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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 2017Z Jul 24, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Wed Jul 24 2024

Day 1
Valid 16Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE UPPER
TEXAS COAST AND SOUTHWEST LOUISIANA...

...16Z Update...

No major changes were made to the Moderate and Slight risk areas.
For the Moderate, the steadiest and heaviest rainfall has just
about ended across the area for the day. A weak disturbance/mesolow
over Texas locally enhanced the forcing resulting in heavy rain
from the southeast tip of Texas near Port Arthur north along the
Sabine River to about the Toledo Bend Reservoir. The heaviest rain
has now shifted into central Louisiana...leaving behind much lighter
rain along much of the Texas Coast. The storms are largely being
driven by anomalously high moisture with PWATs to 2.4 inches and
instability over the Gulf, which continue inland enhanced by the
aforementioned mesolow. With daytime heating, expect the mesolow to
largely dissipate and more widespread instability to result in
decreasing organization to the storms. The Moderate Risk now is
largely to account for an expected redevelopment of shower and
thunderstorm activity tonight over much of the same area along the
TX\LA border near the Gulf Coast.

For the rest of the Southeast, the Slight largely remains the same
as the stalled out front continues to draw plentiful Gulf moisture
northeastward, resulting in training convection over many areas
hard-hit in recent days with heavy rainfall. NASA Sport imagery
shows that for nearly all of the area drawn in the Slight from
Texas to Virginia, soils are at or near saturation with the
exception of central Georgia and southern South Carolina. Thus,
additional expected rainfall due to training storms will result in
widely scattered flash flooding.

For the Marginal out west, the area was greatly expanded with this
update to include much of eastern and southeastern California, as
well as eastern Oregon, central Idaho, and western Montana. In
recent days even isolated storms have been able to produce flooding
due to the sensitivity of the soils in those areas due to terrain,
and a repeat day is expected once again this afternoon. Thus,
isolated flash flooding is likely...though exactly where that
happens is very difficult to say.

Wegman

...Previous Discussion...

...Texas and Louisiana Coast...

After coordination with the local Lake Charles and Houston WFO's, a
targeted Moderate Risk was issued across the Upper Texas coast into
Southwestern LA, including Galveston Island. At the surface, a
coastal trough is positioned just off the Upper TX coastal plain
with a deep moist axis aimed orthogonal to the coast between
Galveston up through the Lower Sabine Valley. The entire area
within the coastal plain is positioned within a very anomalous axis
of elevated moisture with PWAT deviations running between +3-4 from
CRP up through all of Southeastern TX into LA. 00z soundings out of
KLCH depict a very tropical airmass with a deep moisture presence
through the column, basically running from the surface to the
tropopause. Warm cloud layer depth is running around 15.5k feet, a
classic signature of heavy rain potential with a greater propensity
to exude very efficient rainfall rates from warm rain processes.
Even within any deep layer convection, this would spell trouble for
local rates breaching 2"/hr with the sampled environment generally
capable for upwards of 4"/hr within the stronger cell cores. The
latest HREF hourly rate probabilities are indicative of just that
with a corridor of elevated probability signatures between 25-40%
for rates exceeding 3" within the confines of the coast stretching
from Galveston up into places like Port Arthur/Beaumont as the
frictional convergence regime begins towards dawn this morning and
maintains prominence through the early afternoon before
dissipating. The probability fields for total rainfall were the
biggest signal for the risk upgrades with the HREF EAS outputs
considerably bullish within the 2" (45-60%) and 3" (15-30%)
markers. Considering the conservative nature of the EAS due to the
process which it's calculated, this is a significant output that is
typically reserved for those higher end potential events.

Assuming the factors involved and the presence of deep, tropical
moisture advection off the Western Gulf, there was enough of a
signature for a MDT risk upgrade in the location with the highest
probabilities and mean QPF footprint indicating a solid 3-6" with
locally as high as 9" possible, indicated with the LPMM HREF.

