Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Sun Sep 8 2024

Day 1
Valid 01Z Mon Sep 09 2024 - 12Z Mon Sep 09 2024

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

...Southwest...
A localized flash flood risk may persist for a couple more hours
over portions of southern CA, southern NV, southwest UT and
northwest AZ. Convection is generally showing a weakening trend,
although some new cell development is evident over portions of
northwest AZ into southwest UT. Overall, slow moving cells and a
brief cell merger or two could still result in an isolated flash
flood risk through about 03z.

...Deep South Texas...
A Marginal Risk remains for far southeast TX, primarily for the
urban area of Brownsville over to the South Padre Island beaches.
Potential Tropical Cyclone Six is in the western Gulf of Mexico,
with a trough extending north from this system to off the TX
coast. Convection is likely to focus near this trough where
convergence is maximized, and the overall consensus is for this
trough to have some westward movement later tonight and get close
to far southeast TX. There is still a significant amount of
uncertainty with regards to how much convection gets onshore
through 12z Monday. Model guidance remains split, and thus will
just need to keep a close eye on radar trends through the overnight
hours and into Monday morning. There is some chance we could be
looking at 3-5"+ rainfall right along the coast of far southeast
TX...but also a chance these higher totals stay just offshore.
Overall think a Marginal risk remains warranted with this more of a
conditional threat.

...Southeast...
We will maintain a Marginal risk for portions of central and
northern FL. Slow moving convection will continue to pose a
localized heavy rainfall threat for a few more hours...with
activity expected to be on the decline after 03z.

Chenard


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...

...20Z Update Summary...

Only minor changes were necessitated during the D2 time frame with
the SLGT risk maintained across Deep South TX, but brought in
closer to the coast. The MRGL risks across the Southeast and
Western U.S were generally maintained outside small adjustments to
the northern periphery of the risk areas.

Kleebauer

...Coastal Deep South Texas...

20Z Update: The pattern remains conducive for locally heavy
rainfall in-of the South TX coastal plain with the heaviest precip
focus situated east of McAllen towards Brownsville and South Padre
Island. The tropical disturbance to the south will be slowly
progressing northward through the period with the deeper tropical
moisture feed aligned right along the coast with some protrusion
inland as the disturbance jogs northwesterly the second half of
the forecast. Recent ensemble QPF output pins the 2-4" potential
right within Brownsville proper with a sharp western cutoff the QPF
as drier air located to the northwest of the weak circulation
maintains a foothold on the area. Latest HREF neighborhood
probabilities have a pretty solid signal for >3" (80-90+%) and >5"
(50-80%) confined right in that far Southern fringe of the Lone
Star State with a precipitous drop off in the prob fields as you
extend westward. This correlates well with the blended mean QPF and
the recent ML output dictated within the ECMWF AIFS and GFS
Graphcast outputs. There was enough consensus for tightening the
SLGT risk closer to the coastal plain and dropping the northern
fringe of the SLGT a bit further south to match the probability
fields. This is an evolving situation with the future of the
disturbance still in question, so until guidance has a more defined
structure of the tropical system, it will be hard to discern finer
details on the threat. For now, there was enough agreement on the
SLGT risk maintenance with some minor adjustment.

Kleebauer

..Previous Discussion..

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.

Wegman

...Four Corners Region...

20Z Update: One change will be noted this forecast cycle with an
extension of the MRGL risk further north into Northeastern UT
where the core of the heaviest QPF is being depicted on both hi-res
and global deterministic. The signal is in conjunction yo a
trailing shortwave that will cross through the Inter- Mountain
west Monday afternoon while situated on the fringe of the left exit
region of an upper jet cruising into the Southwestern U.S. This
allowed for the primary convective axis to be positioned over the
Wasatch down through Central UT for the period. The rest of the
forecast wavered very little from run to run leading to general
continuity for the remainder of the forecast.

Kleebauer

..Previous Discussion..

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.

Wegman

...Rest of the Gulf Coast...

20Z Update: Little to no changes were made to the MRGL risk area
located across the Southeast. Heavy rain cores will be plausible
in-of much of the FL Peninsula with the heaviest focus over the
interior. A strong signal within the probability fields for >3"
(80-90+%) exists with a sharp decline in the probability for
anything over 5". This limits the upper end of the potential, thus
maintaining the previous MRGL risk signal inherited.

Kleebauer

..Previous Discussion..

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...

...20Z Update...

The tropical disturbance in the Western Gulf will continue to
progress northward through the course of Tuesday into Wednesday
with the western fringe of the precip shield becoming aligned
within the TX coastal plain. This forecast package has seen some
deviation away from the precip extending further west into TX
leading to an adjustment tighter to the coast for the SLGT risk
forecast. The biggest question becomes the organized nature of the
low and how close will it be to the coast of TX to orient enough
moisture to affect to the area of interest within the SLGT. A
further east track of the primary low will create a very focused
corridor of heavy rain right at the coast, limiting the potential
somewhat for greater urbanized zones just inland of the coastline.
Increasing southwest shear on the western quadrant of any organized
system will also have the ability to thwart organized convection
and really hamper the ability for any heavy rain to occur inland
which is noted by a few global deterministic and ensemble members
of the EPS/GEFS combo. This is the period to watch as it pertains
to greater flash flood impacts ramping up with the disturbance in
question. As of now, the only change was that tightening of the
SLGT closer to the coast, but still manage a broad area of coverage
for the risk in question due to some uncertainty and the ensemble
mean QPF depicting the threat still being on the table.

Across the west, the MRGL risk over CO was expanded back to the
west into UT to account for a lingering convective pattern over the
terrain with the primary focus within the Wasatch. The signal is
less enthusiastic on the potential compared to the previous period,
but does fall within the low-end criteria of the MRGL with pockets
of higher QPF located in-of the mountains where flood concerns are
highest. The remainder of the area was relatively unchanged as
scattered thunderstorms are forecast across Western CO with the
more prone slot canyons, arroyos, and burn scars still the zones of
highest flash flood opportunity.

Kleebauer

..Previous Discussions..

...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.

...Inter-Mountain West...

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