FOUS30 KWBC 220749
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Thu Aug 22 2024
Day 1
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S...
...Western U.S...
A continued amplified pattern across the west will evolve further
with a retaining deep moisture fetch arcing through the Desert
Southwest and further into the Inter Mountain territories within UT
and CO. A general +2 to +3 deviation PWAT anomaly will bisect the
region of Northern AZ through Central UT and Western CO by the
afternoon hours today leading to an enhanced convective output
during peak diurnal destabilization. A growing consensus amongst
CAMs members for widespread thunderstorm development with stronger
cores able to produce 1"/hr rates or better aligns well with the
pattern and will induce a greater potential for flash flooding
along the aforementioned areas.
A strong HREF EAS signal for 1" of rainfall exists within the
Southwest portion of UT, including some of the busier National and
State Parks where slot canyon positioning and stronger flash flood
prospects overall create a heightened prospect for impacts during
the expected convective time frame. HREF neighborhood probabilities
for >2" of rainfall are also very high (50-80%) within the local
area of Southwest UT down into the Northern tier of AZ, a signal
generally befitting for flash flood scenarios in recent history.
The threat is not as robust further south into the Mogollon Rim as
the deeper moisture advection regime will be located further north,
but the environment will still be primed for scattered heavy rain
signatures within that zone, so continuity on the southern end of
the SLGT risk was maintained from previous forecast. Further north,
the SLGT risk remains, but the threat is on the higher-end of
potential for the areas near Flagstaff up through that area of
Southwest UT thanks to the alignment of the best instability, deep
layer moisture, and correlated IVT pulse traversing the area.
Pending the evolution of hi-res guidance and radar trends leading
into the afternoon, a targeted Moderate Risk is plausible,
especially within the slot canyon corridor that has the highest
flash flood susceptibility. The areas further north across UT and
CO will see their fair share of convective impacts, especially late
in the afternoon and evening when instability reaches peak and any
mid-level perturbations stemming from convection to the Southwest
advect northeastward allowing enhanced ascent locally as they
maneuver overhead.
Another area of interest will occur across the CO Front Range as
combination of elevated moisture, instability, and a stronger
shortwave ejecting out of the Central Rockies will ignite another
round of heavy thunderstorms to the east of Denver, an area that
has seen impacts as of late with regards to flash flooding. A
secondary QPF max located across the area allowed for a pretty
elevated signal within the 3hr FFG exceedance probabilities
(40-60%), mainly after 00z when LLJ influence peaks the intensity
of the forecasted convection. A MRGL risk is currently located over
the area, but that corridor across Eastern CO, away from the divide
is the focal point for that time frame.
...Florida...
Two separate zones for flash flood potential will exist across FL
this period. The first of which will be situated across the western
coast of the Peninsula with the Clearwater/Tampa/Sarasota corridor
as the primary focus. A quasi-stationary front will meander over
the northern half of FL with smaller mid-level impulses wandering
eastward off the Gulf, creating a multi-wave convective scenario
during the entirety of the forecast. 00z sounding out of KTBW
indicated a sufficiently deep warm cloud layer (>15.5k ft) with
areal PWATs running over 2" with expected continuity during the
period. Mean flow running parallel to the front will create an,
"Express lane" for multiple mid-level shortwaves off the adjacent
Gulf to bisect the area, each carrying their own convective
conglomeration as they move ashore across the region. Rainfall
rates of 2-3"/hr will be plausible in the scenario, impacting an
already water logged area of FL that is still dealing with the
aftermath of Debby. The lower FFG intervals within the urban
corridors, and now surrounding areas with the slow receding waters
create a higher likelihood of flash flooding to occur, especially
with the multitude of convective waves anticipated during the
forecast period. A MRGL risk was added to account for the threat.
The second area of interest lies further northeast, closer to the
Jacksonville metro and surrounding coastal portions of Northeast
FL south of Jacksonville, as well as far Southeast GA where the
recent CAMs output is the most aggressive with regards to heavy
rain prospects. A strong HREF neighborhood probability for >5"
(50-80%) exists within that zone between GA down to Jacksonville
proper with a low-end probability for >8" also depicted within the
prob fields. This all stems from the front location and expected
instability axis to develop during diurnal heating leading to
development along the boundary with slow storm motions anticipated.
In coordination with the Jacksonville WFO, a MRGL risk was added
for that area encompassing Southeastern GA down through Northeast
FL with emphasis on the urban areas along the coastal plain.
Kleebauer
Day 2
Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION AND INTERMOUNTAIN WEST...
The anomalous moisture feed that allowed for the active periods
prior will vacate to the northeast with a skinnier axis of elevated
PWATs generally focused from the AZ/NM state line up through the
Inter Mountain West of UT/CO. This positions the the Four Corners
area up through the Wasatch as the the main targets for heavy
convection during the afternoon and evening Friday before the setup
dwindles the back end of the period. A secondary focus across the
terrain in Southeast AZ over into the NM Bootheel will also be an
area that could see some scattered heavy rain signals with some
support from global deterministic and ensemble members. Current QPF
signal is modest with areal averages closer to 0.25-0.5" with a
maxima focused near the Four Corners over Southwestern CO (>1").
The secondary max is across the Wasatch through the terrain in
Western CO where isolated totals >1" are plausible, but more
sporadic in coverage compared to the Four Corners region. Overall,
the setup is more benign in the grand scheme, especially
considering the previous active periods.
Kleebauer
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