FOUS30 KWBC 040746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sun Oct 4 2026

Day 1
Valid 12Z Sun Oct 04 2026 - 12Z Mon Oct 05 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER TEXAS COAST...

...Southeast to Gulf Coast...
An amplifying mid-level trough will dig and expand across the
eastern third of the CONUS today, but height falls will struggle to
dig as far as the Gulf Coast due to a ridge centered over the FL
Peninsula and backing west into the Gulf. This will result in
squeezed mid-level flow from Texas through the Carolinas, and
although subtle height falls may eventually expand southeast by
Monday morning, the resulting surface front is likely to remain
generally in place with just some wavering through the D1 period.

This front will serve as an axis for both mass convergence and
impressive moisture confluence through the day. The resulting
ascent will be aided by the aforementioned weak height falls, but
more impressively by intense diffluence within the RRQ of a
meridionally arcing and strengthening jet streak pivoting into New
England. This deep layer lift will act upon intense thermodynamics
thanks to a pool of PWs above 2.25-2.5" (above the 99th percentile
according to NAEFS and locally +3 sigma from the climo mean) to
support widespread convection, and although instability may be
somewhat inhibited to generally 500-1000 J/kg due to cloud cover,
the extreme PWs and 15 kft+ warm cloud depths will still support
efficient rainfall rates of 2-3"/hr, with short-term rates even
higher than that thanks to convective organization on 20-30 kts of bulk shear

The guidance is in good agreement this plume of PW will remain
mostly unchanged through the D1 period, leading to multiple rounds
of heavy precipitation. While the axis of heaviest rainfall does
vary amongst the different CAMs, weak impulses drifting from LA to
the western FL Panhandle may help develop a wave of low pressure along
the front to serve as a focus for locally heavier rainfall, and as
Corfidi vectors collapse to around 5 kts and align increasingly
parallel to the weak mean flow, this will support upwind
development and training of rates. This suggests that some areas
will receive 3-5" of rain today, with locally as much as 8"
possible from Mobile Bay, AL eastward to the Emerald and Forgotten
Coasts along the Florida Panhandle. Considered a small MDT risk for
these areas but due to relatively high FFG and modest exceedance
probabilities focused right on the coast, and after coordination
with WFOs MOB and TAE, opted to keep the enhanced slight risk in
place for now and defer a possible upgrade, if needed, to the D1
cycle later today.

Farther southwest along the western Gulf Coast and the entire Texas
Coast, the MRGL risk was joined to encompass the immediate coast of
the entire region due to possible onshore drift of heavy rainfall
through the morning/aftn hours. However, the most significant
rainfall (both of this area and maybe the entire day) is
anticipated along the Lower Texas Coast between Brownsville and
Corpus Christi. Here, convection developing over the Gulf beneath
mid-level impulses will track SW and into impressive moist inflow,
pushing onshore repeatedly thanks to anti-parallel Corfidi
vectors. With rainfall rates likely reaching 2-3"/hr, this could
result in local maxima rainfall in excess of 10" in a few
locations, and both the REFS and HREF indicate a high risk (>50%
chance) for at least 8" of rain. This will fall atop locally more
sensitive soils too since recent rainfall has been significant in
some areas (2-4" last 12 hours, up to 8" last 3 days). While this
region generally has elevated FFG due to sandy soils, after
coordination with WFO BRO and CRP, raised a targeted MDT risk for
the lower Texas Coast where locally significant impacts are
possible should any of this heavy rain track across urban areas or
more sensitive soils.

Finally, there is a tertiary area of locally heavier rainfall
possible along the Georgia coast, centered on the vicinity of
Savannah, GA. While there is a lot of spatial spread among the
various CAMs and associated ensemble clusters (REFS, HREF), there
is a distinct signal for locally more significant rainfall across
this area as well. This is likely to occur in two rounds - one from
pinned sea breeze convection aided by resultant frontogenesis
beneath the ageostrophic circulation of the aforementioned jet
streak - with a second round possibly occurring later at night as a
wave of low pressure moving onshore the FL Panhandle approaches
from the SW. Any convection will again contain 2-3"/hr rain rates,
and this area has been saturated recently leading to locally
compromised FFG of 1-2"/3hrs. This suggests despite the general
sandy soils across Georgia coast, scattered flash flood impacts are
possible where these locally more vulnerable soils become
overwhelmed by repeating rounds of heavy rain.

Weiss
 

Day 2
Valid 12Z Mon Oct 05 2026 - 12Z Tue Oct 06 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE LOWER TEXAS COAST...

