Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Fri Sep 13 2024
Day 1
Valid 16Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND NORTHERN ALABAMA...


16Z update... The latest global and CAM guidance continue to focus
the heaviest QPF across north/north-central Alabama as the remnant
circulation of Francine persist over the region. While these rain
amounts will likely be beneficial to many locations that have been
in a prolonged dry stretch, the rainfall intensity and duration
over sensitive terrain areas may quickly lead to localized flooding
problems. The ongoing Moderate Risk had a small expansion to the
southeast for this update as supported by the latest guidance.
Elsewhere across the region the level of threat still stands as
already identified with the Marginal and Slight Risks in place.

Campbell

...Deep South...

Remnant circulation from Francine will be pivoting back south and
then southeast in retrograde thanks to a strong block from a stout
mid- level ridge positioned over the Great Lakes through
Ontario/Quebec. This will allow for the a migration of elevated
moisture centered under the disturbance to pivot back across MS/AL
where drier air has punched through under prevailing southwest flow
aloft, mainly above 500mb. This is well- documented within the 00z
sounding analysis out of KJAN where moisture is pooled between the
surface to 500mb with a large wedge of drier air focused above. The
sounding from KBMX was beginning to show signs of the dry tongue
and can be noted within the radar presentation as deep convective
development is struggling to initiate this evening. Tomorrow
afternoon and beyond is when the pattern shifts to a more favorable
convective regime across far Eastern MS into much of AL, especially
the northern 2/3's of the state thanks to the stacked synoptic
circulation sliding back overhead. Guidance is keen on the deeper
moisture return as indicated within the 700-500mb RH field leading
to an uptick in regional PWATs back above 1 deviation above normal
with ample mid- level energy accompanying.

The latest 00z HREF remains very aggressive within its depiction
for a broad axis of heavy rainfall in-of Northern and Central AL
during the late- afternoon and evening time frame in conjunction
with the increased upper forcing approaching from the west along
the tightened theta-E gradient located across much of the state.
Surface ridge to the northeast will also begin wedging south into
Northern GA with an axis of convergence developing upstream between
the approaching circulation from now Post-Tropical Francine. This
will place a good portion of AL into the crosshairs for a better
convective training episode as the deep layer flow aligns perfectly
from the southeast. HREF neighborhood probability for >5" is robust
between the area of KHUN down to KBHM indicating a 50-70% prob with
an EAS signal of 50-80% for at least 2", a signal correlated highly
for a higher end risk. Hourly rate probs are also aggressive in the
means with a corridor of elevated probs for between 2-3"/hr within
the HREF, along with some intra-hour rate potential exceeding 4"
given the efficient rainfall processes and expected instability
presence within the eastern fringe of the general circulation. All
these prospects weighted on the decision to maintain the inherited
MDT risk with some minor adjustments to correlate with the blended
mean QPF off the latest HREF, as well as the EAS prob field
coverage of higher probs for at least 2" of rainfall. The changes
made were not extensive in the grand scheme, so the overall
forecast was close to general continuity.

...Coastal Carolinas...

A weak inverted trough located off the Southeast coast will allow
for a targeted axis of heavy rainfall across the Eastern Carolinas,
mainly north of KCHS with a bullseye being forecast along the coast
of Southeast NC. The prevailing easterlies off the Atlantic within
the integrated water vapor field (IWV) from the GFS signal the axis
very well with a nose of elevated PWATs being advected right into
the coastal areas of NC during the first half of the period. 00z
CAMs also signal the threat for a period of heavy rainfall with
some varying degrees of the latitudinal push of where the heaviest
precip would occur. The greatest probabilities for >3" are centered
from Wilmington up through Morehead City into Cape Hatteras.
Considering the area FFG's, the main flooding risks will be
relegated to more urbanized areas where run off potential is
highest. The signal was good enough for a continued MRGL risk with
some expansion further inland, mainly west of Raleigh where some
convergence from the damming high to the north and the prevailing
easterlies may intersect allowing for a small axis of enhanced
precip away from the coastal plain. Not anticipating a large areal
coverage of flash flooding in any case for the region, but a few
pockets are plausible leading to a low to middle ground MRGL for
the forecast.

...Florida Panhandle...

20Z update... A complex of thunderstorms are moving into and downstream
of the Bay County area and have the potential to dump 1 inch of
rain within 15 minutes or a quick 2 to 4 inches. For detailed
information refer to WPC's Mesoscale Precipitation Discussion
#1004. Given this scenario there was a need to upgrade portions of
the panhandle to a Slight Risk for excessive rainfall and flooding
concerns.

Campbell

Deep tropical flow pulling north from the Gulf will allow for an
axis of heavy convection to push north out of the Gulf into the Big
Bend of FL. Areal FFG indices for 1/3/6 hr rates are pretty
substantial, so the threat is low-end within the MRGL risk
category. Still, the signal for amounts >5" is relatively high
right along the coast leading to the MRGL continuation from
previous forecast.

Kleebauer

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt