Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sun Sep 8 2024

Day 1
Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST...

...Summary...

Only a few very minor changes were made to the inherited Marginal
Risk areas across the CONUS.

...Southwest...

A bit more widespread convection is expected with the monsoon this
afternoon across the Southwest. Much of the convection will once
again be focused along the various mountain ranges from the
Peninsular Ranges of southern California through the Sangre de
Cristos of Colorado. As usual in the desert Southwest, somewhat
limited moisture will be the limiting factor, even though areas
could be 2 sigma above normal for atmospheric moisture for this
time of year. Thus, the primary flood threat is for urban areas,
slot canyons, and arroyos where slow moving convection in chaotic
steering flow may result in long-duration localized heavy rain from
the most persistent convection. Any resultant flash flooding will
be isolated.

...Deep South Texas...

The Marginal Risk area was trimmed closer to the coast with this
update, now primarily for the urban area of Brownsville over to the
South Padre Island beaches. A stalled out front will begin to focus
an area of heavy rainfall associated with a slowly developing
tropical low over the western Gulf. Guidance has shifted east a
bit, resulting in lower forecasted rainfall totals for all of Deep
South Texas, with little if any rain expected west of Brownsville
through tonight. However...given the abundance of atmospheric
moisture available any stronger showers and storms will be capable
of very heavy rainfall rates which could cause isolated flash
flooding.

...Southeast...

The stalled out front that has been haunting the Southeast for well
over a week now remains in place from far southern South Carolina
through the northern Florida Peninsula. Besides the front itself
being strong...indicative of a tight gradient between the extremely
humid air mass over the Gulf and an outright autumnal air mass to
its north...instability remains very lacking since this front
hasn't moved in days, so diurnal heating is limited under the
extensive low cloud cover. Thus, almost all of the rain will be
light across this region. This is countered by the aforementioned
abundant amounts of atmospheric moisture available for any
convection that can form. Thus, a low-end Marginal remains in
place, with the greatest threat for flooding over the Florida
Peninsula well south of the front where diurnal heating can
actually make a difference.

Wegman

Day 2
Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024

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

As on Day 1, very few changes were needed to the inherited ERO risk
areas.

...Coastal Deep South Texas...

Weak easterly flow north of a developing tropical low over the
southwestern Gulf will advect increasingly deep tropical moisture
into coastal Deep South Texas on Monday. This is highly likely to
result in much more rainfall for South Padre Island and over to
Brownsville as compared with Day 1/Sunday. Given PWATs that could
exceed 2.5 inches, or 3 sigma above normal for this time of year,
any convection that moves west off the Gulf into the coast will be
capable of very heavy rain rates. Thus, the real question regarding
flood coverage will be how frequent storms producing those rates
move over urban areas. Think right along the coast widely scattered
instances of flash flooding can be reasonably expected. Any
training could quickly result in significant flash flooding. That
said, the system will only really just be getting organized, so
overall rains will likely be more intermittent in most areas. Urban
areas such as Brownsville will be at a higher likelihood for flash
flooding.

...Four Corners Region...

Convective coverage should shift northeastward a bit on Monday.
Thus, the flooding threat shifts into the Four Corners region and
away from the West Coast. Flooding concerns will be greatly
focused in area slot canyons and arroyos of southern Utah and
southwest Colorado Monday afternoon and evening with peak heating.

...Rest of the Gulf Coast...

Rinse and repeat weather pattern will continue, albeit shifted just
a bit south of Sunday's focus. Thus, most of the north Gulf Coast
will get a relative break from the potential for steady heavy rain,
as the heaviest rains shift south a bit away from the coast. The
exception will be into the Florida Peninsula, where a repeat day of
sea breeze showers and thunderstorms can be expected to the south,
with more widespread lighter rain expected further north.

Wegman

Day 3
Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TEXAS,
LOUISIANA, AND MISSISSIPPI GULF COASTS...

...Summary...

The inherited Slight Risk area was greatly expanded northward with
this update in coordination with LIX/Slidell, LA and HGX/Houston,
TX forecast offices.

...Gulf Coast...

In much of the guidance, a strengthening tropical low in the Gulf
will push north up the far western Gulf from Mexico and into the
northwestern Gulf. This will both push the stalled out front that
has been in this area to the north back into the South, and much
better organize its associated rain. The result will be widespread
heavy rainfall along the northwestern Gulf Coast on Tuesday. The
bulk of the heaviest rain will be along much of the Texas Gulf
Coast Tuesday and Tuesday night. Steady heavy rain directly
associated with the low will hold until well into Tuesday night for
the Upper Texas Coast.

Meanwhile into Louisiana and points east, the rain expected in
these areas will be moisture peeling off the low in the western
Gulf and tracking along the stationary front separating the
moisture abundant air mass over the Gulf with the fall-like air
mass over much of the eastern U.S. Thus, forcing will be lesser
into Louisiana and points east than in Texas. The Slight was
introduced not only for the potential for long-duration light to
moderate rainfall, but also due to very favorable antecedent
conditions (saturated soils) from the week plus of consecutive days
of rain from the same stalled front.

As mentioned in previous discussions, while there's good agreement
that the low will be organizing in the western Gulf as it moves
north by Tuesday, the fact that the low is only just beginning to
get its act together as of the time of this writing has resulted in
widely variable solutions as to when and where the heaviest rains
will be along the Gulf Coast by Tuesday. Thus, expect potentially
significant changes in the forecast rainfall footprint in the
coming days as the guidance gets a much better handle on the track
of the low and its associated rainfall.

...Southwest Colorado...

A small Marginal Risk area was added across Southwest Colorado for
lingering monsoonal moisture hitting the same mountain areas as on
previous days' afternoons. Rains will also spread into the
Intermountain Northwest through UT/ID/WY. With better agreement in
the guidance, the Marginal may need to be expanded into this
region with future updates.

Wegman


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