Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 PM EDT Thu Jul 25 2024

Day 1
Valid 01Z Fri Jul 26 2024 - 12Z Fri Jul 26 2024

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

A Moderate Risk for excessive rainfall has been maintained for
this update for much of the Middle and Upper TX Coast, while being
downgraded for the portions of southwestern LA. Heavy rainfall has
come to an end for the bulk of the region (and surroundings), but
the expectation is for refiring of convection once again late
tonight along the coast (where SB CAPE ranges from 1000-2500 J/kg
with precipitable water values of 2.0-2.2 inches, which are near
the 90th percentile). A positively-tilted longwave trough remains
anchored just to the west (over TX), allowing for DPVA from the
southwest (at least partially derived from prior convection over
the terrain of northeastern Mexico). This locked pattern is tending
to result in reinitiation of convection during the overnight
hours, and hi- res guidance once again depicts this potential
tonight (with 18z HREF 40-km neighborhood probabilities for 5"
exceedance of 20-30%). With 1-4" of rainfall (and locally higher)
over the past 24 hours alone, the area remains sensitive
hydrologically (with USGS gauges indicating above normal to near
record streamflows). Some models indicate intense convection
developing farther inland as well (to as far north as College
Station), and a Slight Risk has been maintained in these areas.

Otherwise, a Slight Risk was maintained for portions of the
Carolinas and adjacent portions of eastern GA, where heavy rainfall
is still ongoing (though will likely wane with loss of daytime
heating). A broader Marginal Risk was maintained across the
Southeast, where localized downpours may continue in the presence
of widespread tropical moisture (with a large area of 1.8"+ PWATs).
A Marginal Risk has also been maintained across much of the
Southwest and Intermountain West, where monsoonal convection will
gradually wane with the loss of daytime heating.

Churchill


Day 2
Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024

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

...20Z Update...

No major changes were needed for the Day 2 ERO. The front off the
Carolinas has been resolved in the guidance a bit further east and
off the coast, so the Slight has been adjusted accordingly. Of
course, with a greater amount of the expected rainfall now expected
to fall off the coast, uncertainty as to the potential for flash
flooding along the coast has increased. 12Z HREF guidance still
suggests an over 70% chance of 5 inches of rain in the Neighborhood
Probabilities from Cedar Island, NC southwest to the South Carolina
line. These amounts if they verify would almost certainly result in
widely scattered instances of flash flooding. However, noting the
tendency of the guidance to shift south and the trends already to
shift the precip east, there is just as much potential for
additional shifts eastward resulting in a downgrade to the flooding
potential.

Elsewhere no significant changes were made across the Southeast
regarding the sprawling Marginal risk. Areas of largely
disorganized convection are expected again Friday afternoon, and
there will likely be localized areas where the convection
organizes, resulting in better than isolated instances of flash
flooding. However, there's little confidence elsewhere on where
that will occur, but targeted Slights going forward are certainly
possible. A very similar situation remains in the West with the
higher atmospheric moisture area gradually shifting east with time.

Wegman

...Previous Discussion...

...Carolinas...

The cold front from the previous period 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" across
Southeast NC, including the Wilmington area extending down through
the Low Country and coastal sections of SC. Individual
deterministic output is as expected with some local maxima around
4" within any particular global member leading to ensemble bias
corrected means closer to 2-2.25" over the hardest hit areas. Did
trim back on the northern extent of the previous SLGT with
maintenance further south and southeast across central and eastern
SC up through Southeast NC where the front will slow in forward
progression and become a focal point for Friday afternoon
convection. Best threat for flash flooding will likely occur over
more urbanized zones due to the sandier soils present in the
confines of the current risk area which lead to the continuity,
and not expecting any upgrades in the current setup.

...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. The previous MRGL risk was
kept to general continuity, but did pull the MRGL risk a little
further west into NV due to some linger convective potential from a
trailing shortwave migrating through the region during peak diurnal
instability, aligned with a further west QPF footprint as of the
latest NBM and ensemble bias corrected outputs.

...East Texas through the Lower Mississippi Valley...

Remnant mid-level energy will linger across Eastern TX into the
Lower Mississippi Valley within a persistent anomalous moisture
presence situated over the region. Sufficient buoyancy and
attendant large scale forcing will likely generate periods of
scattered convection capable of producing locally heavy rainfall
over the aforementioned areas. There are some indications that a
much more defined 500mb low will develop over the Southern Plains
which would increase the general forcing pattern allowing for a
more robust convective initiation over the eastern flank of the
mid-level circulation. This was noted via some of the 00z global
deterministic with the ECMWF being the most aggressive with the
solution. There are some hints of the threat becoming more
pronounced across East TX into parts of LA/AR given the recent GFS
Graphcast and ECMWF AIFS ML output, so a targeted upgrade may occur
over the above areas if the consensus grows and remains stable
in the upcoming model iterations. For now, a MRGL was maintained
from previous forecast.

