898
FOUS30 KWBC 050901
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Fri Jul 5 2024

Day 1
Valid 12Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
SOUTHERN AND EASTERN U.S. AND THE GREAT LAKES...

...Southern Plains, Southeast, OH Valley...
Organized convection will likely be ongoing at 12z this morning
across portions of KY and TN. This activity should be pretty
progressive by this time, however high PWs will support heavy
rainfall rates and thus a possible localized flash flood risk.

During the afternoon hours we will likely see scattered convective
development along/ahead of a front from the lower MS Valley all the
way to the southern Appalachians. Not really expecting much large
scale organization of this activity given weaker mid/upper forcing
and minimal deep layer shear. However we will see an impressive
overlap of instability and moisture...with CAPE in the range of
2000-4000 j/kg and PWs well above the climatological 90th
percentile along most of this corridor. Thus convection will be
more than capable of heavy rainfall rates, with high res guidance
supporting localized 2-3"/hr rainfall. Guidance indicates enough
convective coverage to suggest some cell mergers and convective
clusters are probable...which may be enough to locally prolong
heavy rainfall rates and result in an isolated flash flood risk.

Convection will also likely be moving out of northeast NM and into
the TX Panhandle this morning. This activity should shift south
with the front, with the front forecast to stall over central TX.
While no strong convective signal is seen over TX the presence of
the front and above normal moisture suggests a localized flash
flood risk may exist. By later in the day we may see additional
convective development over eastern NM within the easterly post-
frontal regime.

...Great Lakes...
An area of low pressure and strong mid level energy will move
across WI and MI today. Scattered showers and thunderstorms will
accompany this system, although a lack of more impressive moisture
or instability will likely limit rainfall magnitudes. Nonetheless,
localized 2-3" amounts appear probable, which may drive a
localized flash flood risk.

...Northeast...
The weak wave over PA early this morning will track northeast and
likely become a focus for convective development this afternoon
across portions of northeast PA, southeast NY and into portions of
New England. Not seeing enough for a Slight risk, but the 00z HREF
does suggest a more organized area of convection may evolve over
portions of the Hudson valley into western MA/CT, so could end up
with a bit better coverage of 1"+ rainfall here and localized
amounts over 3", which should be enough to drive a localized flash
flood threat.

Chenard

Day 2
Valid 12Z Sat Jul 06 2024 - 12Z Sun Jul 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
CAROLINAS AND THE HAMPTON ROADS OF VIRGINIA...

A slow moving frontal boundary will be a focus for areas of heavy
rainfall across the Gulf Coast region and much of the East Coast.
A Slight risk was maintained with this update across portions of
the Carolinas into southeast VA. An impressive overlap of
instability and moisture is noted across this region...with CAPE
likely over 2000 j/kg and PWs around 2.25" (approaching
climatological 95th percentile values). Some signs that there could
be at least some upper level moisture connection to Hurricane
Beryl by this time as well, potentially adding to rainfall
efficiency. Weaker mid/upper forcing and limited deep layer shear
suggests convection will generally be disorganized and of a pulse
variety. However we should see enough coverage to result in some
cell mergers and clusters, which will locally enhance rainfall
duration. Thus anticipate we will see pretty good coverage of
heavier rainfall totals within the Slight risk area, with scattered
amounts exceeding 3" likely. Dry soil conditions will be a
limiting factor for flooding...however rainfall rates should be
able to overcome this enough to make this a solid Slight risk.

Convective coverage along the central Gulf Coast does not look as
great at the moment. The latest guidance taken verbatim probably
does not support a Marginal risk here, however PWs will remain
elevated, a boundary will be in the vicinity, and areas of heavy
rainfall are likely on day 1 potentially making things more
susceptible by day 2...thus will maintain the Marginal risk.

The stalled front over TX will continue to pose some heavy
rainfall threat, although not expecting coverage to be all that
great. Enough instability and moisture over eastern NM to suggest
another round of terrain influenced diurnal convection is likely.

The Marginal risk was expanded north into New England with this
update. Guidance is in better agreement today that the front will
still be to the west of the area, with plentiful destabilization
during the day Saturday. An approaching mid level shortwave and
stronger deep layer shear suggests a more organized convective
potential. PWs could potentially be approaching early July max
values, so the ingredients are certainly in place for heavy
rainfall. We may very well eventually need a Slight risk over
portions of the region...but still some questions with convective
evolution, and some chance storms stay fairly quick moving off to
the east. Thus did not want to go right to a Slight
risk...preferring to introduce the Marginal and continue to monitor
trends.

A Marginal risk also extends from portions of KS into IA. Broad
troughing remains over the central U.S, and guidance indicates a
mid level shortwave should be diving south within the long wave
trough. There is also a response in the upper jet, and overall
large scale synoptic support for ascent looks pretty good by
Saturday night. Not surprising that we also see a low level
response to this increased forcing, with 850mb moisture transport
seeing a notable uptick. Moisture looks to be a limiting factor, as
still not seeing a great deep layer moisture connection, and so
PWs are only forecast around or slightly above average levels.
Despite only modest moisture...the strong forcing should make up
for this and allow for some potentially heavy rainfall totals.

Chenard


Day 3
Valid 12Z Sun Jul 07 2024 - 12Z Mon Jul 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
KANSAS, OKLAHOMA AND SOUTHERN TEXAS...

...Southern and Central Plains...
Organized convection is likely over portions of KS and OK Sunday
into Sunday night. Quite the impressive model QPF signal...with
both good run to run continuity and model to model agreement. Thus
forecast confidence is above average with this event. Interestingly
the pattern appears to favor a rather progressive convective
complex...and PWs are not forecast to be overly high either. Thus
the impressive QPF from the models, while somewhat surprising, is
likely indicative of the very strong mid/upper level forcing and
low level frontal convergence in play. A look at layer IVT suggests
we may start to see some low level connection to the moisture plume
ahead of Beryl, so this is certainly something to keep an eye on
and could act to enhance rainfall efficiency. Either way this is a
solid Slight risk, with the apparent progressiveness of convection
keeping the risk from going higher...but will need to continue to
monitor.

...South TX...
Heavy rainfall from Beryl will likely begin to impact portions of
South TX Sunday into Sunday night. There is still a large amount
of model spread with the track and speed of Beryl by this time. So
quite a bit of uncertainty exists with the details of what
convection will look like this period. However we are confident
that areas of heavy rain will begin to impact portions of southern
TX, with an uptick in the flash flood risk with time. A Slight
risk remains warranted, with the expectation that an isolated to
scattered flash flood threat will increase into Sunday night ahead
of potential landfall.

...Southeast...
Convection along the stalled front will again pose at least an
isolated flash flood threat over portions of the Southeast Sunday.
PWs will remain above the climatological 90th percentile, and
layered IVT suggest we may see some mid/upper level moisture
connection to Beryl. This could help increase rainfall efficiency
in what will already be a favorable environment for heavy rainfall
rates.

Chenard

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt