FOUS30 KWBC 130755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Fri Sep 13 2024

Day 1
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024

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

...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...

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...

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
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