Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Mon Sep 9 2024
Day 1
Valid 01Z Tue Sep 10 2024 - 12Z Tue Sep 10 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...
...01Z Update...
...Coastal Deep South Texas...
Outer bands from Tropical Storm Francine will continue to cross the
southern tip of Texas near Brownsville this evening with more
substantial bands pushing in over the Lower Rio Grande Valley up
into the King Range area after midnight. The HREF neighborhood
probability for >5" is sufficiently high (70-90%) in the
Brownsville area with slightly less probs further west to
Harlingen. Farther west to McAllen, the probabilities drop off
significantly for accumulations greater than 3". Recent HRRRs
agree with this, so the Slight Risk remains for the Brownsville
area.
..Previous Discussion..
...Louisiana Coast...
Trimmed the north side of the Marginal Risk a bit given radar
trends and recent HRRRs. The 22Z HRRR does have a band of heavy
rain along the MS Coast extending east into southeast LA late
tonight. This may be overdone given the stationary front fairly
south over the northern Gulf, but it seemed within reason to leave
New Orleans in the Marginal.
Nocturnal trends in convection are seen over FL, particularly in
the last hour, so the Marginal Risk has been pulled from the FL
Peninsula.
...Desert Southwest...
High-based convection continues southern UT and northern AZ this
evening. Localized rainfall rates of half inch per hour are likely
to continue for a couple more hours per typical diurnal trends and
recent HRRR runs. Given the slot canyon sensitivity, the Marginal
Risk was trimmed to just these areas of UT/AZ for overnight.
Jackson
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Mon Sep 9 2024
Day 2
Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
GULF COAST WEST OF PENSACOLA..
...20Z Update Summary...
Risk areas across the Gulf coast of Texas were tightened a bit
closer to the coastal plain with the removal of the SLGT over
portions of the TX coast near Corpus Christi. The MRGL risk was
expanded to the southeast over the Southern Rockies.
Kleebauer
...Gulf Coast...
20Z Update: Now Tropical Storm Francine will continue making
headway to the north with a strengthening core, eventually forecast
to become a hurricane within the next 24-48 hours. Guidance
continues to show a general contraction of the storm size as it
matures over the Western Gulf leading to less of a tropical rain
threat over portions of coastal TX as it moves latitudinally. This
will inhibit the western periphery of the heavy rain potential as a
majority of the convective cores around the circulation will be
adjacent to the coast, but just off-shore. There are some
indications that rogue bands could sway further away from the outer
periphery of the circulation leading to isolated instances of
heavier rainfall, especially along the concave portion of the TX
coast, including the Corpus Christi/Port Aransas/Rockport areas up
towards Port Lavaca. Ensemble mean QPF and relevant neighborhood
probabilities for rainfall >3" have deviated away from those areas
being a beneficiary of some of the heavier tropical footprint that
would necessitate a higher risk, including the previous SLGT
forecast. With the consensus growing on the track being slightly
more east of the previous NHC forecast cyclone track, the SLGT risk
located between central Kenedy County up through Port Lavaca was
removed with a maintenance of the MRGL risk to account for the
rogue heavier cells that may protrude the immediate coast in those
areas.
Across the Brownsville area, recent trends have led to a general
continuity over the far southern reaches of TX with a strong signal
for heavy rain likely to affect points along and east of McAllen
with the higher end potential likely situated over Harlingen to
Brownsville and South Padre Island. Additional 2-4" of rain with
locally upwards of 6" will be plausible tomorrow morning before the
primary bands lift north with the forward propagation of Francine.
This will be on top of what will occur in the prior period leading
to storm totals of 4-8" with locally up to 12" for some of the
areas hit by repeating bands. The highest significant flash flood
threat will reside in the urban corridors across Deep South TX,
including Brownsville proper.
