Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sun Sep 15 2024

Day 1
Valid 16Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST INTO THE MISSISSIPPI...

1630Z Excessive Rainfall Discussion Update...
Expanded the Marginal Risk area northward as far as Webb county
along the international border as another area of showers and
thunderstorms were approaching from the west. The better
rates/amounts will likely be confined to areas along the southern
border near the better instability...but the northward expansion of
the Marginal risk area covers the potential for isolated run-off
problems where this afternoon convection overlaps with an area
that received isolated heavy rainfall amounts from convection
overnight. In addition...a few of the high resolution models
suggest the potential for slow moving cells. Any problems should
remain isolated given the flash flood guidance values. Few changes
were needed elswhere with this issuance.

Bann

Previous Excessive Rainfall Discussion...

...Central Gulf Coast into Mississippi...
The setup that has resulted in a convective bands and locally
heavy rainfall the past couple days will continue today, just
shifted a bit to the southwest. Lingering mid level energy and low
level convergence will result in convection today from coastal AL
northwestward across MS and into eastern Arkansas. The persistence
of the convergence will allow for some training/backbuilding
potential locally increasing rainfall totals. The overlap of
strongest convergence and higher instability looks to focus closer
to the Gulf Coast today, across portions of coastal MS/AL and the
western FL Panhandle. The flash flood risk is likely highest across
these areas, with 5"+ HREF neighborhood probabilities as high as
50-80%. HREF guidance indicates one round of potential backbuilding
cells this afternoon, with another round possible late tonight
into early Monday. Given this signal the Slight risk was expanded
into this region, and do consider this area to be a higher end
Slight risk, with locally significant flash flooding a possibility
given what should be very efficient rainfall producers.

The flash flood risk is a bit lower farther north across
Mississippi and Arkansas given weaker instability and what should
generally be more transient convective cells. Nonetheless some
training/backbuilding is still possible, and soil saturation is
still above average due to rainfall from Francine a few days ago.
Thus still think a Slight risk is warranted, with isolated to
scattered flash flooding possible.

....Coastal Carolinas...
An area of low pressure off the southeast coast may bring heavy
rainfall into portions of coastal South Carolina/North Carolina 
between 06z-12z Monday. The GFS remains a fast outlier in bringing
heavy rainfall inland from this system, and thus is not a preferred
solution. While there is still some spread amongst the other model
solutions, some form of consensus is developing. This consensus
would suggest that some bands of locally heavy rainfall may get
into coastal portions of the Carolinas early Monday. Still think
the greater flash flood risk ends up after 12z Monday, but can not
rule out some heavier totals right along the coast by 12z...and
thus will introduce a Marginal risk.

...South TX...
A small Marginal risk was added to far southern Texas. Will likely
be an active convective day across northeast Mexico today, with
easterly low level flow advecting in moisture off the Gulf, and
westerly mid/upper flow bringing in tropical moisture off the
Pacific. Low level convergence should be enhanced in this pattern,
and also have some mid/upper level energy to work with as well. In
general expect the bulk of convective activity to be over northeast
MX where convergence is strongest, however there is at least some
threat far south Texas can see some activity as well. Guidance
indicates that any convection that organizes over south Texas
should drop back south into Mexico, however there is an
opportunity for some slow moving cells later this morning or
afternoon before this propagation takes hold.

...Southwest...
A Marginal risk covers portions of Arizona and New Mexico today
where a localized flash flood risk may exist. This risk area was
reduced in size to just cover areas where convective coverage
should be a bit greater and instability more pronounced. Rainfall
over the Marginal risk area could locally exceed 1" which could
produce an isolated flash flood over any more susceptible basins.


Chenard

Day 2
Valid 12Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CAROLINAS INTO FAR SOUTHEAST VIRGINIA...

...Carolinas into Virginia...
An area of low pressure will likely move into portions of NC on
Monday bringing a heavy rainfall threat to the area. Confidence is
increasing that this system will have a notable core of heavy
rainfall near the center and heavy convective banding to the east
of the center. The threat of flash flooding across portions of
eastern NC appears to be increasing, with locally significant
impacts possible. With that said there remains some model spread
regarding the location of landfall and the structure/intensity of
the system.

Like was mentioned in the day 1 discussion, think the 00z GFS
remains a less likely fast and west outlier with this system on
day 2. After discounting that solution the overall model consensus
improves, although not without some notable spread. The 00z NAM
and Gem Reg are two of the stronger and farther south solutions,
with the 00z CMC and UKMET further east over coastal NC. With the
high res models you have the 00Z 3km NAM and ARW stronger/quicker
and further south near the SC/NC border, and the HRRR and ARW2 a
bit slower/weaker and north. Overall it appears that the earlier
the low begins to deepen the stronger, quicker and further south it
moves ashore. Where as the models with a slower developing low
keep it a bit weaker and east over NC. At the moment do not really
have a strong preference, with things likely becoming clearer today
as the low begins to become more well defined offshore.