...Southeast to Southern Mid Atlantic...

The same quasi-stationary front that has been stuck in the Southern
Mid Atlantic down through the Southeastern U.S will continue to be
a boon for the the region as another round of heavy thunderstorms
will develop within an axis of elevated theta-E's located along the
boundary from LA to Southeastern VA. Several small impulses also
within the stagnant upper pattern will aid in the necessary upper
forcing to promote small cell clusters of storms capable of a
larger areal extent of heavy rainfall that would promote flash
flood concerns within more urbanized corridors across the South,
along with any slower moving convection that gets anchored to
lingering cold pools that are running rampant from the previous
days of convection. A deeper moisture flux will be entering into
the Deep South with an embedded stronger mid-level vorticity maxima
pivoting northeastward after it exits the Gulf. This will spur a
more organized convective cluster downstream over places like
MS/AL/GA. This is causing a well defined bullseye of higher precip
within that corridor and is represented within the probability
fields with the >3" neighborhood output hovering between 40-70%
extending from Southern MS across into the SC Low Country. The
previous SLGT risk was expanded further west to account for the
convective signatures all the way into LA.

...Northeast...

A tongue of elevated theta-E's will be advected north on the lee
side of the upper trough pivoting through the Great Lakes. To the
southwest, a blossoming jet streak will develop over the Southern
Ohio Valley and begin nosing into the interior Northeast by later
this afternoon. At the surface, a cold front will be sweeping
eastward out of Ontario through Western NY and PA leading to
increasing low-level convergence within the axis of greater
instability. All this to say that scattered thunderstorms with
rainfall rates between 1-2"/hr will be plausible across the
interior with the highest chance for flash flood concerns within
Central and Upstate NY where the greatest merger of upper forcing
and repeated convective impacts will occur later this afternoon and
evening before the front kicks the threat downstream. HREF
neighborhood probabilities are generally high for at least 2"
across the Adirondacks back towards the eastern Finger Lakes. The
probabilities for at least 3" are still fairly modest within the
neighborhood output with a bullseye of 40-50% located over the
Adirondacks. The signals is scattered for the highest totals
within the deterministic outputs, so the setup is right on the
higher end of the MRGL and on the cusp of a small SLGT risk area.
For now, maintained continuity but will note the threat of a
targeted upgrade if the trends continue upwards in the next update.


...Midwest and Ohio Valley...

Surface low located over the Great Lakes will begin to occlude and
track northeast through Ontario with a trailing cold front swinging
through the Midwest and Ohio Valley later this morning and beyond.
A general consensus within the CAMs for a round of convection to
develop in-of IA and IL later this morning, spreading southeast
along the confines of the front. Flow will be modestly convergent
along the leading edge of the boundary with some favorable large
scale ascent caused by the northern half of the area sitting within
the left exit region (LER) of a blossoming jet max situated over
KY. Observing the latest forecast soundings off some of the CAMs
indicate a signature for locally heavy rainfall with some potential
training as the flow becomes a more parallel to the front as we
move into the evening hrs. The threat will not persistent however,
as the front will be on the move through the entirety of the
forecast. This has limited the overall potential of the setup to
mainly 1-2" maxes with perhaps as high as 3" as per the latest
HREF neighborhood probabilities of >3" settling between 15-25%
across portions of Central IL with lighter 5-10% markers further
east in IN/OH. This was enough for a continuation of the inherited
MRGL risk with only some minor adjustments necessary to reflect the
latest QPF and probability fields.

...Southwest...

Scattered Monsoonal convection will continue for another period
across the Southwestern U.S with the primary heavy rain footprint
aligned within the Mogollon Rim up through the Great Basin. Areal
SBCAPE values will range from 500-1500 J/kg through a large extent
of the west with PWAT anomalies generally +1 deviations across AZ
and NM with +2 and +3 deviations aligned from Northern CA across
into NV. Highest moisture anomalies will advect eastward through
the period as a shortwave trough across the PAC NW begins to
flatten the northern extent of the ridge sending the more prominent
moisture anomalies into Northern NV, Southern ID, and Western UT.
Current QPF signature within guidance is relatively scattered when
it comes to the higher values, but some 1-2+" totals are likely
within the current setup with the best chance focused across the
Mogollon Rim and the terrain west of I-19 in Southern AZ based on
the latest ensemble QPF output and probability fields. The MRGL
risk from previous forecast was maintained with only a few small
modifications to reflect the instability gradient and forecast QPF
footprint.

Kleebauer


Day 2
Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE MIDDLE
TEXAS COAST THROUGH CENTRAL LOUISIANA...

...2030Z Update...

...Texas & Louisiana...

In coordination with HGX/Houston, TX and LCH/Lake Charles, LA
forecast offices, a Moderate Risk upgrade was introduced with this
update. A couple lows, one in the upper levels south of Brownsville
and a second one off the coast of Brownsville are causing multiple
areas of convection, most of which off the coast. However, the lows
appear likely to combine forces tonight as they track close to due
north. This will bring widespread convection, likely enhanced by
the strengthening nocturnal LLJ, into the Moderate Risk area late
tonight.

As is common with these events, the rainfall will largely be split
between days 1 and 2, but the primary flooding impacts are expected
into the day 2/Thursday period. Expect the heaviest rain along the
Middle and Upper Texas Coast, generally between Matagorda and
Galveston Bays. While this area has not been as hard-hit today, the
likelihood of training storms capable of up to 3 inch per hour
rates is high enough in this area that numerous instances of flash
flooding will be likely starting late tonight but continuing well
into Thursday morning.

Meanwhile up into Louisiana and far east Texas, that same
convection is likely to behave very similarly to the convection
this morning...namely to move well inland from the coast across
central Louisiana before sufficiently weakening so as not to be as
great a flash flooding threat by the time the moisture reaches
northern Louisiana. Thus, the portions of central Louisiana that
are included in the Moderate Risk are more heavily relying on the
antecedent conditions initiated this morning when rainfall amounts
of up to 7 inches were reported in the last 24 hours near
Leesville, LA. Multiple flash flood warnings were issued into
western and central Louisiana this morning. Additional rainfall
expected Thursday, even if lighter in intensity than areas further
south and west, are still likely to result in additional flash
flooding for the area given the favorable antecedent conditions.

In addition to introducing the Moderate Risk, the surrounding
Slight risk was adjusted eastward based on both trends in the
guidance and the favorable antecedent conditions no present into
portions of northern Louisiana.

...Southeast...

There has been a small eastward shift in the axis of heaviest
rainfall across the Carolinas, resulting in a trimming of the
Slight out of portions of central North Carolina and Georgia. There
remains concerns into portions of the western NC Appalachians for
widely scattered instances of flash flooding as well as across all
of SC, so the Slight remains in place for those regions.

...Southwest...

The Marginal Risk area across the Southwest remains largely
unchanged. Persistent anomalous moisture remains across the area,
but should begin to shift east on Thursday. Thus, the likelihood
for convection across California has decreased enough that a
Marginal Risk was not drawn in with this update.

Wegman

...Previous Discussion...

...Southeast to Southern Mid Atlantic...

A cold front will migrate to the southeast pressing through the
southern half of the Mid Atlantic while the at the upper levels a
130kt jet streak will be passing over New England leading to better
large scale forcing within the RER of the jet across the
aforementioned area. Ample pre-frontal instability within anomalous
deep layer moisture will also contribute to the favorable
environment for heavy rain potential, especially as the front
approaches and enhances low-level convergence within the confines
of the boundary. After destabilization will aid in priming the
environment prior to the fronts approach with the initial area of
interest lying over Southeastern VA, eventually shifting focus into
the Carolina's as the front progresses south. This is the area of
greatest significance given the best surface to upper level forcing
presence working in tandem to create a period of widespread heavy
rainfall. Ensemble QPF between 2-3" is common within the confines
of the front from the Hampton Roads area down through Eastern and
Central NC/SC with much of the rain occurring within a short time
frame as the front approaches. This is a signature for heavy hourly
rates that will enhance the flood risk over the region. For that,
the SLGT risk was maintained from the previous forecast.

Across the Southeast, deep moisture presence with several mid-level
perturbations entering the area through the period will assist in
maintaining a scattered thunderstorm signature with heavy rain
threats continuing thanks to the persistent elevated moisture
anomalies and ample instability. The threat is not as pronounced as
what will occur further to the north, but the setup is sufficient
for any cell producing flash flooding, especially over GA/SC where
the best instability is located.

...Southwest and Southern Rockies...

Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. The area
of interest remains across the Mogollon Rim down into Central AZ
where Locally 1-2" with upwards of 3" is possible currently in
that zone due to a stronger axis of instability under which could
end of being near a 600dm ridge on Thursday afternoon. Across the
north, a more prominent moisture advection pattern will continue
through the Interior Mountain West as a shortwave trough over the
PAC NW continues to squash the northern extent of the ridge and
funnel the moisture further into the interior after it began
navigating out of the CA/NV. More widespread convective coverage is
forecast across UT/Southern ID/Western WY as a result leading to
an expansion of flash flood concerns within those areas. The MRGL
risk was generally maintained from previous forecast but did
expand the risk slightly on the northern periphery to account for
the moisture anomalies and accompanying forecast instability across
those northern zones.

Kleebauer


Day 3
Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF THE
EASTERN CAROLINAS...

...2030Z Update...

No major changes were needed to the Slight and Marginal Risk areas
across the country for the Day 3/Friday time frame. The focus will
shift to the Carolinas alone on Friday as activity in Texas greatly
diminishes as compared with Thursday. With some uncertainty on the
northern and southern ends of the Slight, it was expanded north
into the Hampton Roads of VA as well as south to Hilton Head
Island, SC with this update.

Elsewhere the slow eastward drift of the moisture across the West
continues on Friday as well as diminishing amounts, so the Marginal
remains largely the same as inherited, but a bit smaller than
previous days.

Wegman

...Previous Discussion...

...Carolinas...

The cold front from the previous periods will finally make progress
off the eastern seaboard across the northern half of NC while
making slow progress to the south through the rest of NC/SC region.
Scattered thunderstorms within the persistent deep moist and
unstable environment will allow for a period of heavy rain
potential, especially along and ahead of the cold front as it
migrates southward. The heaviest rain will align over Eastern NC
into the South Carolina Low Country, areas where FFGs are a bit
higher and potentially more difficult to flood. Previous rainfall
has lowered the FFG indices, especially as you work from Columbia,
SC to points northeast which plays a role in the current SLGT risk
maintenance. Ensemble QPF output is currently between 1-2" over
the aforementioned area of interest, but can already see the
writing on the wall for locally heavier precip that will cause some
flash flood concerns within more urbanized areas, especially along
and east of I-95. Thus, maintained general continuity with some
adjustments on the northern and southern edges of the risk area.

...Southwest through the Inner Mountain West...

Scattered thunderstorms within a favorable moist axis will linger
through the Southwestern U.S with the northern half of the
convective threat shifting further inland into the Interior before
hitting the proverbial wall at the Central Rockies. Any storm
within the period could exude heavy rainfall with rates generally
topping at 1"/hr, an intensity that could easily cause issues
within the complex terrain out west. The increased moisture
advection back over NM will also open the door for more convective
threats within the remnant burn scars in place over the Sangre de
Cristos, so the MRGL risk was promptly positioned to cover for the
threat. General rainfall maxima is expected to be between 1-2" but
a small chance for locally higher remains, especially from any
cells that get anchored to the terrain.

Kleebauer

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