861
FOUS30 KWBC 200047
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Fri Jul 19 2024

Day 1
Valid 01Z Sat Jul 20 2024 - 12Z Sat Jul 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST, SOUTHERN ROCKIES, CENTRAL HIGH PLAINS, AND EASTERN
CAROLINAS...

...01Z Update...
Primary adjustment was made to the Slight risk area over portions
of Nebraska/Kansas given persistent cell motion and fairly modest
rainfall rates/amounts so far. Ended up adjusting the Slight risk
southward and merged it with the Slight risk covering portions of
New Mexico and Colorado as a 850 mb low takes shape there as seen
on early evening satellite imagery. Other adjustments to the
perimeter of the Marginal risk area tended to be based on short-
term trends seen in satellite/radar imagery and less so on
numerical guidance which seems to be struggling.

Likewise in the Southeast U.S....limited the northward extent of
the Slight/Marginal risk areas somewhat based on satellite imagery
but convection lifting northward through South Carolina will still
encounter an airmass with 70 to 75 degree dewpoints...so did not
trim too much territory out of the risk area.

Bann



...16z Update...

Primary changes for this update include a Slight Risk introduction
for portions of the central High Plains (northwest KS and
surrounding portions of NE/CO) and the expansion of the Slight Risk
over portions of the Southern Rockies to include more of the
Southwest (along the higher terrain of the Mogollon Mountains/Rim
into southwest NM and southeast AZ). For the former, the 12z HREF
PMM indicates the potential for 3-5" localized totals (with 5-yr
ARI exceedance probs between 20-40%). For the latter, 40-km
neighborhood exceedance probs from the HREF indicate ~20% odds of
localized 2" exceedance. Otherwise overall meteorological thinking
remains unchanged from below, and only minor tweaks based on
observational and model trends.

Churchill


...Previous Discussion...

...Carolinas...

The cold front from the past few days will become quasi-stationary
across the Carolinas with a strong low-level convergence signature
focused from Northeast NC down through Central SC in the Piedmont.
Areal PWATs are sufficient for heavy rainfall potential with
anomalies nestled between 1-1.5 deviations above normal with values
between 1.8-2.2" focused over the above area. Current mean QPF is
right around 1-1.5" with some smaller maxima located across
multiple portions of the Carolinas which is reflective of the
overall convective environment. The primary signature of note for
the potential is heavily based in the latest probability fields
with EAS signatures between 25-40% for at least 1" present through
much of the Eastern Carolinas with the max confined to the eastern
extent of the Piedmont down to around Columbia, an urban area more
prone for flash flooding historically. Neighborhood probabilities
for at least 3" show multiple areas between 50-70% with another max
located within the escarpment of SC/NC/TN where models are
depicting some terrain focused convection later this afternoon.
There are some minor signals for at least 5" totals within the
confines of the Eastern Carolinas as well signaling a general
maximum of up to 6" in any cells. Area hodographs indicate a weak
steering pattern which would lead to either slow moving, or even
stationary storms that could enhance the local threat further as
the storm motions would only be accelerated through outflow
propagation. The SLGT risk from the previous forecast was
maintained with some expansion over the northern and southern
fringes to match the recent trends within the probability fields.

...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 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 a few spots seeing
higher amounts are forecast across the terrain down into the
northern extent of the Bootheel near Gila National Forest.
Probabilities for at least 1" are relatively higher when gauging
the neighborhood probability output with some scattered signals for
at least 2" in portions of Central NM. 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 maintain general continuity.

Kleebauer


Day 2
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

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

...20z Update...

The Slight Risk over the Southern Rockies has been expanded
southwestward to include the high terrain of the Mogollon
Mountains/Rim from southwest NM into southeast AZ, as well as lower
elevations downstream into southeast AZ (including Tuscon and
surroundings). The expectation is for a bit more convective
coverage relative to Day 1, due to minor weakening/westward shift
in the upper-level ridge. This should also allow for a bit more
storm movement, which may reduce residence time of higher rates a
bit, but perhaps more significantly this can allow for more storm
propagation into the low elevations of southeast AZ. A skillful
proxy for flash flood impacts across the southwest is the 2"
exceedance threshold from the HREF (40-km neighborhood method),
which exceeds 20% across the bulk expanded Slight Risk. Outside of
this region, changes/tweaks to the outlook were minimal.

