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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0835Z Mar 26, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Tue Mar 26 2024

Day 1
Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024


...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS
OF THE SOUTHEAST...

Convection will be ongoing at 12z across portions AL, with the
heaviest rainfall rates likely over southern AL into the far
western FL Panhandle. Still expect the potential for 1.5"/hr
rainfall into the morning hours, but convection should be steadily
pushing off to the southeast by this time...and a weakening
convective trend is expected by mid morning. For these reasons the
flash flood risk should be on a downward trend as well, with only
isolated issues possible.

Further north, convection over AL should already be weaker by 12z
as it outruns the instability and overall forcing/moisture
transport declines. Showers will move into GA and the southern
Appalachians this afternoon and evening. Limited instability and
decreasing forcing all suggest only modest rainfall amounts, and
indeed areal averaged rainfall is expected to stay in the 0.5" to
1" range. However with the slowing front later today, and some weak
instability present...most guidance does indicate some periodic
embedded heavier convective cells. Still not looking at anything
extreme, but localized 2" swaths are possible.

We will have to keep an eye on potential convective redevelopment
later tonight over portions of the FL Panhandle into southern GA
near the stalled boundary. Low level moisture transport and
mid/upper forcing gradually increase, with an uptick in
instability also expected. FFG is quite high across this area, and
at the moment no guidance is really coming all that close to
exceeding this FFG through 12z Wed. So while there is some
opportunity for convection to over perform given the stationary
boundary and increasing instability, tend to think any flash flood
risk will remain localized in nature through 12z Wed.

Chenard


Day 2
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
northward into the central/eastern Carolinas.

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...along with the exact
axis of heaviest rainfall.

Model guidance has been bouncing west and east with each run, and
at the moment not quite ready to say we have any consistent trend.
A good chunk of the 00z models trended east...such as the GEM reg,
ECMWF, UKMET and RRFS. However the GFS, 3km NAM and FV3LAM remain
west. The 00z experimental RRFS ensemble and 06z HRRR also are
favoring a further east solution. Given all the available data, we
are leaning more towards the eastern model camp, but this is not a
lock, as additional model fluctuations are anticipated. It will
really come down to where the front stalls, and thus where the low
track ends up.

Instability and rainfall rates are the next piece of uncertainty.
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 still 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. The Slight risk aligns well with the higher QPF
signal seen in the 00z ECMWF and UKMET, as well as in the
experimental 00z RRFS (which is forecasting a swath of 3-5" of
rainfall).

Portions of the eastern Carolinas into southeast VA may eventually
need a Slight risk upgrade, but the combination of uncertainty on
the exact rainfall axis along with questions regarding the degree
of instability, suggest keeping the Marginal risk is the way to go
for now. Plus soil saturation and streamflows are generally around
or slightly below average, resulting in relatively high FFG. We
will continue to reassess on future shifts.

...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 1-3" of rain, and with soil
saturation and streamflows running above normal over the area, it
seems plausible that any embedded higher rates could cause some
localized flooding concerns.


Chenard



Day 3
Valid 12Z Thu Mar 28 2024 - 12Z Fri Mar 29 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS
OF THE COASTAL MID-ATLANTIC INTO COASTAL NEW ENGLAND...

An area of low pressure moving up the coast will result in a heavy
rain threat to portions of the eastern Mid-Atlantic into eastern
New England Thursday into Thursday night. As described in the day
2 discussion, there remains uncertainty with the track of this low
and and thus also with the axis of heaviest rainfall. The 00z non-
NCEP guidance has trended east with this cycle, with the GFS and
NAM remaining west. While they do not go out into day 3, the 00z
experimental RRFS ensemble and 06z HRRR are more in line with the
eastern model solutions at the end of their runs. Thus we are
favoring something closer to the more eastern solutions at this
time...although as is typically the case, something somewhere in
between the two is probably most likely. Regardless, with the ERO
being a probabilistic product, we did hedge a bit west of the WPC
deterministic QPF for the western extent of the Marginal risk.
Heavy rainfall is likely along and just inland of wherever the low
does track.

An argument could be made for a Slight risk over eastern NC into
southeast VA given two day rainfall in the 3-5" range in the WPC
forecast and in several deterministic models. However, do think
that by Thursday this rain will likely be on the cool side of the
developing low, and thus should be more stratiform in nature by
this period. For that reason, and the described lingering
uncertainty on the exact axis, think maintaining the Marginal risk
is the way to go for now.

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