Further north, the prospects increase gradually for a locally
significant heavy rain threat, but generally maintained along the
immediate coast as the cyclone jogs north-northeast once it reaches
the latitude of Galveston. The northern periphery of the cyclone
will begin edging onshore of the Upper TX coast over into Southern
LA with multiple bands of heavy rain likely impacting the area from
the lower Sabine over into Morgans City. Depending on the forward
speed of Francine, the timing could be off +/- 4 hours leading to a
significant difference in impacts for the period. The SLGT from
previous forecast was relatively unchanged in its location and
inland placement to account for those uncertainties in the timing.
Regardless, the threat for significant rainfall will be
increasingly steadily over much of Southern LA, including over
towards New Orleans where onshore easterly flow will advect deep
tropical moisture ahead of the cyclone with streaming convection
moving off the Gulf into Southeastern LA.
Kleebauer
...Inter-Mountain West and Southern Rockies...
20Z Update: 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. Regional instability
is enough to maintain a relatively solid pulse convective regime
that would allow for a few stronger cores that if falling over the
wrong area (Slot canyons, burn scars, etc.), could cause some
issues locally. The biggest adjustment from the previous forecast
was an expansion of MRGL risk into Northern CO as recent CAMs and
some of the ensemble mean QPF signaling a threat of isolated
thunderstorms over the Sangre de Cristos, an area that has a
heightened risk of flash flooding due to multiple burn scar areas.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Mon Sep 9 2024
Day 3
Valid 12Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE
MISSISSIPPI DELTA...
...Southeast...
20Z Update: Tropical Storm Francine will likely strengthen to a
hurricane prior to landfall on Wednesday morning with a well-
defined axis of heavy rainfall making a steady progressing through
much of Louisiana and the eastern fringes of TX near the state
lines. The western periphery of Francine will likely be going
through a strong vertical shear pattern that will inhibit the
tropical convective pattern within the western half of the cyclone.
Some of the pivoting bands on the northwestern quad will likely
make progress towards the Lower Sabine which could cause some flood
issues, but the story will be further east where the shear pattern
will be less of a deterrent to the heavy rain footprint with an
increasing consensus of 4-8" with local to 10" situated between the
area of Sabine Pass over to Morgan City and points north. A
secondary maxima over Southeast LA could occur with the prevailing
onshore component on the eastern fringes of Francine's circulation.
The New Orleans metro will be the most susceptible to flash flood
concerns thanks to the urbanization factors with higher run off
potential in the city limits. This area has also experienced a fair
amount of rain leading into the event, so the soils will be primed
relative to what is typical to the area. The MDT risk was
maintained over the eastern extent of LA with only a small
adjustment on the northeast fringe to include the Jackson, MS metro
corridor due to the expected heavy rainfall at the back end of the
period.
The eastern sections of the MRGL and SLGT risk areas will have the
highest threat for locally significant impacts away from the storm
center thanks to the sheared component, even as the storm moves
further inland. It's not until D4 when the storm will begin the
extra-tropical transition leading to a broader expanse of heavier
precip within the western side of the remnant circulation. For D3,
the heaviest rain will remained confined to the core and points
east leading to the sharper drop off in the risk as you move
outside of the TX/LA border.
...Intermountain West...
20Z Update: A potent mid-level shortwave will allow for a broad
axis of upper ascent leading to scattered convection over the
Northern Rockies with a maintenance of the MRGL flash flood risk
over Western MT into Northern ID. Some of the higher QPF may be in
the form of snow, so will have to assess the setup closely for the
snow level forecast to discern if a targeted SLGT risk is still
relevant. The disturbance is fairly robust, so the threat of small
areal upgrade is non-zero.
Kleebauer
..Previous Discussion..