Churchill


...Previous Discussion...

...Southern Rockies...

A pattern of 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. Theta-E ridge is forecast to shift focus across NM
with the tongue of elevated instability generally confined to the
terrain extending from the Bootheel up through the Sangre de
Cristos. Scattered to numerous thunderstorms will be capable of
locally heavy rainfall that will interact with complex terrain and
the burn scars littered across the Southern Rockies down to the
Sacramento Mountains. Areal QPF average between 0.75-1.5" is
forecast with a few locations potentially seeing upwards of 2"
which would easily cause issues no matter where they occur given
the lower FFGs situated across the state. Considering the ongoing
impacts from prior periods and the convective coverage anticipated,
maintained the previous SLGT risk with some broadening of the risk
area to reflect the concerns even outside the terrain focused
areas.

...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.

Kleebauer


Day 3
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...

...20z Update...

Similar to the updates on Days 1 and 2, some expansion of the
Slight Risk was made from the Southern Rockies into the Southwest.
While overall convective coverage and intensity has the potential
to be a bit greater on Sunday, confidence is not as high given the
lack of hi-res guidance during this time frame. The expanded Slight
was confined to the more confident higher terrain areas of NM (and
some surrounding portions of CO/AZ). These areas are where a bias
corrected blend of the ensemble guidance suggests localized totals
of 1-3", and corresponds with many sensitive burn scars which have
been the focus for locally significant flooding impacts. Otherwise,
only minor adjustments were made to the Marginal Risk areas.

Churchill


...Previous Discussion...

...Southern Rockies...

Mid-level ridge axis will begin sliding eastward with building
heights centered over the Lower Colorado River Basin into the
southern portion of the Great Basin. A multitude of mid-level
vortices will continue to plunge down the eastern flank of the
ridge and interact with ample surface destabilization located
within the Southern Rockies. With steep low to mid-level lapse
rates and prominent moisture anomalies running close to 1-2
deviations above normal, convective threats will induce periods of
moderate to heavy rainfall within the general confines of the
Southern Rockies with emphasis on the Sangre de Cristos and the
Northern fringe of the Sacramentos. There will also be an extension
of convective concerns down closer to the NM Bootheel within the
Gila National Forest just to the north of Silver City. Areal QPF
within the means are consistent with 0.75-1.25" with some bias
corrected maxes exceeding 2" in spots, mainly over Northern NM.
With the successive period of heavy rainfall potential within the
terrain and remnant burn scars, the threat remains well within the
SLGT risk range inherited from prior forecast. Thus, outside some
adjustment within the fringes to reflect QPF trends, the SLGT risk
was majority continuity.


...Mid-Atlantic to Central Gulf Coast...

A mid-level trough will begin to carve out across the Central
Plains with a lingering stationary front still bisecting much of
the Southeast into the Southern Mid Atlantic. Broad extent of
thunderstorm chances will remain stagnant in terms of placement and
potential impact with greater effects generally relegated to focal
points near higher surface convergence patterns and deep layer
moisture field present in any given area. Scattered to numerous
thunderstorms are forecast across TX to the Central Gulf Coast
during the afternoon on Sunday with locally heavy rainfall
plausible in any cells that develop. General 1-2" are possible in
any convective signal, however the area over the Central Gulf has
the best chance for heavier returns due to a deeper moisture field
located along the Gulf. These areas do have a higher FFG however,
so the threat is on the lower end of MRGL during the period, along
with the area across TX. Continued threat of heavy rainfall is
forecast across the Southern Mid Atlantic where multiple shortwaves
will enter the region and enhance regional ascent within the
convergence corridor near the stationary front. The main area of
impact is subject to some variability at range due to the forecast
frontal placement being a major part of where training convection
could develop. The MRGL risk across the area was broad but worthy
of the current location given the setup. An upgrade is possible
pending the convective evolution over the previous period.

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