Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Mon Sep 16 2024

Day 1
Valid 16Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEASTERN
NORTH CAROLINA...

...Carolinas...

Potential Tropical Cyclone Eight continues to churn off the
Carolina coast with an ill- defined...north to south elongated 
center of circulation that has largely become exposed with broad
shield of rain to the north. The first push of higher rainfall
rates has started to impact mainly coastal areas of the North
Carolina near the border with South Carolina...with a few ground-
truth in excess of 6 inches. Current thinking is that the potential
for intense rainfall rates will continue to be focused along the
convergent flow of east/northeast flow off the Atlantic and the
flow of drier air wrapping around the southern/eastern flank of the
system. The steepening lapse rates and the added low level
convergence between the two streams should be working in tandem to
support isolated areal coverage of additional 4 inches to 8 inches
of rain mainly confined to areas along the immediate coast. Farther
inland across North Carolina and adjacent South Carolina...rainfall
amounts are not likely to be as high given lower dewpoint
temperatures/precipitable water values but still enough rainfall
that can result in flooding/flash flooding. The amount of dry air
getting pulled eastward/southeastward on the southern flank of
Potential Tropical Cyclone Eight has allowed confining the
Moderate Risk area along and north of the North Carolina/South
Carolina border. The latest HREF signal for >5" remained very
robust with probabilities between 70-90% located across southeastern
North Carolina. As previously noted...greatest concern for flash
flooding remains the urban areas. 

...Northern Plains and Upper Midwest...

Aloft, a shortwave will track from the Central Plains to the
Northern Plains later tonight. This should help renew convection
over portions of the Dakotas to northwestern Minnesota later this
evening. Given ample mid-level ascent conducive for convective
initiation, a pre-frontal surface trough that is expected to
develop over the southern Canadian Prairies that builds into the
Dakotas, and a formidable corridor of high theta-E that bisects the
area, some enhancements of rainfall rates could result in instances
of flooding.

...Western U.S...

12Z upper air analysis indicates a broad closed upper circulation
positioned over northern California with an expectation that it
will continue to strengthen throughout the period. Increasing
moisture on the eastern flank of the mean trough will allow for a
better convective environment as the region enters a strong upper
forcing regime with a correlated instability maximum located over
the Great Basin. The approach of the upper low will initiate a
blossoming of convection across the eastern and northern Sierra,
extending into Nevada and eventually Idaho through this evening.
MUCAPE between 500-1000 J/kg will be most common within the
interior, plenty to induce a swath of enhanced convection over an
area not typically acclimated to see a decent amount of rainfall.
The current forecast calls for a multi-region impact of
convectively driven rainfall that could easily exceed 1" in any
area located from the Great Basin, Four Corners, and portions of
the Southwestern U.S.

...Deep South...

Lingering low-level moisture with the proximity of a quasi-
stationary front across the Deep South will allow for another
round of locally enhanced rainfall that could cause some flash
flood concerns, generally over the Lower Mississippi Valley into
the Central Gulf coast. The signal is more diffuse than previous
days but the probability fields still indicate a formidable
potential for >3", especially along the Gulf coast where the
surface based instability is most prominent. Maintained the low end
Marginal risk area with little change...especially given some of
those areas were hit hardest the past several days from Francine
and its remnants.

Bann

Day 2
Valid 12Z Tue Sep 17 2024 - 12Z Wed Sep 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID ATLANTIC, FLORIDA PANHANDLE, SOUTHERN ROCKIES, AND WESTERN
MONTANA...

...Mid Atlantic...

The remnant cyclone will be perusing through the Mid Atlantic come
Tuesday with its moisture flux steadily pushing northward through
the Central Mid Atlantic, carrying a potential for locally enhanced
rainfall and flash flood prospects. As of this time, the consensus
is still muddled on exactly where the remnant circulation will be
progressing, as well the forecast instability positioning on the
eastern flank of the circulation. PWAT anomalies will be creeping
closer to 1-2 standard deviations above normal, a historical
characteristic for heavy rain and flash flood concerns within the
Mid Atlantic. Ensemble instability fields are generally more
defined in-of the Blue Ridge in VA and across Southeastern VA.
These would likely be the areas with the most interest when it
comes to any higher risk. The shifts within the ensemble mean QPF
add too much variance to conclude the prospects for a higher risk,
at this time. With coordination from the local WFOs across the Mid
Atlantic, a MRGL was maintained from the previous forecast with
some adjustments based on the latest ensemble QPF footprint.