Kleebauer


Day 3
Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF
THE SOUTHEAST, MID MISSISSIPPI VALLEY, SOUTH AND EAST TEXAS,
NORTHERN PLAINS AND UPPER MIDWEST, ALONG WITH PORTIONS OF THE
SOUTHWEST AND INNER MOUNTAIN WEST...

...20Z Update...

No significant changes were needed to the Day 3 Marginals as well.
Convection across the Southeast and Mississippi Valley remain
similarly disorganized as on Day 2 in the guidance. A cold front
working down the Carolinas may help the storms organize ahead of it
into southern SC and GA, but it's possible it will be moving too
fast to result in a long enough duration of heavy rainfall to
result in more than isolated flash flooding. The area will continue
to be monitored should guidance come into better agreement with a
slowing of the storms. One ingredient remains consistent...there
remains plentiful atmospheric moisture from the Gulf in place
across the Southeast, so there's little doubt the storms will be
capable of heavy rainfall, but if they're moving too fast then it
may not rain enough in any one location to result in flash
flooding.

A similar scenario may try to play out again along the Texas Gulf
Coast...with mesolow development that may possibly help organize
the storms that form over an area that has been hard-hit with heavy
rain the past few days. Here too there's a "higher-end Marginal"
threat for flash flooding that may eventually result in a Slight
risk upgrade.

Conditions across the West will continue to deteriorate as far as
potential for flash flooding goes, so the Marginal Risk has been
shrinking in this area day-to-day...however given the terrain and
at least enough moisture in much of the West, it's going to be
really tough to completely eliminate the flash flooding potential.

The Northern Plains Marginal is probably the least confident of the
Marginal Risk areas, as a slow moving front with limited moisture
moves across ND and northern MN. Given the difficulty for this area
to flash flood even with much more favorable dynamics in place, it
seems this area has the best potential for the Marginal to be
downgraded with future forecasts.

Wegman

...Previous Discussion...

...Texas through the Mid Mississippi Valley...

Pending the evolution of the 500mb pattern across the Southern
Plains, the same areas will be subject to a continuation of the
active convective pattern across South and East TX, spreading
northeast into the Mid Mississippi Valley as indicated by a few
global deterministic. Ensemble means are relatively benign in terms
of areal coverage of at least 1" with the highest located along the
TX coast due to potential surface trough refocusing over the
Western Gulf. A deep moist environment with prevalent mid-level
synoptics would certainly support at least the chance of heavy
rainfall and accompanying flash flooding, so the risk area from
previous forecast was relatively unchanged. There could be a future
upgrade if the mid-level pattern progression supports a more
dynamic setup away from the coast, and/or the coastal plains of TX
see an increasing heavy rain footprint that would call for an
upgrade. Until then, the consensus was not clear enough to warrant
an upgrade, so maintained continuity.

...Southeast...

Scattered thunderstorms capable of locally heavy rainfall will be
possible during peak diurnal convection on Saturday. The moisture
anomalies continue to favor a primed environment for a few
thunderstorms creating some isolated flash flood concerns with
totals generally around 1-2" with a chance for 3+ inches in the
strongest cells. This setup is on the lower end of the MRGL
threshold, so some changes are possible in future updates. For now,
with the signature present in a deeper moisture field, the previous
risk forecast was maintained.

...Upper Midwest...

A digging shortwave trough will enter the northern tier of the
Central U.S with increasing large scale ascent over a relatively
sufficient moist axis and accompanying surface based instability.
The threat for locally heavy rainfall will be highest during the
late afternoon and evening on Saturday with some isolated higher
totals that could enhance local flash flood concerns. The previous
forecast was maintained for continuity as the ensemble QPF output
remains generally unchanged from previous forecast issuance.

...West...

Scattered convection across the Rockies down into Southeast AZ will
continue for another period as the presence of sufficient deep
layer moisture extending from the International Border up near the
Northern Rockies will aid in potential heavier convective cores
that could cause issues within the complex terrain aligned over the
outlined area. The area across NM and a few spots over the Central
Rockies will be the highest threat for flash flooding due to burn
scar remnants creating a higher probability for hydrophobic based
run off within the confines of the area. Targeted SLGT risks will
be possible in the future, but the ensemble mean and scattered
nature of convection is too much of a factor to account for a
higher risk area. General continuity was maintained outside some
minor expansions over the northern section of the risk.

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