. Refer
to NHC advisories for forecast track, intensity and tropical watch/
warning information on Francine.
...Northern Rockies to northern High Plains...
Two main regimes are expected to contribute to high-based
thunderstorms, atop well-mixed subcloud layers, with marginal
hail/wind potential (western lobe) and mainly strong/isolated severe
gusts (eastern lobe) in the outlook area, with a relative minimum
but non-zero potential for a severe gust in between.
In the western lobe, scattered afternoon thunderstorms in bands and
clusters should develop as low/middle-level destabilization,
combined with modest but adequate moisture, weakens MLCINH and
strengthens buoyancy. Cooling aloft related to increasing large-
scale lift from DCVA, and in the left-exit region of cyclonically
curved upper-jet segment, will overlie surface heating to steepen
tropospheric lapse rates, with pockets of 250-750 J/kg peak/
preconvective MLCAPE expected to develop. Though north of the
strongest deep shear, the area still will lie under strong difluence
aloft, and 25-35-kt effective-shear magnitudes contributing to some
organization.
Farther east over the northern High Plains, marginal low-level
moisture, recycled from the last continental/polar air intrusion,
will keep moisture weak (with post-mixing surface dewpoints in the
40s to low 50s F), yet still just enough to support isolated to
widely scattered thunderstorms this afternoon into early evening.
Activity may be relatively maximized near the intensifying lee
trough, where low-level mass convergence and strong diurnal heating
will combine to eradicate CINH. Deep subcloud layers with steep
lapse rates will support downdraft accelerations, even with modest
updraft buoyancy (MLCAPE generally 500-1000 J/kg). A moist axis
near and parallel to the front will yield higher CAPE, but in a more
strongly capped air mass where initiation and maintenance of
surface-based convection is less certain.
..Edwards/Kerr.. 09/11/2024
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