Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Tue Sep 10 2024
Day 1
Valid 01Z Wed Sep 11 2024 - 12Z Wed Sep 11 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...
...01Z Update...
Removed the Marginal Risk area from South Texas now that Francine
has had some time to pull away from the region on its northeastward
track. Also removed the Marginal risk area from the Western states
given the modest (at best) rainfall rates during the afternoon and
how the loss of daytime heating will only work to minimize
instability going forward. The expectation is that convection
across Florida should gradually weaken and dissipate overnight but
felt it was too early to remove the risk area with showers and a
few thunderstorms still in the area. Remainder of the outlook
remained on-track and few changes needed.
...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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EDT Wed Sep 11 2024
Day 1
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 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 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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EDT Wed Sep 11 2024
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 rain totals
to 2" and local amounts to 4" appear possible.
Roth
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
