Flash Flooding in Arizona, August 17 to 18, 2021

Area Forecast Discussion
National Weather Service Flagstaff AZ
1009 AM MST Tue Aug 17 2021

.SYNOPSIS...Widespread showers and storms will develop today and
then again on Wednesday. Dryer conditions are then expected for
later in the week and into the weekend. Cooler temperatures are
also expected through the rest of the week.

&&

.UPDATE...A high pressure ridge axis is currently positioned over
northern Arizona. Generally weak winds aloft will lead to
generally slow storm motions. Slow storm motions could lead to
isolated flash flooding. However, given the weak wind shear
aloft, do not expect widespread storm organization or widespread
flash flooding. Will hold off on a flash flood watch for now.

The HREF progs most unstable CAPE (1500-2000 J/KG) will develop
over Coconino and Yavapai counties this afternoon. Sufficient
instability exists for a few storm to produce large hail this
afternoon. However, weak wind shear should prevent widespread
severe weather from occurring. Pulse severe storms are primarily
expected.

No major changes were made to the ongoing forecast. Only
introduced hourly POPs to try and better capture the temporal
evolution of precipitation chances through Wednesday night. The
updated grids have been sent.

&&

.PREV DISCUSSION /222 AM MST/...MCS from this evening has largely
moved out of the area tonight, with only a few lingering showers
through the overnight possible in souther Gila and southern
Yavapai counties. Showers and thunderstorms are expected to
develop once again during the afternoon today. Storms are expected
to initiate mainly over the high terrain during the first half of
the afternoon, before increasing in coverage later in the
afternoon and evening particularly south and west of Flagstaff.
Flash flooding from heavy rainfall will continue be the primary
threat, however a few severe storms with damaging winds and large
hail will be possible as storms become more organized during the
late afternoon and evening.

Wednesday heights begin to fall as a longwave trough begins to dig
into the western United States. Monsoonal moisture surges
northward across much of Arizona as a result. Widespread showers
and thunderstorms expected during the day again on Wednesday,
particularly over northern and eastern Arizona. Additionally,
with an increasing pressure gradient as the trough approaches,
breezy winds are expected as well on Wednesday over much of
northern Arizona, and to a lesser extent again on Thursday.
Cooler temperatures are also expected to continue through at
least the first part of the weekend as the trough passes and in
its wake.

As the trough passes to the north on Thursday, moisture gets
swept away and dryer conditions are expected thereafter in the
wake. Greatest chance for precipitation at this time, looks to be
in the higher terrain over far northern Arizona on Thursday and
Friday. Both GEFS and EPS ensemble guidance indicate that drier
conditions are expected to remain in place through the weekend.
Ensemble guidance only begins to signal a possible return of
monsoonal moisture and warmer temperatures during the first half
of next week.

&&

.AVIATION...For the 18Z package...Another round of scattered to
numerous thunderstorms will develop after 18Z today. A few storms
may be severe producing brief IFR conditions, strong outflow
winds, and hail. Storms will likely continue into the evening
hours, especially across central Arizona. Outside of storm
activity, mainly VFR conditions and southwest winds around 10-15
kts or less. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Scattered to numerous showers and thunderstorms are
expected through Wednesday. Large hail, localized flash flooding and
very gusty outflow winds are the main hazards. Winds outside storms
will be light today, but become breezy out of the southwest
Wednesday. Cooler temperatures today and especially on Wednesday.

Thursday through Saturday...Drier weather is expected Thursday
through Saturday, with below normal temperatures and breezy
southwest winds.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...NONE.

&&

PUBLIC...TM/Humphreys
AVIATION...TM
FIRE WEATHER...MCT

For Northern Arizona weather information visit
weather.gov/flagstaff

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
647 AM MST TUE AUG 17 2021

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

0600 PM     FLASH FLOOD      4 S HUNT                34.55N 109.62W
08/16/2021                   APACHE             AZ   DEPT OF HIGHWAYS

            POST EVENT REPORT FROM MONDAY EVENING. ADOT
            REPORTED A ROAD CLOSURE ALONG HIGHWAY 180A
            AT MILE POST 347. ADDITIONAL FACEBOOK
            REPORTS WITH PHOTOS SHOW FLOODING IN THIS
            AREA WITH AN ESTIMATED DEPTH OF AT LEAST A
            FOOT OF WATER OVER THE HIGHWAY.

&&

MCT