...Florida Panhandle...

Lingering low-level moisture flux tied a quasi-stationary front and
subsequent weak surface reflection along the Gulf Coast will allow
for one more period of isolated to widely scattered thunderstorms
within the Western FL Panhandle, bisecting an area that is
currently under more sensitive FFG indices compared to
climatological averages. The most prone location resides near
Apalachicola where recent 5-day rainfall totals are upwards of
8-15" leading to a fairly water-logged position along the immediate
Gulf coast. Despite the signal not being as impressive as previous
periods, the instability gradient right along the coast could
provide the necessary element to maintain a risk for locally heavy
rainfall, especially any cells tied to the boundary running
parallel to the coast. This is a lower-end MRGL risk categorically,
but with forecasts within the deterministic still pointing towards
cells producing >2" of rainfall, this was enough of a signal to
warrant the MRGL risk inherited from the previous forecast.

...Southern Rockies...

Ejecting shortwave trough out of the Four Corners will provide
ample mid and upper support for a generation of scattered
convection located across the Southern Rockies down into the
Sacramento Mountains in NM. The setup is geared towards locally
enhanced rainfall prospects tied to the terrain with the best
opportunity for impacts likely occurring over burn scars located
within the Sangre de Cristos and the Sacramento Mountains near
Ruidoso. Those two locations are very sensitive, and the setup
would certainly be conducive to providing local impacts within the
two separate MRGL risk areas. Areal QPF average is between
0.25-0.5", but deterministic output still signals potential for
upwards of 1.5-2" of rainfall, plenty to cause issues within the
complex terrain and over any sensitive grounds. A MRGL risk was
added over the above locations to account for the threat.

...Western Montana...

Approaching closed upper low over the Great Basin will pivot
northeastward while tilting negative as it progresses into the
Interior Mountain West, eventually situating over MT by the end of
the forecast cycle. Large scale ascent pattern will be maximized
downstream of the mean trough with ample mid-level moisture
advection being carried poleward into Big Sky Country, pushing PWAT
anomalies into the +2 to +3 standard deviation range. This is a
prominent signal when coupled with a strong upper forcing regime
and is co-located with an increase in boundary layer instability
when assessing the area MUCAPE field(s). Current QPF distribution
signals a scattered array of 1-2" totals with the most precip
footprint of the maxima tied closer to the terrain thanks to the
orographic elements typically favored in these types of setups. The
convectively driven pattern is one that maintains prominence on
local flash flood prospects and still has some room for a targeted
higher risk pending the convective outlook once into better range
of the CAMs. For now, maintained general continuity with some
extension on the southern periphery of the risk area over MT.

Kleebauer

Day 3
Valid 12Z Wed Sep 18 2024 - 12Z Thu Sep 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER NORTHWEST
MONTANA...

...Montana...

Negatively tilted mean trough and closed low over the Northern
Rockies will become vertically stacked over Big Sky Country leading
to a prolific axis of deformation developing in-of the Northern
Rockies encompassing Northwest MT. Ensemble mean QPF is quite
impressive given the synoptic regime with very little surface based
instability to work with. The whole pattern is driven by powerful
upper forcing within a well-defined axis of deformation on the
western side of a broad, occluding surface cyclone. Precip totals
off the latest deterministic are gaudy with a wide swath of 2-4"
with locally as high as 5+" located within the terrain to the east
of Great Falls. The current solution is generating pQPF 90th
percentile output of 100yr ARI's located within the Rockies to the
east of the valley up in Western MT. With multi-day totals
bordering 6-8" across portions of MT, a SLGT risk was added for the
time being as we still have some time prior to eventual impacts.
That said, it's a period to monitor as the current consensus is
pretty bullish with a general agreement to the setup within much of
the 00z deterministic suite. A higher risk is not out of the
question, especially if the magnitude of the forecast rainfall
maintains through multiple model cycles, and/or becomes even more
aggressive in the signal.

...Central Mid Atlantic...

The remnants of the tropical disturbance will linger across the Mid
Atlantic with some locally heavy rainfall prospects still in the
forecast. There is a great amount of uncertainty on specifics, so
the current forecast is based off the latest ensemble blend with a
focus on the potential most likely closer to the coast where
instability is highest during the period. For now, wanted to
maintain some continuity with the prospects still non-zero with
more time to assess and hone in more as we move closer into the
period.

Kleebauer


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