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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0014Z Mar 27, 2024)
 
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Excessive Rainfall Discussion...UPDATED
NWS Weather Prediction Center College Park MD
813 PM EDT Tue Mar 26 2024

Day 1
Valid 01Z Wed Mar 27 2024 - 12Z Wed Mar 27 2024


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

Potential convective redevelopment is expected later tonight over
portions of the FL Panhandle, FL Big Bend, and GA near a stalled
front. Low level moisture transport and mid/upper forcing gradually
increase, with an uptick in instability also expected. Flash flood
guidance is quite high across this area, and 18z HREF probabilities
suggest they'll be approached, but probably not be exceeded, into
Wednesday morning. While there is opportunity for convection to
over perform given the stationary boundary and increasing
instability, believe any flash flood risk will remain localized in
nature.

Roth


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...

...20Z Excessive Rainfall Discussion...
Only minor adjustments were made to the on-going Excessive
Rainfall Outlook. The focus for any heavy rainfall looks to be in
proximity to a cold front that slows its forward motion as it
approaches from the west. Low pressure forms along the front late
Wednesday night begins to move northward along the Carolina
coastline. Flash Flood Guidance is high along that path...and
the previous Slight risk embedded within a broader Marginal Risk
area still looks reasonable. A slight nudge eastward was warranted
given latest WPC QPF and consensus QPF. No changes were needed
across the Western U.S..

Bann


...08Z Excessive Rainfall Discussion...

...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...

...20Z Excessive Rainfall Discussion...
Previous outlook still on-track and no changes were made to the
Marginal Risk area from the coastal Mid-Atlantic to Coastal New
England other than a subtle eastward shift.

Bann

...08Z Excessive Rainfall Discussion... 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