FOUS30 KWBC 150911
QPFERD

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

Day 1
Valid 12Z 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...

...Central Gulf Coast into MS...
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 AR. 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 further north across MS and AR
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 SC/NC 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 TX. Will likely be
an active convective day across northeast MX 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 TX can see some activity as well. Guidance 
indicates that any convection that organizes over south TX should 
drop back south into MX, 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 AZ and NM 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 further 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
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