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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0800Z Mar 25, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Mon Mar 25 2024

Day 1
Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST...

The inherited Slight risk still looks in pretty good shape.
Convection will be ongoing at 12z across portions of northeast TX
up towards the OK/AR border. As we go through the morning hours we
should see an uptick in convective intensity as this line moves
east into AR and southern MO. Increasing forcing from the west is
expected to tighten and strengthen low level convergence helping
intensify convection. Activity is expected to have an eastward
movement, however cell motions will have a fairly large component
parallel to the low level boundary for a period of time this
morning and afternoon, which should result in some brief training
of cells.

Guidance supports 1-3" of rain along an axis from western to
central AR into southern MO, with both the HREF and experimental
REFS ensembles showing a 20-60% chance of localized amounts
exceeding 3". Hourly rainfall will likely exceed 1", but 2"/hr
probabilities are low...with most high res guidance peaking hourly
rates around 1.5". Limited instability and a limited duration of
heavy rainfall are both minimizing factors for flash flooding.
However do think the degree of moisture transport and forcing,
combined with the brief window of opportunity for training as the
wave to the west digs, all support an isolated to scattered flash
flood risk here.

There may be a corridor with a relative min in QPF over the eastern
half of AR, before activity ramps back up again over MS during the
overnight hours. Impressive IVT forecast over MS/AL tonight, with
values approaching the climatological max for late March. The low
level jet is also impressive, with 850mb wind speeds over the 99th
percentile. These factors, combined with the strong mid/upper
forcing, should be enough to overcome the marginal instability in
place and result in strong organized convection capable of heavy
rainfall rates.

High res models easily support 1"/hr rainfall with this activity,
with localized swaths exceeding 2"/hr appearing likely. Thus rates
will be there for flash flooding...with the main question being
whether the duration is there as this convection will likely be
moving off to the east at a decent clip. However, given the very
strong low level southerly inflow, would expect to see some brief
backbuilding/training of cells...especially on the south and
southwest flank of the activity. Given the impressive moisture and
forcing in play, suspect that rainfall magnitudes from this event
will verify on the higher end of the model envelope over far
northeast LA into central MS and western AL. Of the two high res
ensembles, the experimental REFS is wetter than the HREF, and thus
am leaning in that direction, which would support a swath of 3" of
rain (REFS neighborhood probabilities of exceeding 3" are 40-60%).
This should be enough to result in isolated to scattered instances
of flash flooding. In the end the progressive nature of the system
should keep this from getting any higher than a Slight risk event.

The Marginal risk still extends northward into northeast MO, IL
and southern WI...where 1-3" of rain is expected. Rainfall rates
will decrease with northeast extent over this corridor, which is
why the risk level is Slight in southern MO, but just Marginal
further north and east.

Chenard


Day 2
Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA PANHANDLE INTO SOUTHEAST AL AND SOUTHWEST GEORGIA...

The risk of flash flooding does look to decrease as this event
moves into Tuesday over the Southeast. The better forcing lifts
off to the north and IVT and the low level jet weaken. With that
said, convection will likely be ongoing at 12z Tuesday over AL
with locally heavy rain, and this activity will move into GA and
the southern Appalachians through the day as well. Eventually the
front and moisture plume are expected to slow down and/or stall
out, so while instability and forcing is weaker by this time, can
not rule out some continued locally heavy rainfall.

One area where a higher flash flood risk could exist remains over
portions of southern AL into the FL Panhandle. Instability is
higher here, and so expect intense convection to persist here
Tuesday morning into the afternoon hours. Activity should be
progressive off to the east, but 1-2"/hr totals are still probable.
The front stalls out by later in the day, and while forcing and
moisture transport decrease by this time, instability will remain
to go along with the lingering low level convergence. So while most
guidance is not that aggressive with convective development by
later in the day into the overnight, the potential still appears to
be there for additional development. Overall this Slight is lower
confidence than day 1. Current QPFs from the models support more of
a Marginal risk...however given the setup and ingredients, there
is some potential for convection to over perform near this slowing
front. Thus think maintaining the Slight risk is the best option at
this time.

Chenard


Day 3
Valid 12Z Wed Mar 27 2024 - 12Z Thu Mar 28 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA...

...Southeast...

A swath of heavy rainfall appears likely Wednesday into Wednesday
night across portions of the FL Panhandle, into southeast GA and
north into the central/eastern Carolinas. At the moment the models
are in pretty good agreement with the axis of this swath...although
given we are 3 days out the uncertainty is probably a bit greater
than the 00z deterministic guidance would indicate. Either way,
confidence is indeed growing for a swath of 1-3" of rainfall
across this corridor.

Increasing forcing from the west will move over a stalled out
front...likely resulting in cyclogenesis and a swath of heavy
rainfall. PWs are forecast near or above the climatological 90th
percentile, so we will likely have the moisture in place for
excessive rainfall...and the duration also looks to be there given
the stalled nature of the front. The main uncertainty comes down to
the degree of instability and rainfall rates. At the moment it
would appear like instability may be lacking over most of this
corridor, which may act to temper rainfall rates...and thus even
1-3" of rainfall would probably just be worthy of a Marginal risk.
The 00z GFS is outputting 4-6" of rain over portions of SC/NC...but
instability is limited, and only a small portion of this rain is
convective QPF out of the GFS. Thus tend to think this specific
forecast is overdone.

The best chance of more robust and persistent instability is
probably closer to the Gulf Coast. It is here where some better
moisture coming in off the Gulf may sustain enough instability for
more vigorous convection. Even this is uncertain to a degree, but
confidence is a bit higher here for higher rainfall rates and the
potential for 3"+ total rainfall. Thus for now think the best
strategy for the ERO is to start small with the Slight risk, and
keep it confined to where confidence is higher on better rainfall
rate potential. Then keep the rest of the corridor in a Marginal
risk and continue to monitor trends over the coming days and
adjust as necessary.

...Northwest CA into southwest OR...
We will maintain a Marginal risk across these areas, although this
is probably a lower end risk. IVT is not all that strong, and the
system is progressive, both of which will be limiting factors for
excessive rainfall. However upper forcing is pretty good, and it
does look like we may be able to generate 0.5"/hr rainfall rates
with some weak instability working into the backside of the cold
front. Still look on track for 1-2" of rain, with localized 3"
amounts. Considered removing the risk, but soil saturation and
streamflows are running above normal over the area, so seems
plausible that any embedded higher rates could case some localized
flooding concerns.


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