Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Sun Jul 21 2024

Day 1
Valid 01Z Mon Jul 22 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST AND THE WESTERN CAROLINAS...

...Western Carolinas...
Some southward shift to the Slight Risk area was done per radar and
18z HREF trends.  Another round of scattered to widespread
convection is ongoing in and near the Carolinas with some
locations setup to see a greater emphasis for heavy rainfall. A few
weak mid- level perturbations will advect northeast within the
mean flow.  The coupling of increased upper support with a strong
diurnal destabilization pattern has promoted the threat of
stronger cell cores with heavy rain potential due to anomalous
moisture lingering along and south of the quasi- stationary front
to the north. Convection firing along the terrain of the
escarpment up through the Appalachians of NC should drift to the
east towards lower elevations. Locally enhanced rainfall exceeding
2" in an hour remains high for much of Western NC down into the
far Upstate portion SC to the north of GSP in the short term.
This area has seen its fair share of heavy rain in the past 24-48
hrs at times, so the latest FFGs are lower than climo.


...Southwest and Southern Rockies...
Divergence aloft between an upper low offshore the Northwest and an
upper level trough across the Plains combined with daytime heating
has led to convective development in- of the Southern Rockies and
adjacent terrain of NM due to primed environmental destabilization
coinciding with a progression of mid- level vortices streaming down
the eastern flank of the ridge across the Western U.S. A slight
weakening of the ridge will allow for an expansion of the
convective threat to migrate westward into the Mogollon Rim with
some deterministic output signaling some formidable amounts within
the terrain. The primary areas of interest within the current SLGT
will reside within the complex terrain and areas surrounding the
remnant burn scars located within the Sacramento's and Sangre de
Cristos. Secondary areas of focus include the NM Bootheel,
Southeast AZ terrain around the Huachucas, as well as the Eastern
NM High Plains into the Northwest Permian Basin of Texas.

The latter of the aforementioned areas has a focus along a remnant
outflow that bisect the Caprock down into the Permian Basin,
outlined by a marginal theta-E gradient in place from Clines
Corner, NM down close to I-20 around the Midland/Odessa corridor. A
shortwave currently analyzed over CO will continue to make headway
to the south around the eastern flank of the ridge eventually
aiding in convective initiation across east-central NM by the
afternoon. Multi-cell clusters of thunderstorms have been
congealing and driven south by prevailing cold pools, riding right
along the theta-E gradient. Storms will be able to hold together
and impact a large area encompassing the Caprock of Eastern NM down
through the Northwest Permian Basin before potentially collapsing
tonight. There is a chance this holds together to the I-20
corridor and provides some heavier rain within the confines of the
Midland/Odessa area, but the probability is lower compared to the
Northwestern areas up across Southeastern NM up through the Caprock
along the TX/NM state lines. Despite a very dry signal within the
soil moisture availability over the region, rainfall rates in
excess of 1.5"/hr will be plausible given the elevated moisture
presence as noted within the latest NAEFS PWAT anomalies
approaching 1-1.5 deviations above normal across much of Southeast
NM extending southeastward into the quasi-stationary front aligned
near I-10. Some adjustments were made due to radar reflectivity and
18z HREF trends.


...Oklahoma...
A compact but well-defined surface wave will linger
within the base of the mean trough carved out across the Mid-
Mississippi Valley back down into the northern fringes of the
Southern Plains in OK.  Signals for increasing low-level
convergence within the confines of low are present, leading to a
narrow corridor of heavy rain that could see totals breach 4"
within a short period of time. The main threat is confined within
the small circulation with the northern fringes of the low being
the prime focus for where modest training could occur.  The areal
extent of flash flood concerns is small due to the compact nature
of the setup.

Roth/Kleebauer


Day 2
Valid 12Z Mon Jul 22 2024 - 12Z Tue Jul 23 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
SOUTHERN ROCKIES, TEXAS THROUGH THE ARKLATEX...

21Z Excessive Rainfall Discussion
The large scale pattern continues to shift slowly during the
period...with a quasi-stationary front continuing to be one of the
main features to focus and sustain convection capable of producing
downpours that results in flash flooding. The previous outlook
still had the support of the HREF probability guidance...so only
minor tweaks were needed,

Bann


Previous Excessive Rainfall Discussion

...Southern Plains...

A quasi-stationary front will bisect much of the state of Texas
with a westward extension into the Southwest TX terrain back
through the Big Bend, Edwards Plateau, Hill Country, all the way
out to the ArklaTex. Mid-level energy from the northwest will
navigate southeast towards the higher terrain south of I-20,
migrating slowly eastward within the confines of the front. The
added upper forcing in conjunction with the increasing low-level
convergence along the stationary front will induce a swath of
stronger thunderstorms capable of significant rainfall within any
cell core. Latest NAEFS PWAT anomalies signals a solid +2
deviations from climo for the moisture field present across much of
the central portions of TX back into the Stockton Plateau. Cell
initiation across the higher terrain out west will lead to
convective clustering with outflow generation stemming from areal
thunderstorm coverage, eventually propagating to the east into the
I-35 corridor in Central TX by later Monday afternoon and evening.
Precip totals within the initial cell development are generally
between 1-2", but some higher totals exceeding 4" will be possible
across the Lower Trans Pecos through the Southern Edwards Plateau.
The heaviest core of precip will lie within the eastern extent of
the Edwards Plateau across into Hill Country until it reaches the
I-35 corridor near Austin/San Antonio. This is where ensemble mean
QPF has been the most consistent for totals exceeding 2" with
indications of up to 3" in areal average QPF within Hill Country
and individual deterministic output exceeding 6" in places hit with
repeated cells. Mean storm motions within the confines of the
boundary are weak meaning slow moving convective clusters with
rainfall rates between 2-3"/hr are possible along that frontal
boundary. The SLGT risk from previous forecast was maintained but
expanded to the west to include more of the Stockton Plateau given
the latest trends within the mean QPF and favorable environment
over the area leading into the evolving event.

Further to the northeast through Northeast TX into the ArklaTex,
energy from the southwest will eventually advect northeastward with
convective generation during the afternoon and evening hours
becoming more organized with the additional upper support. Some
cell clusters will be capable of locally heavy rainfall extending
along the stationary front with some convective training plausible
due to similar conditions from upstream. Totals are not as prolific
within the means across the above region, but some totals of 2-4"
are not out of the question, so felt there was no reason to make
significant adjustments to the previously inherited SLGT risk.

...Southwest and Southern Rockies...

Scattered thunderstorms will develop once again within the confines
of the Sangre de Cristos with highest impacts situated over the
complex terrain and any burn scars within the mountain chain.
Totals are not expected to be as prolific compared to recent days
with less of a favorable mid-level pattern and lower SBCAPE
forecast. Regardless, considering the expected convective
development and the very sensitive nature of the flash flooding
potential within the burn scars, there was enough merit to continue
the focused SLGT risk across the Mountains with an extension down
into the Sacramento's due to the ongoing issues caused by the burn
scarring near Ruidoso. This is a lower end SLGT risk threshold with
impact based reasoning for the risk continuity.

Across the rest of the Southwestern U.S, the upper pattern will
shift to have less ridge potency leading to an expansion of the
convective risks a bit further west to include the Lower Colorado
River valley between CA/AZ/NV. The best threat will still be across
the Mogollon Rim where scattered thunderstorm coverage will likely
spawn some totals exceeding 1" within a short period of time along
the terrain of central AZ. The coverage and moisture anomalies were
not primed enough to warrant an upgrade at this time, but the
threat is still a mid to higher end MRGL risk, on the cusp of a
potential upgrade if the setup becomes more pronounced within the
means.

...Southeast to Southern Mid Atlantic...

Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. The one area of note for a potentially more
impactful setup is across the Central Mid Atlantic where a stronger
shortwave will advect northeast out of the Carolina's with
increased upper forcing traversing the DC/Baltimore metro area
during the middle of peak diurnal instability. Recent deterministic
output is a bit more robust compared to recent forecasts and is in
agreement with the ML output based within the GFS Graphcast and
ECMWF AIFS. It will be interesting to see the trends as we move
into the CAMs window because there could be a targeted SLGT risk
within the urban corridor if the signal holds. A MRGL risk is in
effect, but will be monitoring closely.

Kleebauer


Day 3
Valid 12Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
TEXAS...

21Z Excessive Rainfall Discussion
Few changes needed to on-going EROs given latest deterministic and
ensemble runs from 12Z.

Bann

Previous Excessive Rainfall Discussion

...Southern Plains...

A repeat of heavy convection across TX is forecast as the quasi-
stationary front within deep moisture anomalies remains parked
across much of Central and Eastern TX. Latest ensemble bias
corrected QPF is signaling an additional 2-3" possible within the
I-35 corridor with some scattered heavy rain signals all the way
back into the Central RGV from Del Rio down towards Laredo. Weak
mean storm motions and rates likely pushing 2-3"/hr will allow for
more significant flooding potential over areas that will have been
hit the prior period. In fact, 48-hr QPF could top 6" in spots
within the two successive periods which would allow for significant
issues to arise if it falls across the I-35 corridor. A SLGT risk
was maintained and expanded across the western flank of the risk
area to account for trends in the ensemble QPF footprint and better
instability fields located out towards the RGV. If the previous
period ends up with more significant impacts, this could lead to an
upgrade in the forecast risk over portions of Central TX.


...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. A MRGL
risk is in effect across much of the climatologically favored
areas in the Southwestern Monsoon.

...Upper Midwest...

Shortwave trough moving through the Great Lakes will allow for the
initiation of organized convection as it pivots through the region.
Modest instability and moisture anomalies in the confines of the
Lakes will lead to some scattered heavy rain signals as the
disturbance traverses the area. The progressive nature of the
system will limit widespread flash flooding concerns, but a few
isolated heavier cores could pose some issues for more urbanized
areas across Northern and Central WI into the Michigan UP. A low-
end MRGL is in effect for the above area.

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