419
ACUS01 KWNS 250606
SWODY1
SPC AC 250604

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0104 AM CDT Wed Sep 25 2024

Valid 251200Z - 261200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
GULF COAST INTO THE SOUTHERN/CENTRAL APPALACHIANS AND FLORIDA
PENINSULA...

...SUMMARY...
Isolated severe storms are possible later today from parts of the
Southeast and central/southern Appalachians to the Texas/Louisiana
Gulf Coast. Some tornado threat may develop across the Florida
Peninsula by this evening, in association with Tropical Cyclone
Helene.

...Synopsis...
The upper-level pattern across much of the CONUS will become rather
complex today and tonight. Tropical Cyclone Helene is forecast to
intensify as a hurricane as it begins to move north-northeastward
across the eastern Gulf of Mexico; see NHC forecasts and advisories
for more information. A mid/upper-level low that will initially be
over the mid MS Valley is expected to slow down and eventually move
southward, as it begins to interact with Helene. A northern-stream
mid/upper-level trough is expected to amplify as it moves
east-southeastward toward the lower Great Lakes region. A seasonably
strong mid/upper-level shortwave trough will move across the
Northwest through the day and into the evening.

...Parts of the TX Gulf Coast into the Southeast and southern
Appalachians...
Widespread precipitation is expected from parts of the Southeast
into the southern Appalachians, due to the combined influence of the
mid/upper-level flow to the west and moisture streaming north from
the Gulf of Mexico and Helene. While this will tend to limit
available buoyancy with time, a few strong storms will be possible
with initial development, with preconvective MLCAPE of around 1000
J/kg and generally favorable deep-layer shear. Locally damaging
gusts and perhaps a tornado could accompany the strongest storms,
before low/midlevel lapse rates and buoyancy become increasingly
weak with time, and an outflow-reinforced front sags southward
across the region.

Compared to areas farther east, somewhat stronger diurnal
destabilization will be possible from parts of central/east TX into
LA, and perhaps into MS, with MLCAPE potentially increasing above
1500 J/kg. Isolated to widely scattered storm development will be
possible near a southward-moving cold front. Effective shear will
generally range from 25-35 kt, and a few modestly organized storms
are possible. Isolated damaging gusts could accompany the strongest
storms within this regime.

...Florida Peninsula and Keys...
Helene`s wind field is forecast to be rather large, especially on
the east side of the circulation. This will result in strengthening
low-level shear/SRH across the Florida Peninsula during the day and
especially into the evening/overnight. Instability will likely
remain rather weak, and details regarding the intensity of any outer
rain bands remain uncertain, but some tropical-cyclone tornado
threat could evolve across parts of the peninsula and Keys,
especially later in the period.

...Upper Ohio Valley into the lower Great Lakes...
Weak to locally moderate buoyancy may develop later today from parts
of the upper OH Valley into the lower Great Lakes region, between a
weakening cold front approaching from the west, and a slow-moving
warm front draped from central PA into western NY. Moderate
southwesterly midlevel flow will support effective shear of 35-45
kt, sufficient for storm organization. However, with midlevel lapse
rates expected to remain rather weak, storm intensity and any
attendant severe threat could be relatively limited. Confidence in
sufficient destabilization is too low to add probabilities at this
time.

...Eastern WA into northern ID and northwest MT...
While buoyancy is expected to remain quite weak, relatively steep
midlevel lapse rates will support thunderstorm potential from
eastern WA into northern ID and northwest MT, in advance of the
approaching shortwave trough. Most guidance suggests that
instability will remain too weak for an organized severe threat,
though rather strong deep-layer flow/shear could support a strong
storm or two with locally gusty winds and small hail. Severe
probabilities may eventually be needed if confidence increases
regarding the development of sufficient instability.

..Dean/Thornton.. 09/25/2024

$$