523
FOUS30 KWBC 060034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
834 PM EDT Fri Jul 5 2024

Day 1
Valid 01Z Sat Jul 06 2024 - 12Z Sat Jul 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE DEEP SOUTH...

01Z Update:

...Northeast...
Convection continues to develop ahead of an approaching shortwave
trough over the Northeast, with a couple focus areas of more
organized convection, one near NYC metro through southern New
England while another area is developing across southern NY near
BGM. The expectation for the remainder of the period is that a few
lines/clusters of storms will move through, relatively quickly, but
there is still some potential for training/repeating rounds that
could drop a quick 1-2" and cause isolated flash flooding, so the
Marginal Risk and the WPC MPD remain in effect.

...Deep South...
In the very warm and very moist airmass, deep convection developed
across portions of Louisiana and Mississippi early this afternoon.
Effective boundaries and outflows along with a favorable upper
level pattern will continue the threat for slow moving, intense
thunderstorms across portions of Louisiana through late this
evening. 18Z HREF continues to highlight potential for isolated
2-3" hourly totals with convection near New Orleans as well as with
activity in northern Louisiana that should move slowly southward
into the evening hours. Some potential for a quick 2-4" of rain
based on the latest HREF probabilities and the Slight Risk was
adjusted to the trends.

Further east into Alabama and Georgia, more isolated evening
convection is expected but given the very moist airmass and
environmental ingredients, isolated flash flooding will remain
possible.

...New Mexico...
Convection firing off the terrain in eastern New Mexico will
continue to pose a heavy rainfall and flash flood risk into the
late evening hours. While coverage will remain on the isolated
side, intense rain rates over sensitive areas justify continuing
the Marginal Risk.

Taylor

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...

...20Z Update...

The previously inherited SLGT risk across the Carolinas into
Southeast VA was maintained with little to no changes as guidances
maintains relative continuity on both coverage and magnitude of
expected convection. 12z HREF probability fields indicate localized
amounts exceeding 3" possible on Saturday afternoon into the
evening before the loss of diurnal heating puts an end to the
primary convective threat. There was reasonable coverage of 1-2"
totals scattered across the I-95 corridor between Raleigh down to
Columbia, SC within the blended mean forecast off the HREF with
significant overlap of the highest QPF signatures from the previous
ensemble bias corrected QPF footprint. Pending cell mergers and
established outflows, higher totals could fall outside the SLGT in
the MRGL risk area, especially across SC. There was little reason
to deviate from the previous forecast given the above conditions
and signals.

Elsewhere, the biggest changes occurred over the Plains where the
MRGL risk was expanded in all directions with the biggest
additions over the Northern Plains and through parts of the TX/OK
Panhandles. This was more in line with the First Guess Fields and
the convective QPF footprint within the latest hi-res ensemble.
There`s a greater signal further north across the Dakotas due to
the favorable upper forcing pattern transpiring thanks to a
shortwave trough pushing into the region. A stronger thermodynamic
profile exists further south into the Southern Plains, but the
signal is more isolated, but still within an environment favorable
for locally heavy rainfall with rates of 1-2"/hr possible.

MRGL risk across New England was maintained with some lean towards
a SLGT risk addition, but wanted to give one more cycle of guidance
to assess the setup and add a targeted SLGT after the convective
evolution later this evening. Some of tomorrows impacts are
contingent on the convective pattern at the end of D1, but there is
some First Guess field support for the SLGT potential, albeit a
little large in coverage than what could possibly be issued.

Kleebauer

...Previous Discussion...

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, AS WELL AS THE CENTRAL AND SOUTHERN COAST OF
TEXAS...

...20Z Update...

The previous SLGT risk across South TX was expanded up the coast in
the latest update. For more on this change, please refer to the,
"Coastal Texas" sub-heading below...

Elsewhere, the signal for significant rainfall has grown across
portions of the Central and Southern Plains with the primary axis
of focus lying between southern KS through the western half of OK,
excluding the Panhandle. Ensemble QPF between 2-4" is now the base
forecast across the outlined region with a strong mid-level
disturbance and surface convergence pattern the main reason for
such a prolific signal. Latest IVT forecasts continue to show
somewhat of a connection with the moisture associated from Beryl
streaming out ahead of the cyclone with some prolific theta-E
advection towards the end of the forecast moving into the primary
region of convergence. If this were to verify, an upgrade could be
plausible for parts of the Southern Plains with the best chance
residing over western OK down into the Red River Valley.

There were no changes across the Southeast where scattered
convection with isolated stronger cores could allow for locally
heavy rainfall with flash flood threat confined to more urbanized
settings within the confines of the Carolinas down towards the FL
Panhandle.

...Coastal Texas...

Biggest shift in the forecast comes across the Central and South
TX coastal plain where recent trends in the track of Beryl have
allowed for an expansion of the previous SLGT risk further up the
TX coast while encompassing the risk area closer to the coast as
the storm motion and compact storm structure is forecast to keep
the heaviest precip focused along the coast. There has been a
significant shift in some of the recent deterministic that now
aligns well with the storms track based off recent ML output, and
favors the right side of the 12z Hurricane Model tracks. The core
of the cyclone is now forecast to remain over water through the D3
forecast period with the outer bands Beryl likely protruding inland
as far as the immediate coast with highest certainty and further
inland more likely after the current D3. Totals based on recent
ensemble means have increased as a result further up into the
middle TX coast with the heaviest now situated between Corpus up to
Freeport. There is still some uncertainty in the northern extent
of the track as some guidance continues to fall even further north
of the current forecast, so this forecast is subject to shifts in
the exact location of the SLGT risk, but likely still confined to
the coastal plain.

Kleebauer

...Previous Discussion...

...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.

...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