FOUS30 KWBC 201551
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Sat Jul 20 2024

Day 1
Valid 16Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

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

16Z Update...
Overall...changes to the on-going Excessive Rainfall Outlook were
pretty minor. Broad area of deep moisture and resulting instability
remained in place across the Southeast US with weak and difficult-
to-time shortwaves embedded within the mean flow. Flow looks to
remain confluent...which will help support cell development and
fuel the potential for isolated to widely scattered downpours
throughout the day and evening. There were some signals from the
12Z HREF probabilities for 1 to 2 inch per hour rates over portions
of northern Iowa into far southern Minnesota. The previously issued
Marginal did not reach quite that far north...so extended the area
a bit to cover the potential.

Over the western US...models still tended to support the previously
issued Marginal risk area as well as the embedded Slight Risk area
extending from southeast Arizona across much of New Mexico into
southern Colorado. Except for a few nudges...the area remained
unchanged from the initial issuance. 

Bann

...Southeast Virginia...

A well-defined shortwave analyzed across the AL/GA line will advect
northeast through the early portion of the period, eventually
closing off at 85H as it progresses into Southern VA by the early
afternoon period. At the surface, a quasi-stationary front will
lift northward towards the vicinity of the Hampton Roads corridor
with a surface reflection developing over the NC/VA border and
riding along the confines of the boundary. Increasingly backed flow
within the boundary layer will promote the advection of deeper low
to mid-level moisture that will align with the frontal boundary as
the surface wave approaches with generally more favorable support
within the 850-700mb height field. The combination of the above
variables will allow for a stronger low-level convergence zone
aligned within close proximity to the wavy frontal positioning
transposed over the region. The latest HREF and correlating CAMs
output have all increased the signal within the QPF footprint with
the ensemble means generally depicting an axis of heavier precip
from I-85 to the east towards the I-64 corridor and adjacent
coastal areas. QPF maxima in exceedance of 5" is common within the
deterministic output along with the HREF blended mean signifying a
stripe of 2-3+" within the confines of Williamsburg down close to
Norfolk. HREF neighborhood probabilities were also fairly stout
when it comes to the upper quartile of the threat as the >3" probs
sit at a widespread 40-70% over the entire stretch of Southeast VA
from I-85 to I-64 with a bullseye of 80% located along I-64. The
>5" probability field is less robust out towards the I-85 area up
to Richmond, but there's still a significant corridor of 30-60%
probabilities situated across that I-64 area down close to Hampton
Roads.

The signals were pronounced enough to warrant a targeted upgrade
across the aforementioned area with the alignment of the risk
comprising of the higher forecast probabilities for at least 3"
with the best threat likely over the I-64 area southeast of
Richmond. The general proxy of the front and the aided surface wave
bisecting the area will likely target an area under or just to the
north of the general circulation. The track of any 850mb low
development will only add to the prospects of increasing low-level
convergence signatures that would spawn an axis of very heavy
rainfall (>2"/hr) within the northern periphery of that circulation
as well. A SLGT risk was added to account for the evolving setup.

...Southwest and Southern Rockies...

General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel over into
Southeast AZ as noted within the convective QPF footprint. The
area will see another round of diurnally driven convection with
more mid-level perturbations rounding the eastern flank of the
ridge, moving overhead during the peak of diurnal heating. This
will create an enhancement within the large scale ascent pattern
along with the already favorable instability field with the theta-E
ridge bisecting much of Central and Northern NM. Local totals of
1-2" with some locally enhanced amounts are forecast across the
terrain down into the northern extent of the Bootheel near Gila
National Forest. HREF neighborhood probabilities for at least 1"
are very high with an areal coverage exceeding 70% that encompasses
much of north-central NM, the Sacramento Mountains, and the
Huachucas in Southeastern AZ. The 2" probabilities are also higher
than previous days with an expanse of 15-30% outputs with a few
bullseyes over 30% within the aforementioned zones above. The
combination of multi- day impact within complex terrain and remnant
burn scars exacerbating the flash flood concerns was plenty of
reason to extend the SLGT risk and maintain general continuity.

Kleebauer


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...

...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
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