FOUS30 KWBC 090830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Mon Sep 9 2024

Day 1
Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...

...Coastal Deep South Texas...

The Slight Risk for the coastal sections of Cameron, Willacy, and
southeast Kenedy Counties remains largely unchanged, though trimmed
a hair on the west/inland side with this morning's forecast update.
The northern edge of the convection associated with Potential
Tropical Cyclone Six (hereafter, PTC6), will try to move into the
coast today into tonight. A strong front remains draped over South
Texas, along the coast, but extending west/inland over Deep South
Texas. This front is keeping dry air nearby, greatly limiting any
westward extent to the rainfall, but is also a source of forcing
for the very deep tropical moisture currently in place over most of
the Gulf. Thus, as PTC6 organizes today into tonight, waves of rain
may move into the coast over South Padre Island and Brownsville.
Widely scattered instances of flash flooding are probable,
especially where the storms are the most persistent today.

...Louisiana through Florida Gulf Coast...

Few changes were made to the Marginal Risk area...which will be
largely in place for afternoon sea breeze convection as deep
tropical moisture associated with PTC6 move east across the
Peninsula and along the Gulf Coast. There has been a nominal
southward shift in the guidance, so the Marginal was trimmed to
just the immediate Gulf Coast except over the Florida Peninsula
where sea breezes and outflow boundaries will be much more of a
factor in forcing storms capable of producing heavy rain and
resultant flash flooding.

...Four Corners...

No changes were made to the Marginal Risk area, where isolated
flash flooding is possible especially in slot canyons and arroyos.
Guidance has backed off a bit on the areal coverage of convection
in this region, so this area is a low-end Marginal.

Wegman


Day 2
Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ALL OF THE GULF
COAST WEST OF PENSACOLA..

...Gulf Coast...

Potential Tropical Cyclone 6 (PTC6) will continue organizing over
the western Gulf based on the latest NHC forecast as it tracks
north and east over the northwestern Gulf. Heavy rains associated
therewith will impact all of the Gulf Coast, especially into
Louisiana and Texas. The latest guidance has been shifting the
axis of heaviest rainfall east, in response to very dry air north
of a strong front that will be guiding PTC6 northeastward. Thus,
the westward extent of the associated rainfall will be greatly
limited. Thus, the inherited Slight Risk was trimmed from the west,
especially over Texas. With that said, any heavy rain associated
with PTC6 will have well above average amounts of atmospheric
moisture to work with, making it more common for prodigious
rainfall rates to occur. Much of southern Louisiana has been very
wet in recent weeks, so saturated soils will only quicken the onset
of flooding once the heavy rain gets going. 

Into Florida, the same stalled out front will shear some of the
moisture from PTC6's circulation, adding fuel for diurnally driven
showers and storms once again on Tuesday afternoon. Isolated flash
flooding will be possible once again.

...Utah/Colorado...

No changes were made to the inherited Marginal Risk area, where
numerous showers and storms may develop Tuesday afternoon. The most
concentrated and heaviest showers and storms will be in the
Marginal Risk area from eastern Utah into western Colorado.
Isolated flash flooding will be possible in flood prone areas such
as slot canyons.

Wegman


Day 3
Valid 12Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE
MISSISSIPPI DELTA...

Potential Tropical Cyclone Six (PTC6) is forecast to continue
strengthening up until landfall in southwest Louisiana on
Wednesday. Abundant associated moisture will advect north ahead of
and alongside the low center, resulting in multiple inches of rain
over almost all of Louisiana and adjacent southern Mississippi. For
Louisiana, much of the southern half of the state has seen abundant
rainfall over the past couple weeks, resulting in highly saturated
soils. A hurricane making landfall here will cause most of the rain
to convert to runoff. Thus, in coordination with LCH/Lake Charles,
LA; LIX/Slidell, LA; JAN/Jackson, MS, and SHV/Shreveport, LA
forecast offices, a Moderate Risk upgrade was introduced with this
update. The greatest risk of significant and potentially life-
threatening flash flooding will be in the hardest hit areas of the
I-10 corridor from New Orleans west through Baton Rouge to
Lafayette. A higher-end Moderate is considered in effect for this
portion of the I-10 corridor.

Considerable uncertainty persists on the north and west side of the
ERO risk areas. To the west, a notable and persistent drying trend
continues, partially due to eastward shifts in the track and
partially due to disagreement as to whether PTC6 will begin
extratropical transition around landfall. With the center expected
to track along the spine of Louisiana, locations to the west will
be on the dry side of the storm, which will quickly cut down on
rainfall totals. Given these trends, the Slight was trimmed on the
western side. To the north, the concern will be more about rapidly
deteriorating antecedent conditions, as northern Louisiana,
Mississippi, and Arkansas have all been much drier than points
further south towards the Gulf Coast. This in turn will make the
watersheds much more able to absorb heavy rainfall before
significant flooding commences. Thus, the Slight and Marginal Risk
areas are quite conservative, as the first few inches of rain
should be largely beneficial in these areas.

The aforementioned eastward shift in track should mean wetter
conditions into Alabama and the Florida Panhandle, and the Slight
was expanded east for that reason, though Alabama has been
similarly dry away from the Gulf Coast as its neighbors to the 
west.

With PTC6 really still trying to form, expect continued adjustments
in the track and expected rainfall associated therewith.
Uncertainty with locations and amounts have also precluded some
areas well away from the current forecast track of the center from
being upgraded, but may be with future updates, especially east of
the track.

...Intermountain West...

A Marginal Risk was introduced for this region as a strong digging
shortwave trough interacts with topography and above normal
moisture for this region. Forecast amounts may be high enough to
potentially need a targeted Slight with future updates in this
area.

Wegman
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