FOUS30 KWBC 100727
QPFERD

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

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

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

...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
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