The above described uncertainties will keep us from upgrading to a
MDT risk at this time. However do consider this a higher end
Slight risk across portions of southeast NC. Wile the details still
need to be ironed out, what we can say is that the flash flood
risk from this system is increasing and locally significant flash
flooding is possible over portions of eastern NC (and possibly
northeast SC depending on the track). Just about every model
solution brings a low pressure ashore with locally heavy rainfall
near and east of the center...however enough spread still exists
with the track and intensity of the system to keep the risk level
at Slight for now. Once the low develops and confidence on the
evolution of the system continues to increase, a MDT risk upgrade
may be needed.

...Central Gulf Coast...
Some flash flood risk will likely linger across portions of the
central Gulf Coast into southeast AR on Monday. However the pattern
becomes gradually less favorable with time, and thus currently
expecting that only a localized flash flood risk will linger into
Monday.

...Southwest to Four Corners... A strong deep layered low moving
into the CA will result in an uptick in large scale forcing over
portions of the Southwest on Monday. Broad south to southwesterly
flow ahead of this feature will also advect moisture into the
region, and should have plenty of instability and moisture in
place for locally heavy rainfall within convection. Given the
increased forcing convective coverage should be notable over the
region, suggesting at least an isolated flash flood threat. Cell
motions should be fairly quick off to the northeast limiting the
duration of heaviest rainfall and the overall flash flood risk.
However with the persistent forcing in place areas could see
multiple convective cells/clusters move through over the period
adding to rainfall totals.


...NV/ID...
An axis of higher areal averaged rainfall is likely closer to the
stronger forcing underneath the deep layered low over portions of
northern NV, southeast OR and into ID. On Monday morning there is
some signal for potential training rainfall bands over portions of
NV, although instability looks rather weak, suggesting rainfall
rates may struggle to get high enough for flash flooding.
Nonetheless will need to be monitored and some areas of 1-2" of
rainfall are possible, which for some of this area could be around
a 5yr ARI event. Thus even without higher rates some areal flooding
is a possibility. By Monday afternoon we should see some increase
in instability over the broader region likely allowing for some
heavier convective cells. However currently expecting these cells
to be rather quick moving. Thus overall think a Marginal risk
should suffice, with localized flooding concerns a possibility.


...ND/MN...
A Marginal risk was added across portions of ND and MN. Model
guidance shows a lifting warm front, mid level vort energy moving
through, and divergent flow aloft between two jet streaks...likely
enough forcing to get convective development Monday into Monday
night. Convective details differ from model to model, but the
overall ingredients are similar and instability should be
plentiful. Overall the threat is there for an axis of training
and/or backbuilding convection within the Marginal risk area, and
some flash flooding could evolve.

Chenard

Day 3
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, IDAHO AND MONTANA...

...Mid-Atlantic...
The coastal low pressure system described in the day 2 discussion
will move inland across the Mid-Atlantic on Tuesday. Just like on
the previous days the GFS is an outlier and not a preferred
solution. Taking a non-GFS consensus yields a broad area of heavy
rain potential over the Mid-Atlantic. Despite areal averaged QPF of
1-3" in the WPC QPF, and several models showing even higher
totals, the risk level was left at Marginal for now. The system
should be weakening fairly quickly as it moves inland, so there
remain some questions with regards to the structure of the system
by Tuesday.

Expect the heavy rainfall near the center of the low to lose
intensity as the low weakens and the area of rain moves away from
the better instability. However if the low moves towards the
central Appalachians then upslope enhancement could make up for
this to some extent and drive a flash flood risk. Thus can not
rule out a Slight risk upgrade over portions of western/northern
VA, but questions with the Storm track, structure and instability
suggest keeping things at a Marginal level is best for now.

We should still be seeing some convective banding east of the
center across the coastal plain from eastern VA into MD and DE.
However, still some questions regarding the degree of instability
present and how transient the convective bands will end up being.

Overall think a broad Slight risk might be overkill at the moment
given these questions. At least some flash flood risk likely
exists and the Marginal risk covers that threat. As confidence
increases upgrades may need to be considered near the center of the
low (if instability ends up persisting and/or terrain influences
are maximized), and another area somewhere along the coastal plain
if enough instability and persistent banding look more likely.

...FL Panhandle...
The risk of flash flooding across the Southeast continues to
decrease on Tuesday as forcing weakens and dry air wraps in behind
the Atlantic coastal low. However some risk of backbuilding
convection may persist along portions of the FL Panhandle.
Confidence is lower by this point, but enough of a threat to
continue with the Marginal risk.

...ID and MT...
The strong deep layer low that was over CA/NV on Monday will shift
northeastward into ID and MT on Tuesday. Overall looks to be
impressive forcing over the region and expect a rather widespread
1-1.5" rainfall. However most of this rainfall will more than
likely be stratiform in nature as the low deepens and forcing
persists. The lack of more intense rainfall rates should limit the
flash flood risk, although some areal flooding concerns are
possible given the widespread nature of the moderate to heavy
rainfall. The better instability and risk of higher rates will be
over eastern MT, but expect convection will tend to be quick
moving here. Thus this area should see lower coverage of heavy
rainfall, but a better chance of localized flash flooding given the
higher rate potential.


Chenard


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