Flash Flood in Arizona, August 15 to 16, 2022

Area Forecast Discussion
National Weather Service Flagstaff AZ
232 PM MST Mon Aug 15 2022

.SYNOPSIS...An active monsoon pattern will continue across
northern and central Arizona through the upcoming week. The
coverage of rain and storms may abate some Tuesday and
Wednesday...but scattered to numerous showers and storms will
resume mid-week and onward. Heavy rainfall leading to flash
flooding will continue to remain a threat. It will be cooler this
weekend with daytime highs nearly 15 degrees below normal.

&&

.DISCUSSION...Convection continues to blossom this afternoon
across the High Country as rapid clearing earlier and a subtle
synoptic scale ascent continues to impinge on the area. The
current Flash Flood Watch (effective thru 11 PM Arizona Time or
Midnight Navajo Nation Time) appears to be on track and no changes
are anticipated at this time. Remember, dangerous and life-
threatening flash flooding can still transpire even if
thunderstorms are miles away from your location. Be alert to
rapidly changing weather conditions and get out of slot canyons
and swimming holes if a warning is issued!

As the main ascent lifts northward this evening, we should see a
gradual downturn in the coverage of convection. Where skies clear,
there will be the potential for patchy fog. HREF visibility
probabilities suggest that this potential is less than 20%, but
given the lack of rainfall data in the 12 UTC runs, we'll maintain
patchy fog mainly across the western Coconino Plateau back
eastward toward the White Mountains. I don't anticipate fog to be
dense, but trends will be monitored.

The coverage of showers and thunderstorms is expected to be
subdued some on Tuesday thanks to widespread mid/upper level cloud
cover. PoPs are advertised between 30%-60%, however, there's one
caveat. If cloud cover erodes rapidly and we experience a bit
more in the way of daytime heating, then the coverage will be
greater than advertised. There are some hints at mid-level
ridging in the wake of today's upper low and that may tend to
suppress some activity.

Most of Wednesday may offer a brief respite from widespread
showers and thunderstorms (though the lower range of PoPs is still
between 15%-30%). The most concentrated areas of showers and
thunderstorms (thus PoPs between 60%-85%) are across the Mogollon
Rim and into the White Mountains. As an inverted trough/easterly
wave translates across southern New Mexico into adjacent portions
of Arizona, some loosely organized convective complexes may slide
westward. The tail end of some HREF members do support higher PoPs
south of the Show Low/St. Johns area and 70-85% PoPs have been
painted here.

Thursday and into the Weekend will feature likely our next period
of widespread PoPs. Initial glances at the broad brushed 80%-90%
PoPs from the NBM appeared to be a little overdone. However,
taking a look at the overall synoptic setup, this may not be too
far-fetched. Just about all of NWP moisture progs (including
ensemble PWATs) show a surge of moisture streaming northward into
northern Arizona. A diffuse trough from the northwest will also
act upon what will likely be an anomalously moist (PWATs pushing 2
standard deviations above the normal)...almost akin to a
Predecessor Rainfall Event (PRE) across the SE. With favorable
antecedent conditions (many areas have seen close to 200% of
normal of precip thus far), runoff efficiency will be high and
it's highly probable that we'll need a Flash Flood Watch.

The other weather story will be the much cooler conditions. Given
the 80-90% NBM PoPs (especially Friday through Sunday), I've
nudged high temperatures down closer toward the NBM 25th/50th
guidance. In fact, the Extreme Forecast Index (EFI) from the ECWMF
is approaching its "coldest" forecasts for this stretch of time
(this weekend) compared to historical forecasts. This translates
to temperatures being near 15 degrees below normal with highs in
the 50s across the peaks to 60s and 70s along the rim. Areas
across the LCR and Yavapai County Deserts will stay mild--- though
still below normal with temperatures in the 80s.

&&

.AVIATION...For the 00 UTC TAF package. SHRA/TSRA will be in
progress across northern AZ at the start of the TAF period. The
main hazards will be brief heavy rainfall (resulting in MVFR vsby)
as well as gusty/erratic winds, especially at PGA. Convection is
expected to diminish ~06 UTC at all terminals. MVFR BR remains a
possibility, but this threat is less than 20% (though greatest at
FLG and SEZ). Another round of SHRA/TSRA is on tap for Tue with
the greatest potential (~50%) at GCN and FLG.

&&

.FIRE WEATHER...Showers and thunderstorms are forecast on Tuesday.
Gusty, erratic winds, and flash flooding can be expected near
storms. The coverage of rain and storms will diminish some on
Wednesday with the highest rain chances across the Mogollon Rim
and White Mountains.

Thursday through Saturday...The coverage of showers and
thunderstorms will increase with the threat for gusty and erratic
outflow winds and flash flooding increasing.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 11 PM MST this evening for AZZ004>007-009>013-
015-038>040-104>107-109>113-115-138>140.

&&

PUBLIC...Bain
AVIATION...Bain
FIRE WEATHER...Bain

For Northern Arizona weather information visit
weather.gov/flagstaff

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
318 PM MST MON AUG 15 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0249 PM     FLASH FLOOD      4 NW DONEY PARK         35.31N 111.56W
08/15/2022                   COCONINO           AZ   COUNTY OFFICIAL

            N. COPELAND LN OVERTOPPED WITH MUDDY WATER
            AND DEBRIS 8 INCHES DEEP AND 10 TO 15 FEET
            WIDE. ALSO, MINOR OVERTOPPING OF HENRY DR.
            NEAR CAMPBELL.

&&

BAK