995
FOUS30 KWBC 051714
QPFERD

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

Day 1
Valid 16Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024

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

...16Z Update...

A SLGT risk was added across portions of the Deep South, mainly
over Southern MS and Southeast LA where the best signals for
elevated rainfall rates and totals intersect. More details in the,
"Southeast" sub-heading below...

Elsewhere, changes were minimal with general shifts in the MRGL
risk areas based on the latest QPF footprint based on the 12z HREF
mean and blended mean, as well as adjusting based on recent radar
trends where rainfall has exited areas that were previously
outlined.

...Southeast...

A very anomalous environment is currently present across portions
of the Deep South as noted via the 12z soundings out of KJAN and
KLIX indicating PWATs running towards the top end of climatology.
In fact, the 2.43" reading out of KJAN was tied for 10th all time
within the month of July, and highest daily max, exceeding the
previous record by over 0.1". This has aided in deep, moist
sounding with formidable EL heights nestled way up into the column.
This is textbook for locally heavy rainfall prospects that are
conducive for flash flooding, especially when coupled with slow
mean storm motions progged outside propagation of outflows. Latest
12z HREF was aggressive with signatures for heavy rain totals
exceeding 3" and 5" respectively with the latter running between
30-50% neighborhood probabilities over Southern MS with the max of
60% settled along Lake Pontchartrain, including the city of New
Orleans. Rates upwards of 2-3"/hr are likely given the antecedent
environment and correlates well with the elevated probability
fields from both the HREF and the 90th percentile of the NBM.
Considering the slow-moving nature of the convection, along with a
tropical-like airmass settled over the region highlighted, a SLGT
risk was added to account for the potential with the main focus
being urbanized flooding.

...Southern Plains...

Thunderstorm clusters across portions of North TX and Oklahoma
will continue through the afternoon with a general enhancement
expected due to increased destabilization from diurnal heating, as
well as added surface based ascent along the slow progressing cold
front shunting southward through the period. The best potential
for heavy rain will lie along the boundary as the front will be a
focal point for cell cores to lock on train along the boundary in
question. Guidance continues to signal a maximum centered from the
Concho Valley and points east and northeast into North TX with
local totals upwards of 2-4" plausible. The convective coverage
will be highest north of I-20, but the heaviest cells will likely
lie along and south of the interstate due to better pre-frontal
destabilization and convergence later this afternoon. The signal
for localized flooding extended into the Lower Trans Pecos, back up
through eastern NM where a small mid-level perturbation over
Northeast NM will likely set off a few stronger cells that will
propagate southeast on the eastern flank of the western ridge. For
those prospects alone, there was little reason to deviate from the
previous forecast, thus maintained continuity. Coverage and
expected rainfall intensities were not high enough to warrant an
upgrade at this time, however pending radar evolution and obs, a
targeted SLGT risk cannot be ruled out in later updates.

...Ohio Valley...

Complex moving east over KY/TN will continue to progress east, and
then northeast entering into southern WV by later this afternoon.
Sufficient mid-level shear will be good enough to sustain
reasonable updrafts with cells likely firing quickly and sustaining
forward motions through the complex terrain over the state. The
mean storm motion should limit more significant flood potential,
however the environment and area of interest is conducive for
localized flood prospects, especially urban areas and within the
complex terrain within the central and southern Appalachians. HREF
blended mean is suggesting local totals of 1-2"possible over
eastern KY up through much of WV, enough to warrant a continuation
of the MRGL risk in place.

Kleebauer

...Previous Discussion...

...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