A Marginal Risk was introduced for this region as a strong digging
shortwave trough interacts with topography and above normal
moisture for this region. Forecast amounts may be high enough to
potentially need a targeted Slight with future updates in this
area.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Sep 9 2024
The core of the heaviest rainfall associated with a weakening TS
Francine is forecast to be across eastern Arkansas and into western
Tennessee and northern Mississippi by Thursday, and the QPF from
the ECMWF/GFS best matched the NHC track and is overall more
progressive compared to earlier forecasts. Therefore, the Slight
Risk area on Day 4/Thursday has been nudged northward some from
overnight continuity, and also extends eastward over western
Alabama where the inflow band may reside and result in locally
higher rainfall totals as well. Given the recent dry conditions and
remaining uncertainty in the swath of heaviest QPF, there is no
need for any Moderate Risk areas for the Day 4 time period at this
time, but may be needed for future updates. For the Day 5/Friday
time period, the overall QPF decreases as the low lifts farther
north and steadily weakens across the Ohio Valley region, where a
Marginal Risk is planned. This Marginal Risk also includes the
southern Appalachians where moist upslope flow could produce even
higher rainfall totals, and southward towards the Florida panhandle
along the front/inflow.
Otherwise, the trough arriving over the West will result in
numerous showers across the Northern Rockies. The heaviest QPF from
this will likely be over northwestern Montana, with snow for the
highest mountain ridges. Closed upper low passage and increased
ensemble enhanced QPF probabilities seems to also support a
Marginal Risk ERO area for Day 4/Thursday. Showers and some storms
then reach the central/northern Plains by Friday, while the Great
Lakes and the Northeast U.S. remains mostly dry.
Temperatures will still feel more like August across much of the
north-central U.S. through the end of the week and into the weekend
for both highs and lows, with potential +20 degree anomalies for
lows across North Dakota and northern Minnesota by Friday morning.
Meanwhile across the West Coast region and the Intermountain West,
a return to refreshingly cooler temperatures is expected by midweek
in response to the building upper trough, and feeling more like
October. Widespread highs in the 60s can be expected for the lower
elevations behind the cold front. It will likely be cooler than
average across portions of the Mid-South for daytime highs to close
out the week owing to increased clouds and rainfall.
Santorelli/Hamrick
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Sep 9 2024
The core of the heaviest rainfall associated with a weakening TS
Francine is forecast to be across eastern Arkansas and into western
Tennessee and northern Mississippi by Thursday, and the QPF from
the ECMWF/GFS best matched the NHC track and is overall more
progressive compared to earlier forecasts. Therefore, the Slight
Risk area on Day 4/Thursday has been nudged northward some from
overnight continuity, and also extends eastward over western
Alabama where the inflow band may reside and result in locally
higher rainfall totals as well. Given the recent dry conditions and
remaining uncertainty in the swath of heaviest QPF, there is no
need for any Moderate Risk areas for the Day 4 time period at this
time, but may be needed for future updates. For the Day 5/Friday
time period, the overall QPF decreases as the low lifts farther
north and steadily weakens across the Ohio Valley region, where a
Marginal Risk is planned. This Marginal Risk also includes the
southern Appalachians where moist upslope flow could produce even
higher rainfall totals, and southward towards the Florida panhandle
along the front/inflow.
Otherwise, the trough arriving over the West will result in
numerous showers across the Northern Rockies. The heaviest QPF from
this will likely be over northwestern Montana, with snow for the
highest mountain ridges. Closed upper low passage and increased
ensemble enhanced QPF probabilities seems to also support a
Marginal Risk ERO area for Day 4/Thursday. Showers and some storms
then reach the central/northern Plains by Friday, while the Great
Lakes and the Northeast U.S. remains mostly dry.
Temperatures will still feel more like August across much of the
north-central U.S. through the end of the week and into the weekend
for both highs and lows, with potential +20 degree anomalies for
lows across North Dakota and northern Minnesota by Friday morning.
Meanwhile across the West Coast region and the Intermountain West,
a return to refreshingly cooler temperatures is expected by midweek
in response to the building upper trough, and feeling more like
October. Widespread highs in the 60s can be expected for the lower
elevations behind the cold front. It will likely be cooler than
average across portions of the Mid-South for daytime highs to close
out the week owing to increased clouds and rainfall.
Santorelli/Hamrick