...Texas Coast...
Another day of heavy rainfall is likely across the Lower Texas
Coast and into parts of South Texas, likely increasing or
exacerbating flash flooding impacts anticipated during D1.

A surface front will track southward into the Gulf Sunday and
linger along the Lower Texas Coast Monday. While the accompanying
ribbon of instability may be quite compressed by Monday, PWs as
high as 2.5" and deep warm cloud depths will maintain a favorable
thermodynamic environment for heavy rainfall. Additionally, an axis
of 700-500mb deformation just offshore will arc back towards the
region, resulting in a narrow corridor of enhanced ascent to drive
streaks of convection with heavy rain. The HREF (when available)
and REFS both indicate a 20-30% chance of rainfall rates exceeding
2"/hr, which through training thanks to continued anti-parallel
Corfidi vectors may produce an additional 3-5" with locally higher
amounts (REFS 5"/24hr probabilities above 50%). For this reason a
SLGT risk has been added for a narrow corridor of the BRO CWA.

...Southeast...
A similar environment persists farther east along the Gulf Coast
from the Florida Panhandle and extending northeast into coastal
South Carolina. A cold front will be drifting slowly southeast
across this area as height falls south of an expansive mid-level
trough inexorably work their way south towards the Gulf Coast. Weak
impulses moving through the flow aloft will help generate waves of
low pressure along this front, and where these waves move northeast
they will enhance the rainfall through locally accelerated flow and
stronger ascent. Regardless, efficient rainfall rates of 2-3"/hr
are likely within a plume of exceptional tropical moisture (PWs
above 2.25" and deep warm cloud depths. Considered a SLGT risk
where the guidance supports areas of heavier rainfall, but there
continues to be some uncertainty as to how much instability will
persist north of the front (depending on how far south it
progresses), and anafrontal rainfall may not be heavy enough for a
SLGT risk upgrade. While training is likely along the CAPE
gradient which through these rain rates could lead to 2-4" of rain
atop saturated soils from D1, opted to maintain the MRGL risk at
this time due to this uncertainty. However, a SLGT risk upgrade may
be needed if signals continue to support another day of heavy rain on Monday.

Weiss
 

Day 3
Valid 12Z Tue Oct 06 2026 - 12Z Wed Oct 07 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
FLORIDA AND SOUTHEAST GEORGIA...

The cold front from Monday will stall across the northern Florida
Peninsula Tuesday as a stationary front. This front will meander
subtly north/south through D3, remaining entrenched within weak
mid-level flow and a plume of anomalous PWs progged to be over
2.25", or above the 97th climatological percentile according to
NAEFS. Weak mid-level shortwaves traversing slowly atop the
boundary will enhance ascent, and despite modest instability, the
overlap of sufficient ascent within the deep tropical moisture will
result in yet another day of heavy rain as convection trains WSW to
ENE on 0-6km mean winds parallel to the boundary. With rain rates
expected to reach 1-2"/hr, at least isolated flash flood impacts
are expected, especially where rain moves across areas that also
experience heavy rain on D2 /Monday./ Effective bulk shear of 20-30
kts may support some modest organization which could enhance the
risk to drive a targeted upgrade to a SLGT risk, but at this time
maintained the MRGL risk due to uncertainty in the intensity and footprint.

Weiss
 

Day 4 and Day 5
Valid 12Z Wed Oct 07 2026 - 12Z Fri Oct 09 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL WEDNESDAY ACROSS
PORTIONS OF THE FLORIDA PENINSULA...

A stationary surface front will remain draped roughly halfway
across the FL Peninsula Tuesday through Thursday and will continue
to be a focus for potentially heavy rainfall through at least
Wednesday night (current Day 4). The axis of highest PWs
(2.25-2.5" and mid-level shortwave energy will be be in the
front's vicinity, as well as instability from the nearby Gulf. Deep
west/west-southwesterly flow will enhanced the risk for training
cells coming off the eastern Gulf across central FL, while anti-
parallel flow to the front's north across northern FL bringing in
instability from the subtropical Atlantic and could lead to very
high precipitation efficiency. Effective bulk shear values over
20kts would allow for some degree of organization on Wednesday.
However on Thursday, the guidance shows the front become a little
more diffuse as the subtropical ridge and a more DNVA regime
begins to build across the FL Peninsula. This would explain why the
00Z operational global QPFs (GFS/ECMWF/UKMet/CMC) are lower on Day
5 compared to Day 4. As a result, at least for now, we did not
extend the Marginal Risk area into the Day 5 ERO.

Hurley/Roth
$$
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