FOUS30 KWBC 140809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Sat Sep 14 2024

Day 1
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...

Current radar/satellite composite depicts a broad cyclonic gyre
situated across the Southeastern U.S with a north-south band of
convection bisecting the a good portion AL into Western TN around
the northeast periphery of the remnant cyclone. The surface
reflection from what was Francine will become more diffuse over the
next 6 hrs with a degrading surface signature, leaving much of the
flow tied to the leftover mid and upper lows associated with the
storm. That said, the cyclonic spin from the tandem 850-500mb
low(s) will allow for a continued favorable moisture stream around
the circulation with small mid-level perturbations stuck over the
same areas allowing for locally enhanced ascent within the confines
of AL/GA/TN. PWATs generally residing within the +1 standard
deviation anomaly across the Deep South will be sufficient enough
within the pattern to yield another round of scattered to
widespread convection across similar areas that were impacted
today. The difference is the magnitude of the convective pattern
will be a bit less vigorous thanks to the weakening mid and upper
low centers as denoted within all the recent guidance height
fields. Still, the combination of the moisture anomaly and semi-
favorable ascent under the remnant mid and upper lows will be
plenty to offer another round of convection across the Deep South. 

To the northeast, a stout surface ridge will nose down further into
the interior Southeast with a formidable easterly regime focused
in-of Northern GA down into the Northwest side of AL. The
combination of the cyclonic flow from the mid-level remnants and
wedging associated with the surface ridge will create a tightened
axis of convergence within the state of AL down into Western GA.
The consensus this evening within the 00z CAMs suite was very well
represented within the HREF mean QPF and associated probability
fields, highlighting the semi-narrow corridor of higher rainfall
potential located just to the south and west of Huntsville,
bisecting much of North-Central AL down into the Southwest side of
GA. This area has been the beneficiary of some significant rainfall
the past 24-48 hrs leading to a degraded FFG footprint where flash
flood prospects will be greater, relative to average. Signals for
1-2"/hr rainfall rates would be sufficient given the antecedent
conditions across the area, and with the setup likely to garner
some locally enhanced rainfall rates >2"/hr as indicated within the
prob fields, there's a higher likelihood for continued flash flood
concerns within the confines of those areas in AL and GA mentioned
above. A higher end SLGT risk is the forecast for those areas
extending from Northwestern AL near the AL/MS line, southeast
through Columbus, GA, an area that includes the Birmingham metro.
SLGT risk extends further northwest and southeast, respectively
with a broad MRGL extension around the periphery as heavy rain
concerns exist from as far north as Western KY to as far Southeast
as Northeastern FL. 

...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
anticipated this afternoon, the 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 500-1250 J/kg off the 00z 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 with only minor adjustments based within the
HREF blended mean QPF footprint, and aligning with the edges of the
10% or great probabilities within the neighborhood probability
output for >2" rainfall potential. This area will have more 
favorability later on D2.

...South Florida...

Surface trough located in-of South FL will maintain a prevailing
westerly flow across the Southern part of the Peninsula, placing
the mean steering pattern pointed towards the population centers 
along the Southeast FL coast in time for the sea breeze convective
initiation this afternoon. Area PWATs are very elevated this
evening with the 00z sounding out of KMFL depicting a solid 2.40"
PWAT reading placing the area within the +1-2 standard deviation
anomaly for the period. This environment will be conducive for
convective generation just inland along the penetrating sea breeze
in the afternoon with locally strong convective cores producing
rainfall rates between 2-3"/hr on initiation, moving towards the
coast thanks to the prevailing steering flow. HREF probabilities
for >3" are solidly between 50-80% along the population centers
extending from PBI down through MFL with the highest probs situated
between PBI and FLL. Lower end 5+" probabilities are also
highlighting the area of interest, so the threat is within the
middle grounds for flash flood concerns leading to an addition of a
MRGL risk situated along the Southeast FL coast encompassing the
population centers. 

Kleebauer


Day 2
Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE SOUTHEAST U.S...

...Southeast...

Despite the remnant mid and upper circulation from Francine 
degrading in its presentation, another period of enhanced
convective activity will spawn underneath the primary upper forcing
lingering over the Southeastern U.S. There's a bit more emphasis on
an axis of convection developing across portions of the Lower
Mississippi Valley in-of AR and MS, extending southeast into parts
of the Deep South, mainly Southern MS and AL. Areal PWATs will
remain elevated within +1 standard deviations, enough of an anomaly
to spur up some potential heavier convective cores that could
result in some flash flood prospects, especially over any urban
zones where run off potential is greatest. Ensemble bias corrected
QPF and the 75th percentile of the NBM indicate a fairly stout axis
of heavier precip likely to occur under the influence of the upper
low and remnant energy pivoting around the general circulation. The
primary concerns will continue to be those locally heavy rates that
will reach between 1-2"/hr with some intra-hour potential for
heavier rates that typically cause more issues in this part of the
country. A SLGT risk was added to the previous forecast with an
adjustment southwest to the overall SLGT/MRGL risk areas to align
with the current trends and ensemble forecast consensus on the
heaviest precip location. The MRGL expands into the FL Panhandle to
cover the area that has been hit pretty hard in the past 36 hours
as isolated storms are still forecast for those areas that have
seen copious amounts of rainfall in the above time span. 

...Southwest...

Remnant moisture from now Tropical Storm Ileana will continue to
advect northward into the rest of the Desert Southwest with a
trajectory towards the Four Corners, wrapping back eastward into NM
during the afternoon hours Sunday. The ensemble QPF footprint upped
the total precip forecast compared to the previous periods, a trend
that was noted when assessing the QPF differential from 12z to 00z.
This places the convective pattern towards those areas we've been
highlighting all summer for flash flood concerns thanks to a
plethora of burn scars located within the terrain across parts of
NM. The previous forecast was relatively unchanged over AZ into the
Four Corners, but the MRGL risk was expanded to include much of NM
with a focus into the terrain over the Sangre de Cristos and the
Sacramento Mountains. General QPF average between 0.25-0.75" is
expected, but maxima of >1" will be plausible anywhere within the
confines of the forecast risk area. A targeted SLGT risk is still
possible, but the scattered nature of the convection forecast
doesn't instill enough confidence at this time. A reassessment with
the addition of CAMs guidance could sway that the other way, so
stay tuned for future updates. 

Kleebauer
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