FOUS30 KWBC 211555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sun Jul 21 2024

Day 1  Valid 16Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

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

...16Z Excessive Rainfall Discussion...
Few changes needed. There was a southeastward extension of the 
Slight Risk a bit farther into Texas than before given a subtle 
increase in QPF shown by the NBM and reflected by the HREF 
probabilities. Otherwise...the 21/12Z suite of guidance largely 
supported the on- going ERO/ERD.

Bann

...Western Carolina's...

General pattern persistence across the Southern Mid Atlantic will
yield another round of scattered to widespread convection in-of
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, encroaching on the Piedmont of the
Carolina's by later this morning. The coupling of increased upper
support with a strong diurnal destabilization pattern will promote
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. Recent trends within the HREF and
associated CAMs have been for an increase in heavy rainfall for
points along and west of I-85 where the mid-level ascent will be
maximized along with the favored instability. Convection will
likely fire along the terrain of the escarpment up through the
Appalachians of NC before drifting to the east and northeast away
from the terrain. Probabilities for locally enhanced rainfall
exceeding 2" is very high (>70%) for much of Western NC down into
the far Upstate portion SC to the north of GSP. There's some hints
of upwards of 5" within the probability fields and individual CAMs
members along the escarpment up through areas like Boone points
east towards Greensboro. 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. In coordination with the surrounding WFOs that bridge
coverage in the Western Carolina's, have introduced a SLGT risk
across the area encompassing much of the area west of the I-85
corridor.

...Southwest and Southern Rockies...

A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of 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 is a newer development
within the latest ensemble means with 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 cluster of thunderstorms will congeal by
prevailing cold pools and migrate to the southeast, riding right
along the theta-E gradient that will maintain presence through the
afternoon. There's a growing consensus that 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 in the evening. 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. HREF
probabilities are highest for at least 2" within the confines
above, including some >70% output being displayed within the
Northwest Permian up through Lea and Roosevelt Counties in NM.
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.

The previous SLGT risk was maintained, but did allow for an eastern
extension to account for the increasing organized convective threat
aimed for portions of Southeastern NM down into West Texas.

...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. Current IR satellite indicates a blossoming of convection
over Northeastern OK this evening that will lead to some locally
heavy rainfall close to Tulsa and surrounding locales. Hi-res
deterministic is consistent on the surface reflection sticking
around through the first half of the period with a short term
degradation of the convective field after 12z, but comes back in
earnest due to the addition of the diurnal destabilization along a
surface trough extending near and south of the surface low. Signals
for increasing low-level convergence within the confines of low
are present within several CAMs outputs later this morning and
afternoon 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. There's a small footprint within the ensemble means of 2+"
just south of the Tulsa metro with accompanying neighborhood
probabilities of >5" up between 25-40% in the same area. Whether
that's the exact location or not, the areal extent of flash flood
concerns is small due to the compact nature of the setup. A MRGL
risk was maintained from previous forecast, but want to make
mention the threat could trend towards more locally significant
impacts where the training convective pattern establishes itself.
Look for future MPD's on the threat as we move towards the late
morning and early afternoon hours.

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...

...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
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