272
FOUS30 KWBC 202346
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
746 PM EDT Sat Jul 20 2024

Day 1
Valid 01Z Sun Jul 21 2024 - 12Z Sun Jul 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST AND EASTERN NORTH CAROLINA...


...Eastern North Carolina...
A shortwave trough moving eastward from the central Appalachians
into the Mid-South is allowing for diffluence aloft near a front.
Earlier convection across VA has moved east out to sea, and a new
convection is attempting to form between central and northeast NC
at this time. The latest HREF shows the strongest linger signal
near the NC convection, so realigned the Slight southward. Hourly
rain totals as high as 3" and with local amounts to 5" are possible
here into the overnight hours, with the threat waning during the
early morning hours of Sunday.


...Southwest and Southern Rockies...
An upper level high combined with monsoon moisture have led to
thunderstorm activity this afternoon, with the most concentrated
activity across portions of NM and southeast AZ.  Local totals of
1-2" with some locally enhanced amounts remain in the forecast
overnight across southern NM and southeast AZ, with activity
generally on the wane near and after 9 pm MST as CIN sets in.
Activity across much of UT has waned, except near the southern
border with NV. Risk areas were realigned based on HREF and radar
reflectivity trends.  The combination of multi- day impact within
complex terrain and remnant burn scars exacerbating the flash flood
concerns was reason to maintain the southern portion of the
previously assigned Slight Risk, with convective and HREF trends
supporting removal of the northern portion of the risk areas.


...In and near Oklahoma...
The HREF guidance shows activity flaring through the overnight and
early morning hours on Sunday as an upper level disturbance moves
into the area and interacts with 1.5-1.75" precipitable water
values and MU CAPE of 2000+ J/kg, which would take a while to
erode. Effective bulk shear sits around 30 kts, which is sufficient
organize thunderstorms. Hourly rain totals to 2" with local amounts
to 4" look possible here where cells merge and train, and near any
random mesocyclones. The southeast half of OK has been quite dry
this past week, leaving their flash flood guidance values quite
high, though values are more modest across central OK. Left the
threat area as Marginal but Slight Risk impacts are possible if
heavy rainfall aligns with urban areas.

Roth


Day 2
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
SOUTHERN ROCKIES AND SOUTHWEST...

...21Z Update...
Overall pattern remains largely unchanged from previous
outlook...with a corridor of anomalous moisture draped across the
Southeast US and the expectation of moderate to locally heavy
rainfall in the Southwest US. Changes in the Southwest tended to
fit adjustments made in the WPC deterministic QPF, ensemble
guidance and input from affected offices. In the southeastern
US...made only minor adjustments given the fact there were no
strong or compelling signals to latch onto.

Bann

Previous Excessive Rainfall Discussion

...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 ensemble mean QPF was
not as robust compared even with the addition of the elevated
deterministic signals leading to less confidence of higher impacts
that would necessitate an expansion of a higher risk across parts
of Central AZ. The previous update does have the eastern most
confines of the Mogollon Rim within the inherited SLGT risk which
does jive with the latest NBM probability fields signaling the best
threat further east away from the ridge center. QPF mean around
0.75-1.25+" was the target precip depiction for where to outline
the SLGT risk which aligns with the lower FFG exceedance markers
across the region. The highest probabilities for those totals
within the NBM are mainly in NM with only some of AZ seeing those
types of better outputs. This solidified the SLGT risk from
previous forecast with some minor modifications based on the QPF
footprint. 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 Sacramentos and Sangre de
Cristos. SBCAPE between 800-1200 J/kg is forecast within the
ensemble means with some max of 1500+ J/kg showing up within the
terrain of NM and AZ. Expect a general continuity for the next
succession of updates with perhaps that westward expansion of the
SLGT into the Mogollon Rim if the short range guidance trends
upward with the heavy rain signal.

...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. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.

...Southern Plains...

The eastern extent of the broad ridge axis across the Western U.S
will help send down multiple mid-level perturbations that will
allow for convective development upstream over the Eastern NM High
Plains, eventually propagating south and east along expected
outflow generation and mean steering flow out of the north-
northwest. There`s a growing signal for organized multi-cell
clusters to move off the Caprock and enter portions of West TX and
the nearby Upper Trans Pecos. Deterministic output is subject to
variability in the location and magnitude of any convective cluster
that materialized and enters the area. The signal for potential is
present and was enough to expand the western MRGL risk area further
southeast to account for the threat of more organized heavy
rainfall. This is within the lower end of the risk threshold for
now, but will be subject to expansion or removal based on later
trends within numerical output, especially in the hi-res window.

Further to the southeast across Central and Eastern TX, an
approaching front will become a focal point for diurnally driven
convective development in-of the boundary itself as the environment
it moves into becomes increasingly favorable for cell generation.
The convergence signal is not nearly as impressive as areas further
to the east, but the ensemble mean QPF footprint dictates a
smattering of smaller QPF maxima that would indicate some locally
enhanced rainfall potential. Mean QPF is only around 0.75-1.25" in
spatial coverage, but some deterministic output is much more robust
with 2-4+" bullseyes in portions of Hill Country, Concho Valley,
out into the I-35 corridor. This is a lower end MRGL risk
opportunity within the region, but the upper quartile of outcomes
could spell for some local flash flood concerns if everything
breaks right (or wrong depending on perspective).

Kleebauer


Day 3
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 AND SOUTHERN PLAINS INTO THE ARKLATEX...

21Z Excessive Rainfall Discussion
Few changes needed with the primary focus being a quasi-stationary
front and deep moisture in place over the southeastern US and broad
cyclonic flow aloft over the Southwest.

Bann


Previous Excessive Rainfall Discussion

...Southern Plains...

A quasi-stationary front will position itself across the Southern
Plains extending to the northeast through portions of the Southeast
and Tennessee Valley. A stronger mid-level vorticity maxima will swing
down from the northwest and interact with the surface front
allowing for a much more favorable ascent pattern and low-level
convergence field across the area the frontal boundary bisects.
Ensemble bias corrected QPF and the National Blend are indicative
of more organized convective regime over much of Central and
Northeast TX with a western extension back to the Lower Trans Pecos
with some inferences of stronger convection developing in-of the
eastern Stockton Plateau. The better shear and low to mid-level
convergence pattern is further to the east with eyes on the I-35
corridor up closer to the ArklaTex, well documented within the mean
QPF output in the ensembles, as well as the probability fields in
the upper quartile output in the NBM. With the increasing signal
over the past succession of model runs and the upper pattern
breaking towards a more favorable pattern benefiting the Southern
Plains, the inherited SLGT risk was maintained with an extension to
the northeast through the ArklaTex as mid-level vorticity and
potential weak surface wave riding along the front could enhance
the convective regime further away from the initiation point in
TX.

...Southern Rockies...

Western ridge will be in the process of breaking down in intensity
with less of a direct connection of mid-level shortwaves and
confined perturbations sneaking down the Continental Divide.
Theta-E ridge that was ever persistent within the confines of NM
will weaken in conjunction with the evolving mid and upper pattern.
There`s still some lingering moisture located across the Southern
Rockies with enough diurnal destabilization characteristics to drum
up another round of scattered convection across the Sangre de
Cristos and adjacent higher terrain of south-central CO down
through Northern NM. With the heightened risk of flash flooding due
to complex terrain and area burn scars, a targeted SLGT risk was
maintained within the above area reflecting the threat. Mean QPF
signals are not as robust as previous days, so the risk is closer
to the lower end of the SLGT threshold, but enough of a signal with
other factors to contribute to the maintenance of the previous
forecast.

Kleebauer

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt