Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Tue Sep 3 2024

Day 1
Valid 01Z Wed Sep 04 2024 - 12Z Wed Sep 04 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTH TEXAS...

...01Z Update...
Removed the Marginal risk area from the northern Rockies given
radar trends. With loss of daytime heating...little additional
instability is expected and the potential for any additional heavy
rainfall has diminished.

Trimmed some area of the Slight and Marginal risk on the western
and northern side...leaving the focus across the central portion of
the state south- and eastward to the Gulf coast. High resolution
convective allowing models continue to suggest some organization to
the convection into the overnight hours with potential for heavy to
excessive rainfall.

Introduced a Marginal risk in the northeast corner of the Florida
peninsula into nearby coastal Georgia where a weak circulation
riding along a quasi stationary boundary could help pull enough
ingredients together for localized heavy rainfall later tonight and
into the early morning hours. Also in Florida, realigned a
previously issued Marginal risk area and confined it closer to the
Apalachee Bay area where radar has been showing a persistent band
of showers over the Gulf waters that were beginning to drift
northward. The expectation is that the threat of heaviest rainfall
and any associated excessive rainfall should be close to the coast.

Bann


...13Z Outlook Update...

...Texas...
Slight risk areas were combined and expanded to include more of
south Texas (near the Del Rio area). This morning, slow-moving
convection has materialized and produced areas of 1-2.5 inch/hr
rain rates just east of the Rio Grande. These storms are in a
sufficiently moist/unstable environment with weak flow aloft for
continued heavy rainfall. Low FFG thresholds (near zero in spots)
were also noted. The flash flood threat should persist for most of
the D1 forecast period under this regime. Reference MPD #0968 for
additional mesoscale details.

...Southeastern Mississippi eastward to northern Florida...
Latest CAMs indicate a robust axis of convection materializing amid
weak flow aloft from the Pine Belt of Mississippi eastward to the
Jacksonville Metro area. HREF probabilities of 24-hr, 3 inch+
precipitation totals maximize near Jacksonville, where models
depict a persistent fetch of deep convection originating along a
frontal boundary over the Gulf Stream. Multiple rounds of
precipitation are expected today through the early overnight
hours especially across northeastern Florida. Localized flash
flood potential is expected where rainfall can occur over
sensitive/urbanized locales. The Marginal risk has been expanded
northwestward and eastward to include areas of isolated flash flood
potential.

The remainder of the forecast is unchanged. Refer to the previous
discussion below for more details.

Cook

...Previous Discussion...

...Texas...

The mid-level vort is expected to exit the region although
precipitable water values will remain high. Flooding issues will be
possible in the area with a widespread 1 to 1.5 inches forecast
with locally higher amounts. A Slight Risk remains in effect for
central portions of the state although it was expanded y 1 tier of
counties to the south to reflect the latest trends in the guidance.
A slow moving tropical low will continue to bring periods of heavy
rain in the form of slow- moving showers and thunderstorms from
the Gulf into the adjacent Texas Coast. Most areas will not see
steady rain the entire time, but very localized areas of training
storms resulting in flash flooding. The heaviest rains in the
state are expected to be along the Gulf Coast from Galveston south
and west through Corpus Christi. Locally heavy rains to 2 inches
per hour will be possible.

...Mississippi and Alabama Gulf Coast...

The Marginal Risk was maintained for the potential of slow moving
and training storms to the east of the low along a stalling front
into coastal Mississippi and Alabama. The abundance of atmospheric
moisture and instability, as well as, the slow-moving nature of
the synoptic systems increases confidence that an isolated flash
flood or two could develop along this stretch of coast.

...Idaho/Montana...

There is potential for isolated to scattered heavy rainfall as an
advancing shortwave encounters an anomalously moist airmass. PW
values rising to near 1 inch will support storms capable of heavy
rain that may produce isolated flash flooding should the storms
impact particularly flood sensitive or more urban areas.

Campbell


Day 2
Valid 12Z Wed Sep 04 2024 - 12Z Thu Sep 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI
DELTA REGION AND ADJACENT GULF COAST...

...20Z Update...

