Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Fri Sep 13 2024

Day 1
Valid 01Z Sat Sep 14 2024 - 12Z Sat Sep 14 2024

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

...0100 UTC Update...
Minor changes were made to the Day 1 ERO based on the latest
observational trends and current HRRR/HREF guidance. We removed the
targeted Slight Risk along the FL Big Bend, while pared the Slight
Risk a bit across parts of central GA.

Hurley

...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...
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 2
Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEASTERN U.S...

...Southeast...

21Z update... The latest guidance continues to suggest an
incremental southward trend with the QPF swath associated with
Francine. This resulted in a minute expansion of the southern
boundary of the Slight Risk across southern Georgia and Alabama.
Consensus maintains a NW to SE orientated axis of moderate to heavy
rain from western Tennessee to southeast Georgia mainly in the 1 to
3 inch range, although local maximums may be closer to 4 or 5
inches.

Campbell

The remnant mid and upper circulation from Francine will get stuck
under the blocking pattern positioned across the northern and
eastern U.S with little propagation of any kind during the period.
This will allow for a continuation of broad, cyclonic flow across
the Southeast with a steady convergence pattern positioned over
parts of the Deep South. A steady southeast flow pattern will
preside over much of AL and Western GA with a sharp convergence
signal focused over the central portion of the outlined area thanks
to the persistent anti-cyclonic "wedging" occurring around the
southern flank of the ridge, banking against the cyclonic regime
centered around Francine's remnants. Ensemble QPF distribution is a
bit further south compared to the previous forecast allowing for
an adjustment in that direction for the SLGT risk prospects. Totals
are generally favored to be within 1-3" overall, but some pockets
of higher totals are depicted within the 00z deterministic suite
leading to some locally impactful flash flood prospects, especially
when coupled with the previous period(s) of rainfall lowering the
area FFG's across AL/GA.

The SLGT risk is on the higher-end of the spectrum with a higher
risk potential relying on the contingency of how the D1 period
plays out across the region. There's a non-zero chance of an
upgrade, but the slowly degrading axis of instability may thwart
the potential all together. Regardless, the SLGT risk is sufficient
just given the variables at hand.

...Arizona...

21Z update... The level of threat for excessive rainfall remains in
good order and no changes were made at this time.

Campbell

A surge of deep tropical moisture from now Tropical Storm Ileana
will make progress into the southern portion of AZ by the middle to
end of the forecast period. With sufficient diurnal heating
Saturday afternoon, environment across Southern and Southeast AZ
will destabilize allowing for isolated to scattered thunderstorms
to initiate in-of the terrain situated within Pima/Santa Cruz
counties to possibly as far east as Cochise county in proxy to the
Huachuca mountains. MUCAPE forecast is relatively favorable for
convective development with the ensemble SBCAPE forecast between
750-1250 J/kg off the latest HREF mean. The combination of
increasing moisture anomalies and environmental buoyancy adds
favor to an isolated flash flood risk, especially within the
terrain and adjacent locales. A MRGL risk was maintained from
previous forecast, but trimmed on the northern edge given the
expected timing of the moisture push to be delayed for a greater
convective threat within Pinal/Maricopa counties. This area will
have more favorability later on D3.

Kleebauer

Day 3
Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID AND LOWER MISSISSIPPI VALLEYS INTO THE SOUTHEAST, AS WELL
AS THE DESERT SOUTHWEST...

...Southeast...

21Z update... The level of threat for excessive rainfall remains in
good order and no changes were made at this time.

Campbell

The remnant circulation and moisture from Francine will stick
around for one more period to round out the weekend with scattered
thunderstorm activity prevailing across much of the Southeast up
through the Mid-Mississippi Valley. Amounts currently are lower
given the reduced ascent pattern as the upper energy starts
becoming more diffuse in its representation, stunting the potential
for greater convective impact compared to previous periods. Local
totals >2" are still plausible in the setup and the fact there will
be overlap in some of the harder hit locations from the D1-2 time
frames, there could end up being a targeted SLGT risk just based
off the ensemble QPF distribution. For now, a broad MRGL will
suffice given the expected synoptic pattern and convective environment
forecast.

...Southwest...

21Z update... Leading up to and during this period the mid/upper
atmosphere will be getting moisture from Tropical Cyclone Ileana
which is currently tracking northward along the Gulf of
California/Baja California. The lower atmosphere across much of
Arizona and New Mexico has been recently dry which will greatly
inhibit convective rains reaching the ground, at least initially,
or reduce rainfall amounts/efficiency during this period. The model
spread also reduces confidence on where the actual heaviest QPF
will occur. There is still some locations that may see isolated
heavy amounts and very localized run off or flooding concerns. The
Marginal Risk was nudged a small amount to the east across west-
central New Mexico.

Campbell

Elevated moisture returns over the Desert Southwest will aid in
more widespread thunderstorm development during peak diurnal
instability leading to scattered flash flood concerns across much
of AZ, Western NM, into the Four Corners. The primary areas of
interest will be the favored complex terrain locations, urbanized
zones, slot canyons, and remnant burn scars littered across the
region. A targeted SLGT is possible in future updates, especially
across the terrain in AZ.

Kleebauer

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt