Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Tue Jul 23 2024

Day 1
Valid 16Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY AS WELL AS THE
SOUTHERN MID ATLANTIC...

...16Z Update...

No major changes were needed to most of the ERO risk areas for
today. The guidance remains somewhat steady state highlighting
portions of south central and southeast Texas as well as the
Carolinas for the most rain this afternoon through tonight. This
continues to match well with the inherited Slight risk areas.

One of the two changes of note was to trim back the Slight and
Marginals around the Big Bend of Texas, as much of the expected
rain has already happened. The front causing the rain will continue
to sag south and east today, so no redevelopment of heavy rain is
expected along the portion of the Rio Grande north and west of Del
Rio.

The other change was to expand the Slight in NC and VA westward to
include more of the flood-sensitive mountains along the NC/TN
border as well as portions of southwest Virginia. The front to the
west is likely to direct training storms northeastward into the
region later this afternoon into this evening. Given the saturated
soils and the sensitivity to flooding of the terrain of this
region, the upgrade appears necessary.

Wegman

...Previous Discussion...

...Southern Plains and Lower Mississippi Valley...

Two distinct areas of interest for the D1 period with different
synoptic and mesoscale evolutions to impact each area. The first
area will be across Central TX within the Edwards Plateau, Hill
Country, to the I-35 corridor. A persistent shortwave embedded
within the base of the trough located over the Southern Plains will
enhance regional upper forcing through the first half of the
forecast period, interacting with a weak quasi-stationary boundary
bisecting the aforementioned area. As high pressure noses in from
the north, a tightening theta-E gradient will transpire within
the confines of the shortwave impulse. Enough favorable large scale
forcing within a weakly capped environment will allow for the
initiation of a line of convection located within the proxy of the
front. Models are in agreement on the development and overall
impacts expected within the line of convection promoting locally
heavy rainfall within areas that were impacted over the previous 24
hrs. Area FFGs are much lower within the Edwards Plateau out
through Hill Country after yesterdays storms leading to a lower
threshold for impacts from flash flooding as the top layer of soil
moisture now sits between 60-80% according to the latest NASA SPoRT
viewer. Neighborhood probabilities for at least 3" are up around
50-80% within a large portion of the above area, extending
eastward to the I-35 corridor from Killeen down through New
Braunfels. >5" probabilities are also present, but more scattered
in the presentation with percentages closer to 15-25% leading to a
somewhat capped upper potential. This was sufficient for the
current SLGT risk continuity with minimal change from the previous
forecast.

Further southeast into the Texas coastal plain and the adjacent
Lower Mississippi Valley, the pattern will yield a much different
impact scenario thanks to the evolving upper levels leading to a
persistent Gulf moisture advection regime within a developing
coastal trough. Small impulses will round the western periphery of
a mid-level ridge extending through the Gulf, making their presence
known as they enter the proximity of the middle and upper TX
coasts. Bands of heavier rain with tropical origin in the moisture
field will promote efficient rain makers as they work their way
onshore. Just away from the coast, adequate surface buoyancy with
deep moisture presence will allow for the generation of more
thunderstorms just away from the coast leading to a larger QPF
footprint between the two convective initiation standpoints. As of
now, the prospects for flash flooding are highest along the Upper
TX coast between Houston to the southwest corner of LA as all
deterministic, ML, and associated ensemble means have overlapped
with the best focus of heavy rainfall around that corridor in
question. Areal average of 2-4" will be forecast within that span
of the coastal plain with QPF maximum upwards of 7-8" possible if
one particular area sees extended training. Probability fields
indicating at least 5" are running between 25-40% across an area
between Matagorda through Port Arthur right along the coast. The
probabilities are a bit less for the >5" totals away from the
immediate coast, but still generally between 10-20% for places like
Houston and Beaumont. This is bordering on a higher risk category,
but the protrusion of heaviest rainfall being mainly at the coast
leaves this on the fence, but well within the upper bound of the
SLGT risk. Will be monitoring the progression of the setup closely
as a targeted MDT is possible, especially if conditions allow for
the population centers within Houston/Galveston up towards Port
Arthur and Beaumont.

Some of the moisture entrainment within the northwest flank of the
ridge in the Gulf will be pulled inland to the northeast over LA
and MS leading to an expected band of heavy thunderstorms capable
of rates between 2-3"/hr at peak and totals pushing 2-5" locally
extending from Lake Charles up through Jackson, MS. The prospects
for localized flooding within urban corridors has grown from recent
updates allowing for an extension of the SLGT risk over TX to be
pulled more northeast to account for the threat.

...Southern Mid Atlantic...

Another day of scattered to widespread convective impact
anticipated across the Southern Mid Atlantic with the heavy rain
threat continuing within Central and Northeast NC up through the VA
Tidewater. Heavy rain is likely within any cell development thanks
to a persistent +2 deviation PWAT anomaly situated south of the
quasi-stationary front bisecting the Central Mid Atlantic. Multiple
smaller impulses will advect from the southwest with the mean flow
aloft along with entering into the RER of an upper jet streak
forming to the north. This will correlate to cell initiation
within more favorable upper dynamics and modestly buoyant
environment in place over the Southeast VA through the Carolinas
creating another threat of heavy rainfall and repeated impact of
cells along the corridor from the SC Piedmont and points northeast.
HREF EAS probabilities between 25-40% for at least 1" covers a
large expanse of the Southern Mid Atlantic, a traditional symbol
for agreement within the CAMs on a widespread convective heavy
rain threat and within the threshold for a SLGT risk upgrade. Based
on the setup, repeated threat continuing of heavy rain and flash
flood concerns, and the probability fields insinuating more 3+"
totals locally in Southern VA and through NC, a SLGT risk was
issued over the aforementioned area(s).

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of the southern half of NV leading to
more coverage of the MRGL risk across those areas. QPF means are
generally light, but some of the higher end outputs within the
deterministic suite are upwards of 1-2", especially across the
Mogollon Rim. This is a classic Monsoonal setup with the favored
terrain being the primary focus for the period. The MRGL risk
inherited was maintained with full continuity as ensemble mean QPF
did not change much in terms of magnitude and precip placement from
previous forecast.

...Midwest...

Shortwave trough across the Great Lakes will continue to push
southeastward with modest upper forcing under the trough axis.
Modest instability will linger across the Midwest heading into
today with some trailing vorticity maxima entering Central Midwest
by late morning and afternoon. Convection will develop upstream
over the Upper Midwest and pivot south into Central and Southern
WI, IA, Northern IL, and eventually spread eastward into parts of
Michigan. Isolated heavy rain signals within the CAMs backed by
some modest probabilities for 1-2" and locally up to 3" promote a
general continuation of the MRGL risk in place. Only some minor
adjustments were made to the previous forecast given the relatively
stable ensemble QPF footprint.

Kleebauer


Day 2
Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA, AS WELL AS PORTIONS OF THE
SOUTHEAST...

...2030Z Update...

...Middle Texas Coast to Southwestern Louisiana...

A slow-moving disturbance in the western Gulf at the southern end
of a stationary front draped across the Southeast will be the focus
for heavy rain along much of the western Gulf Coast on Wednesday.
The front will make slow but steady progress east across interior
Texas, but a persistent low-level jet (LLJ) of abundant Gulf
moisture and instability will continue to pump that moisture into
the front across east Texas and western Louisiana. There has been a
small eastward nudge to the forecast axis of heaviest rain. Much of
the heaviest rain will remain off the coast of Texas, but the
southerly flow will threaten much of the immediate coast especially
through Wednesday morning. Due to less instability inland because
it's the coolest time of the day, it's expected that much of the
heaviest rain will fail to make it very far inland before weakening
and becoming much less of a heavy rain threat. That said, much of
the area has seen multiple days of heavy rain, so rivers and
streams...already running high in some areas from previous
months...remain unusually susceptible to additional flash flooding
should heavy rain occur over an extended period of time. Given the
favorable pattern for training convection, this is certainly
possible.

On the flip side of the flooding threat coin is the susceptibility
of the CAMs to have a north bias...meaning frequently heavy rain
occurs south of there the guidance suggests it will be. Any
southward deviation of the heaviest rain from where it's depicted
now and the entirety of the heavy rain remains off the coast. Thus,
there is a high bust potential. It's important to note with about
90 percent of the forecast heaviest 24-hour rainfall already off
the coast, that the guidance appears to (rightly) suggest that the
Gulf is the primary fuel source for the rainfall. Thus, it's
reasonable to expect that if moving inland results in a rapidly
decreasing supply of "fuel" for the storms, then it's a very
reasonable expectation that the storms don't even try to move
inland. It appears most likely that the storms that do impact
coastal communities will move into southwestern Louisiana, rather
than Texas. This is due to the predominant southwesterly flow
supporting the storms tracking northeast...and the Louisiana coast
would be the first land northeastward moving storms would
encounter. Ultimately the positioning of the front, which would
locally enhance lift, will determine how much rain occurs into
Texas.

...Southeast...

The Slight risk area across the Southeast was expanded in multiple
directions with this update. Guidance indicates increasing
likelihood for heavy rain across central Alabama. The area has been
hit with multiple days of scattered thunderstorms that have locally
produced heavy rain and saturated soils. NASA Sport imagery shows
much of central Alabama well above normal for soil moisture and
above 70% saturation in the near-surface soils. Thus, expect a
renewed round of heavy rain in the area to have a higher-than-
normal likelihood of resulting in flash flooding. The Slight was
expanded southwest roughly following where the lower FFGs may
coincide with heavy rain.

The Slight was also expanded west to include the mountains of the
western Carolinas. Low FFGs and terrain considerations will both
locally increase the likelihood of flash flooding should heavy rain
impact the area. Given the well above normal atmospheric moisture
expected, as PWATs approach 2 inches, the storms will be quite
capable of heavy rainfall. The good news is the storms will be
moving along to the northeast, so it will take some training of
multiple cells to produce significant flash flooding.

Finally, the Slight was also expanded to include Hampton Roads of
southeast Virginia and the portion of northeastern North Carolina
north of Albemarle Sound. This was largely due to both ongoing and
expected convection that will impact this area on top of the
multiple days of storms that have already impacted this region.

...Southwest...

Monsoonal moisture will result in increased coverage of showers and
storms, primarily across Arizona and the Mogollon Rim again
Wednesday afternoon as compared to today. However, limited
organization should keep any resultant flash flooding isolated, so
the Marginal remains in place with few changes.

Wegman

...Previous Discussion...

...Texas and Louisiana...

A continuation of the pattern from the previous period will lead to
more shortwave impulses exiting out of the western Gulf as they
navigate northward between a broad upper trough to the northwest
and the persistent ridge off the southeast in the Gulf and Western
Caribbean. A coastal trough will strengthen at the surface right
along the mid and upper TX coast leading to a more defined axis of
surface convergence in-of the coastal plain. Moisture anomalies
will be poking closer to +3 deviations with area PWATs generally
running between 2.2-2.5 along the TX coast all the way into
Southwest LA. There is still some modest uncertainty as to where
the best axis of heavy rainfall will preside for the D2 period, but
the consensus is slowly merging towards the Upper TX coast with a
grazing of the Mid TX coast for the heaviest rain potential. Recent
ensemble QPF has an axis of 2-4" located right along the immediate
coast beginning near Matagorda, expanding to the northeast to as
far north as Lake Charles. Ensemble probabilities within both the
GEFS and ECENS are targeting the Middle TX coastal areas to the
southwest of Houston with the best probabilities for upwards of 2"
while the NBM has a split maxima of one overlapping the area the
GEFS and ECENS are targeting along with one to the north over Port
Arthur into Lake Charles. The split comes for a discrepancy in the
handling of a more robust shortwave that ejects to the north later
in the period. Due to that discrepancy, a potential for an upgrade
was passed on to take more time for guidance to come into agreement
on where the focal point for the highest totals will occur. There
is enough merit given the environment that if any area is favored
for at least 4" within the mean, and probabilities favor the threat
of over 5-6+", there could be a targeted MDT risk in future
updates. For now, a broad higher end SLGT has been maintained with
emphasis on the Middle and Upper TX coast, including near Houston
proper.

...Southeast...

A stronger mid-level impulse will eject northeast out of LA into
through the Deep South within a corridor of moderate instability
and deep moist access. The favorable upper forcing and accompanying
thermodynamic presence has allowed for guidance to generate a
widespread area of higher QPF along the confines of the impulses
path. This is consistent within the ensemble bias corrected QPF
footprint with 1.5-2.5" totals in the ensemble positioned from
east-central AL through Central GA/SC into extreme southern NC.
Considering the prospects for 2-3+"/hr rates within the convective
zone Wednesday afternoon, a SLGT risk addition was warranted to
cover for the potential.

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.

...Great Lakes and Ohio Valley...

Surface low across MI will push northeast into Canada with a
trailing cold front progressing east and southeast over the course
of Wednesday. Isolated to scattered thunderstorms will develop
along and ahead of the cold front with some modest cell cores
plausible given the increased surface and upper forcing pattern.
QPF footprint is overall on the weaker side compared to what you
would want to see regarding more considerable flash flood threats,
but the environment is capable for some totals of 2-3" in a very
short time which if it falls within any of the urban corridors
around the Lakes could spell some isolated flash flood concerns.
The threat overall is low-end within the MRGL threshold, but enough
to warrant a continuation from previous forecast.


Kleebauer


Day 3
Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHEAST
TEXAS, WESTERN LOUISIANA, AS WELL AS PORTIONS OF THE SOUTHEAST...

...2030Z Update...

...Southeast Texas into Western Louisiana...

Much of the rainfall expected into this region for the day Thursday
will occur Thursday morning, before the usual weakening of the LLJ
result in decreasing coverage and intensity of convection by
midday. Nonetheless expect a renewed southerly push due to the
peaking of the LLJ's intensity, which will result in the stationary
front over eastern Texas briefly becoming a warm front as it
retrogrades westward. The heaviest convection will be in eastern
Texas, including the Houston area since this will be the area where
the aforementioned front will be aligned parallel with and be
closest to the Gulf. Much of the Upper Texas Coast is considered in
a higher-end Slight, both due to expected convection and soils
remaining saturated from previous days' rainfall.

...Southeast...

The Slight Risk area was expanded well to the south with this
update, and now includes much of northern GA, all of SC, and all of
coastal NC. This is largely due to a small eastward shift in the
axis of heaviest rainfall expected to impact the area during peak
heating Thursday afternoon as the front forcing the storms makes a
small eastward jaunt across this area. With well above normal
moisture in place, unidirectional southwesterly flow favorable for
training, and well-saturated soils from prior rainfall, flash
flooding will be widely scattered in this area...with the greatest
risk in any urban areas from Raleigh east.

Wegman

...Previous Discussion...

...Texas and Louisiana...

A general persistence in the pattern will yield another active day
across the Texas and Southwest LA coastal plain. There's still not
enough agreement on the potential for an upgrade beyond the current
SLGT risk forecast. Pending the outcome from the previous periods,
a targeted upgrade is not out of the question, but will not commit
until we have more consistency in the ensemble QPF output and we
have some idea of impacts prior to the period in question. For now,
another 2-4" with locally higher amounts is plausible along the TX
coast from Corpus Christi up through Lake Charles. Multi-day totals
could be exceeding 8" in spots after this time frame, so it bears
monitoring in the coming days.

...Carolina's to the Virginia Tidewater...

A continuation of the active pattern across the Carolina's into
Southeast VA will unfortunately resume for another day as the area
sees an introduction of a stronger shortwave out of the Southeast
along with entering into the favorable RER of a 125-130kt jet max
positioned across the Northeastern U.S. The combination of the
maximized upper forcing within a very moist, unstable airmass sets
the stage for another round of heavy rainfall with cell cores
capable of exuding another 2-3"/hr in rain rates with slower cell
propagation as noted by the forecast Corfidi Vectors in NC hovering
around 7-10kts within the upshear component of any storms. Ensemble
bias corrected QPF signatures of 2-3" are already showing up across
the Central and Eastern Carolina's with the northern extent peaking
up into the Hampton Roads area. This area will have been through a
multitude of heavy rain events in the past week, so another day
will lead to issues, especially with one of this expected
magnitude. A SLGt risk was introduced across much of SC/NC up
through Southeastern VA as a result.

...Southern Rockies and Southwest...

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. One area
of interest is across the Mogollon Rim down into Central AZ.
Locally 1-2" with upwards of 3" is possible currently in that zone
due to stronger axis of instability under which could end of being
a 600dm ridge on Thursday afternoon. A MRGL risk is in effect
across much of the climatologically favored areas in the
Southwestern Monsoon, but the Mogollon Rim and potentially over
Northwest UT will be favored for an upgrade if the current forecast
holds.

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