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

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

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEASTERN
NORTH CAROLINA AND THE NORTHWESTERN CORNER OF SOUTH CAROLINA...

...Carolinas...

PTC8 continues to churn off the Carolina coast with an ill-defined
center of circulation and convective clustering to the north of
the analyzed low off the coast of SC. The beginning wave of
tropical rainfall has started across Northeast SC into Southeast
NC with rates anticipated to pick up as the deeper tropical
moisture flux located offshore begins to propagate into the coast.
There has been relatively modest uncertainty to exactly where the
heaviest rainfall will occur due to a less defined surface low
center that provides more consensus on the specific track and
subsequent heavy rain axis. One of the notable observations through
the past 6-8 hrs has been the QPF shield displaced further north
away from the analyzed surface center provided by NHC. A lot of
this is likely due to the moderate shear impinging the southern
flank of the circulation allowing for an offset of the precip field
to the north and northeast of the primary circulation. This has
caused issues within the NWP suite of forecasts, especially higher
resolution guidance that tends to need a more defined surface
initiation to outline short range prospects within the precip
field and track of the surface reflection. There is enough data to
project the current QPF swath to be displaced further north than
some of the CAMs and global deterministic output with a surface
reflection likely to come ashore a bit further north than even the
00z consensus. For that, the trend is for 4-8" totals with locally
up to 10" possible across the Northeastern corner of SC through
Southeast NC, an agreed axis within the neighborhood probability
fields from the latest HREF. The signal for >5" is very robust with
probabilities between 70-90% located across Southeastern NC with
the strongest signal located near Wilmington, an area of greatest
concern considering the urbanization factors.

The storm will induce some pretty good rainfall across Northern SC
and much of NC through the periods as the circulation moves inland
and progresses westward into the Carolina Piedmont. Bands of
tropical rainfall with efficient warm rain processes will pivot
inland with the prospects of flash flooding increasing over areas
that see training. As a result, a broad SLGT risk is centered
across much of North Carolina and the northern periphery of South
Carolina. A targeted MDT risk is in place over the northeast corner
of SC and much of Southeastern NC as the consensus for higher
rainfall amounts is most prolific within that zone.

...Northern Plains and Upper Midwest...

Aloft, a shortwave will eject out of the Central Plains and push
quickly to the northeast, entering the Northern Plains of the
Dakotas into Northwestern MN by later this evening providing ample
mid-level ascent conducive for convective initiation. At the
surface, a pre-frontal trough will develop in-of the Southern
Canadian Prairies, shifting southward into Northern ND and MN by
early this evening, setting the stage for a strong surface
convergence axis located across Eastern ND and Northwest MN. This
is thanks to the tandem mid-level ascent pattern and convergence
signature located within a formidable theta-E tongue bisecting the
area. The signal has grown considerably in the past succession of
runs with the latest CAMs all generally robust with its
presentation of locally enhanced rainfall located over the above
regions. With the mean flow generally parallel to the pre-frontal,
surface based trough, there is a growing concern for training cells
that develop upstream over ND and progress east-northeast along the
slow-moving trough axis. The 00z HREF blended mean QPF is quite
robust in its depiction with an areal average of 1.75-2.5" of total
precip located from just west of Grand Forks up through the
International border in Northern MN. A bullseye of 3-5" is co-
located within the axis of the surface trough positioning extending
over much of Northwestern MN, an area that typically does not favor
these types of convective outputs. HREF EAS probabilities for 1"
(80-90+%) and 2" (30-50%) are incredibly high for an event that is
not tied to a strong organized synoptic complex, only adding
emphasis to the potential with such high agreement within this
evenings CAMs. Neighborhood probs are just as impressive with a
strong >3" signal (70-90+%) and modest >5" (15-30%) output with the
latter tied closer to the Canadian border where the model agreement
is strongest for the heavier totals. A SLGT risk was added given
the favorable forecast from this evening and a strengthening
consensus amongst CAMs members for a locally enhanced flash flood
threat across the northern tier of the ND/MN area.

...Western U.S...

Upper air analysis indicates a broad closed upper circulation
positioned over Northern California with an expectation to
strengthen as we progress through 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
NV and eventually ID as we progress through today and 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. A fairly robust and widespread probability
for >1" (70+%) is well documented within the 00z HREF neighborhood
probability output over the above locations, a strong enough signal
to warrant a continuation of the previous MRGL risk forecast.

...Deep South...

Lingering low-level moisture with the proximity of a quasi-
stationary front across the Deep South will allow for another
period 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
periods, but the probability fields still indicate a formidable
potential for >3", especially along the Gulf coast where the
surface based instability is most prominent. This is a low-end MRGL
risk with a better chance for flash flooding to occur over those
areas that have been hit hardest the past several days from
Francine and its remnants.

Kleebauer

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