As 12Z guidance rolled in today, the precipitation shield was not
as progressive as previous forecast cycles. A blend of CAMs/global
guidance has shifted the focus for heavier rainfall to be along the
Upper Texas coast where they will be located closest to an
organizing and elongated area of low pressure in the western Gulf
of Mexico. NAEFS continues to depict a swath of PWs that are
topping the 99th climatological percentile from as far south as
Brownsville to as far east as Pensacola. The area most at risk will
likely be found from the Upper Texas coast to southern Louisiana where
the combination of surface-850mb theta-e advection directly into
or quasi-parallel to the stationary front will trigger more
widespread thunderstorm activity. Southeast TX also still sports
>80% soil moisture percentiles within the 0-40cm layer according to
NASA SPoRT-LIS, suggesting soils there would be more sensitive to
2-3"/hr rainfall rates.

The other adjustment to the forecast was to expand the Marginal
Risk east over the Florida Panhandle where an influx of anomalous
PWs and modest instability along the stalled frontal boundary may
trigger additional thunderstorms capable of producing torrential
downpours. Areas most at risk for possible flash flooding will be
areas whose soils are more saturated following today's thunderstorm
activity and as far east as the Jacksonville metro area.

Mullinax

--Previous Discussion--

...Gulf Coast and Lower Mississippi Valley...

The surface front will continue to be draped west to east across
the South, albeit further south than previous forecast cycles. This
frontal boundary will provide focus for showers and thunderstorms
to spread across the entire Gulf Coast from south Texas all the
way to the Florida Peninsula on Wednesday. Most of the guidance
shows 1 to 3 inches will concentrate over this part of the region
thus increasing the risk for excessive rainfall and local flooding
potential.

There is the potential for a few locations for local
maximums to exceed 6 inches near northern Louisiana/Arkansas.
Onshore flow across southeast Louisiana and southern Mississippi
could yield 1 to 4 inches as well. The focus for the heaviest rains
will be in the central Gulf Coast from Louisiana east through the
Florida Panhandle. Locally heavy rains are likely, especially from
New Orleans east through Pensacola, where a higher-end Slight is in
place. With increasing amounts of rain forecast in and around New
Orleans, continued increases in rainfall there may require an
eventual targeted Moderate Risk should trends continue.

Given the trends, the back edge of the Marginal Risk was trimmed
eastward toward the Texas coast in addition to reducing the
northern boundary by 1 tier of counties across Alabama and
Mississippi.


Campbell/Wegman


Day 3
Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL
GULF COAST...

...20Z Update...

Similar to the Day 2 update, guidance has trended slower with the
progression of the QPF associated with an elongated area of low
pressure in the northwest Gulf of Mexico. This led to the threat
areas being drawn further west this cycle with the focus for heavy
rainfall being centered over southern Louisiana. However, there
remains a notable amount of spread in both the northern extent of
the QPF axis and in rainfall totals. The GEFS remains more of a
northern outlier compared to ECENS/GEPS members. While dispersion
is on the higher side on Day 3, there is a better consensus for
heavier rainfall along the central Gulf Coast given the bulk of PWs
>99th climatological percentile residing along the Gulf Coast. The
region is also closer to the surface-850mb southeasterly flow that
will sustain a steadfast regime of low level theta-e advection
into the nearby stationary frontal boundary. Additional forecast
changes are likely given so much of this setup lies with the
development of low pressure along the Gulf Coast.

Elsewhere, a Marginal Risk was added over portions of the Southern
Rockies due to the PWs topping 0.75" and up to 500 J/kg of MUCAPE
out ahead of an approaching cold front. Given the region's
sensitive soils, both from saturated soils and burn scars, opted to
issue a Marginal Risk for the localized flash flood potential
there Thursday afternoon and evening.

Mullinax

--Previous Discussion--

...Gulf Coast and Lower Mississippi Valley...

The area of precipitation mentioned for the Day 2 period will
shift eastward across the central Gulf Coast with the heaviest QPF
still focusing across portions of southeast Louisiana, southern
Alabama, southern Mississippi and clipping western portions of the
Florida panhandle. Again, there was a notable southern shift in
placement within a majority of the guidance from previous
cycles.coastlineThe inherited Marginal Risk areas was removed
from southern Tennessee and northern portions of Alabama,
Mississippi and northwest Georgia while trimmed eastward across
central Louisiana. The Slight Risk spans along the Gulf Coast from
west-central Louisiana to eastern Choctawhatchee Bay Florida.

Campbell


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