Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Tue Sep 10 2024

Day 1
Valid 16Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024

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

...16Z Update...

...Central Gulf Coast and Florida...

Rainfall along the far south Texas Gulf coast should come to an
end by early afternoon, with the expectation that the risk for any
additional heavier rainfall should remain along the immediate
central and upper Texas Gulf Coast as Francine tracks to the north-
northeast. The greater threat will be with bands of convection
already focused along the central Gulf coast far to the northeast
of Francine, with additional bands expected later this evening and
overnight as the center of the system approaches from the
southwest. Updated 12Z hi-res guidance shows remarkably good
agreement on the heaviest rainfall footprint along the central Gulf
Coast, stretching from near Cameron, LA east through southeastern
LA and to the immediate coast of MS. Areal average totals through
early tomorrow morning are on the order of 2-3" with locally higher
amounts of 3-6" possible with training bands.

Additional storms are expected further east in vicinity of a
quasi-stationary frontal boundary draped through central Florida.
Enhanced moisture in the area from Francine will help contribute to
some locally heavy downpours with rain rates of 2-3" per hour,
which could lead to some isolated flooding/ponding issues in urban
areas.

...Intermountain West...

General footprint for the current Marginal Risk remains the same
with a small northwestward extension into southwestern Wyoming to
cover additional areas of scattered convection expected this
afternoon. Latest hi-res guidance indicates the potential for some
locally heavy storm total rainfall of 0.5-1", possibly as high as
1.5", as well as some low-end HREF probabilities (20-30%) of
exceeding 2-year ARIs. These totals may be enough to lead to some
isolated concerns for flash flooding in the usual more terrain
sensitive areas such as slot canyons, dry washes, and any burn
scars.

Putnam

...Previous Discussion...

...Gulf Coast...
Tropical Storm Francine will continue making headway to the north
and northeast towards Louisiana, with NHC forecasting the system to
become a hurricane.  The combination of dry/cool air to the west
and northwest of the storm will lead to a tight rainfall gradient
on its west side. Further enhancing the western gradient will be
the increasing vertical wind shear with time out of the west-
southwest, which will attempt to shift convection downshear and
downshear left of its center (to its northeast). With QPF
continuing to trend downward, the Slight Risk for the TX coast was
removed this cycle.  Across the immediate Brownsville and Port
Isabel area, recent heavy rains could make them more sensitive to
additional rainfall this morning. Even though the risk area was
reduced to Marginal, Slight Risk impacts cannot be ruled out.

Prospects increase gradually along the central Gulf Coast for
heavy rainfall. While initial bands from the periphery of tropical
cyclones can quickly erode existing instability ashore, portions of
Acadiana and Southeast Louisiana along with far southern MS have
had an exceeding wet week from a cyclone that ultimately merged in
with Francine, so keeping the inherited Slight Risk makes sense.
Also, guidance tends to bias slow with tropical cyclone
recurvature, and this Slight Risk area can account for some
possible track acceleration between now and its coastal approach.
As with any warm core cyclone, the low-level inflow will exceed
the mean flow, aiding precipitation efficiency. Plentiful effective
bulk shear will organize convection into curved bands within its
cyclonic circulation.  Hourly rain totals to 3" will be possible
with any bands that manage to train.


...Florida...
An unusually far south synoptic front for Mid-September focuses
very high moisture east of Francine where, at times, effective bulk
shear could flirt with 25 kts. Where semi- organized and organized
convection manages to form, hourly rain totals to 3" and local
amounts to 6" are considered possible where storms merge, train, or
get tied to the boundary. The 00z NAM and 00z Canadian were
especially bullish with local maxima, both remained widely
scattered to isolated in coverage of the heaviest rainfall. This
kept the risk at the Marginal level.

Another area with sufficient instability and moisture is behind
the front near the northeast FL coast. Adding to possible issues
there is low level and upper level flow trying to zero out. Short
bands of efficient convection, which could lead to hourly totals to
2" and local amounts to 4", are possible. Kept this area in the
Marginal Risk to help support their flood watch.


...Inter-Mountain West and Southern Rockies...
A scattered convective footprint is anticipated across portions of
the West with the main threat residing over the Southern Rockies
into the Wasatch of UT. The threat is on the lower end of
probabilities considering the middling signal within the latest
HREF for >1" chances over the region. However, with precipitable
water values near 0.75" and sufficient instability, localized flash
flooding can't be ruled out in any dry washes/arroyos, burn scars,
slot canyons, or steep topography where hourly rain totals manage
to reach 1.5". The Marginal Risk area maintained showed good
continuity.

Roth



Day 2
Valid 12Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...

...Southeast...
Francine should begin extratropical transition and become highly
sheared around, if not before, the time of landfall Wednesday
afternoon/evening, with a developing dry slot possible. This should
lead to a convective comma head and an inflow band/baroclinic
trough/developing front becoming increasing removed to the east of
its center which would both be foci for heavy/excessive rainfall.
The cities of Lafayette, Baton Rouge, and New Orleans (along with
their suburbs) would be most susceptible for flash flooding as
hourly rain totals to 4" (if not more) should be possible,
particularly within the trailing band/forming front to its
southeast. Acadiana and Southeast Louisiana are more susceptible
due to heavy rainfall from this past week caused by the cyclone
which merged in with Francine, so the soils will be more primed
relative to what is typical to the area. Continuity was generally
maintained in the risk areas.


...Intermountain West...
A deep layer cyclone moving into MT brings moderate to strong
upslope flow into western portions of the state, along with MU CAPE
of ~750 J/kg. Precipitable water values are sufficient for heavy
rain-related issues, so increased the risk area to Slight for
portions of western MT. Hourly rain totals to 1.5" with local
amounts to 3" are expected, which would be most impactful in burn
scars. Some of the QPF would be in the form of snow at the highest
elevations of the northern Continental Divide.

Roth


Day 3
Valid 12Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR THE MID-SOUTH...

Mid-South/Southeast...
Francine is expected to be fully extratropical, with a well-
developed comma head and trailing front. Guidance suggests that MU
CAPE will be modest, 250-500 J/kg. Effective bulk shear should
continue to be sufficient for convective organization where enough
instability develops/exists.  Precipitable water values remain
near 2", so hourly rain totals up to 1-2" should be possible, which
would be most problematic in urban areas and initially beneficial
elsewhere. This region has been quite dry lately. With the volume
of rain expected, and the concept of this being on the cusp of a
flash flood and longer duration flood event, kept the Slight Risk
area which showed minimal change from continuity. Across the
Southeast, there is a risk of training convection along Francine's
cold front, but its progression is expected to limit issues. Still,
hourly rain totals to 2" and local amounts to 4" should be
possible.


Western MT, Idaho Stovepipe & Vicinity...
One or two surface lows are expected to lift northward this
period through MT, drawing in sufficient upslope flow, moisture,
and instability from the east to be concerned about the potential
for organized convection. The guidance signal is not cohesive as of
yet, so left the risk across the area as Marginal. Should the
guidance get more agreeable or confidence increase, a Slight Risk
could become necessary in later updates. Hourly rain totals to 2"
and local amounts to 4" appear possible.

Roth


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