Flash Flood Warning 57, NWS Flagstaff, AZ

Area Forecast Discussion
National Weather Service Flagstaff AZ
920 AM MST Sat Sep 2 2023

.SYNOPSIS...Monsoonal moisture will provide one more day of
showers and storms before we start trending dry Sunday into the
start of next work week. Temperatures remain below normal before
gradually moderating through the middle of next week.

&&

.UPDATE...A few showers are being observed this morning across
northern Arizona. We've updated the short term precip chances in
the forecast to account for these. We remain in a moist,
monsoonal air mass today with FGZ precipitable water values just
under 1". This will be the last day for appreciable rain and
thunderstorm chances. Similar to yesterday, we're also seeing
substantial cloud cover that will delay or inhibit afternoon
convection. We're starting to see some clearing across eastern
Arizona, and expect to see some of this nudge westward allowing
for some instability to develop. All in all, convection looks
pretty scattered today with no definitive feature to organize
things. The best coverage looks to be generally west of a
Flagstaff to Page line where the deepest moisture remains. This,
combined with recent rains, will lead to this area seeing the
greatest flash flood threat today. The Flash FLood Watch for
western Arizona continues through 11 PM MST tonight. Storms will
dissipate this evening with most activity ending by 9 or 10 PM
MST.

&&

.PREV DISCUSSION /301 AM MST/...Today: continued favorable
monsoonal flow will usher in high PWATS. The 00Z sounding
yesterday measured a PWAT of 0.8 inches and guidance this morning
indicates that PWATS could swell to above an inch in areas north
of the Mogollon Rim. While cloud cover yesterday contributed to
limited instability, HREF projections show clearing taking place
late this morning into the early afternoon to get CAPE anywhere
from 500-1500 j/kg available. So long as the clearing actually
happens and the PWATs remain elevated, storms should be able to
develop this afternoon west of the I17/HW89 corridor. In fact,
local WRF projections show that we could have training
thunderstorm problems for areas in the west. Meanwhile, the east
looks to deal with more typical monsoon storm development and have
enough storm motion to not lead to increased flooding concerns.
Sitting in the middle of all that activity is the actual I17 and
HW89 corridor (including Flagstaff, Sedona, Camp Verde, etc) that
look to mainly make it through today mostly dry, minus any
isolated showers and storms that might develop. Sky conditions in
the east should definitely be clearer so confidence in storm
development is higher there. Temperatures are going to remain
below normal.

Sunday: notably pulled POPs down for Sunday. PWATs will be
steadily dropping as more zonal flow takes hold with the western
trough shifting east. There will still be some isolated shower
and storm activity, but nothing to run home about. Despite the
decreasing moisture, the recent drop in temperatures combined
with more zonal flow will help keep the cooler temperatures
around.

Next Week: zonal flow for the first part of the week,
transitioning to unfavorable ridging for precip later in the week
will mean dry weather for us. Temperatures will gradually moderate
through the week, eventually returning to seasonal normal
readings by roughly Wednesday and start to trend above normal. As
of now, any rain chances remain beyond the extended forecast
period, returning some time next weekend into the following week
should ensemble guidance hold-up.

&&

.AVIATION...For the 18Z TAF Package...Periodic MVFR cigs possible
this morning. Then, expect scattered SHRA/TSRA to develop this
afternoon with the greatest coverage south of a KGCN - KSOW line.
Mostly VFR expected through the day, except for pockets of
MVFR/IFR in TSRA. Storms diminish after 02Z with VFR area-wide
expected thereafter. Surface winds SW 10-20 kts, with stronger
gusts to 30 kts possible near TSRA. Aviation discussion not
updated for TAF amendments.

&&

.FIRE WEATHER...Saturday and Sunday...Remaining seasonably cool with
scattered to numerous showers and thunderstorms on Saturday becoming
more isolated on Sunday. Away from showers and storms, expect mainly
southwest winds at 10 to 20 mph.

Monday through Wednesday...Dry, breezy and slighty warmer. Daily
southwest winds 15 to 25 mph, with gusts to 30 mph.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this evening for AZZ004-006>008-015-037-038.

&&

PUBLIC...MCT/RKR
AVIATION...MCT
FIRE WEATHER...Peterson

For Northern Arizona weather information visit
weather.gov/flagstaff