FOUS30 KWBC 111603
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1203 PM EDT Wed Sep 11 2024

Day 1
Valid 16Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024

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

...16Z update...

The latest NHC track is very similar to the previous issuance 
therefore no significant adjustments were needed for the Moderate 
and Slight Risk areas that follow the core path of Francine as it 
approaches the coast, makes landfall and then moves inland. The 
latest guidance did show a decrease in QPF across portions of 
eastern Texas and Oklahoma therefore the Marginal Risk area was 
trimmed eastward by 1-2 tiers of counties to reflect the reduction 
in threat for excessive rainfall.

The latest hi-res and global guidance continues to show convective
activity firing up along the higher terrain areas of western
Montana and central Idaho, with the potential for hourly rainfall
rates to pulse up to 1.5 inches/hour. The inherited Slight Risk
area encompasses where the majority of the guidance suggest an
elevated threat for excessive rainfall and isolated to scattered
instances of flash flooding concerns

Campbell

...Southeast...

Francine should begin extratropical transition and become highly
sheared around, if not before, the time of landfall late on
Wednesday, with a developing dry slot possible. This should cause
sheared convection initially expected to prefer the north and
northeast quadrants -- near and left of track -- and eventually a
convective comma head and an inflow band/baroclinic trough/
developing front becoming increasing removed to the east of its
center which would be foci for heavy/excessive rainfall. The
cities of 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. Southern Louisiana is
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, though gradients were
sharpened across LA and the slower track for Francine inland caused
some retreat of the northern fringes of 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 the Slight Risk area remains for portions
of western MT. Hourly rainfall rates up to 1.5 inches/hour will be
possible as individual cells pass through along with a daily
accumulations up to 3 inches in isolated locations; which would be
most impactful in the vicinity of burn scars. Some of the QPF 
would be in the form of snow at the highest elevations of the 
northern Continental Divide.

Roth/Campbell


Day 2
Valid 12Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024

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

...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 which shows a greater signal for excessive rainfall
overnight. Broadened the risk area to encompass the possible
training rain band, as enough moisture, instability, and possible
stalling to be concerned about hourly rain totals to 3".

...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 Slight Risk raised by continuity has been
maintained across portions of western MT. Hourly rainfall rates may
intensify up to 2 inches/hour during this period, with a few 
isolated locations possibly accumulating up to 4 inches by the end
of the day.

Roth/Campbell


Day 3
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MID-SOUTH AND SOUTHEAST...

A broad Marginal Risk of excessive rainfall exists for portions of
the Mid-South and Southeast with the weakening extratropical/Post-
Tropical Cyclone Francine. The guidance isn't showing a great deal
of overlap in potential heavy rain areas, so left the risk area
as-in, but in theory, portions of northern FL and the Southeast,
possibly including the southernmost Appalachians, have potential
for hourly rain totals to 2.5" and local amounts of 5" as enough
instability and low- level inflow/effective bulk shear exists for
possible issues. Should the guidance align on this idea QPF-wise
further, a Slight Risk could be added in later cycles.

Roth
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