Flash Flood Warning 14, NWS Flagstaff, AZ

Area Forecast Discussion
National Weather Service Flagstaff AZ
256 PM MST Sat Jul 29 2023

.SYNOPSIS...The most active monsoon pattern of the year so far is
settling into place across the region. Shower and thunderstorm
activity will increase into Tuesday before a drying trend begins
Wednesday. The coolest high temperatures in quite some time are
expected over the next few days in concordance with the increase
in moisture.

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.DISCUSSION...The development of a robust monsoonal flow over the
past week is dramatically increasing shower and thunderstorm
activity over northern Arizona. A favorable placement of the
subtropical high pressure ridge has induced persistent moist,
southerly flow through the Southwest. This buildup of moisture
both aloft and at the surface is evidenced by the increasing daily
MCS activity to our south, and the northward surges of high dew
points from the Gulf of California.

The GFS and European ensembles both portend a drying trend
beginning Wednesday, as a result of our subtropical high shifting
eastward. Until then, however, the most active monsoon pattern of
the year so far is in place. Mesoscale models are painting a wet
picture for not only the high terrain, but the desert lowlands. It
looks like convection will initiate each day in the late morning
hours over the high terrain before spreading into the low
elevations during the afternoon hours. This includes locations
that have yet to see substantial rainfall this season, such as the
canyon country near the Arizona-Utah border. Heavy rainfall (1"
per hour), small hail, frequent lightning, and gusty winds
(greater than 30 mph) are expected with some storms.

It is possible that convection organizes over Yavapai County each
evening, as storms slide into the lower elevations, tapping into
deeper moisture. Relatively large dew point depressions at the
surface in the deserts may encourage robust cold pool formation.
Strengthening cold pools may help organize convection and create a
scenario for damaging downbursts and severe winds.

In spots where local terrain favors near-stationary or
backbuilding storms, multiple inches of rain may fall within a few
hours. This is something we have seen in some areas lately,
especially in Yavapai County. We expect instances like this to
increase in the next few days. As a result, the risk of flash
flooding is growing, with areas in and around fire scars and
rocky or saturated soils being the most vulnerable. Remember,
NEVER drive through floodwater. Murky waters covering usually dry
washes and low-water crossing conceal hidden threats, including
dangerously deep waters and washed out surfaces. If planning on
outdoor activities, understand the risk for thunderstorms before
heading out. Never enter a slot canyon if you hear thunder or see
a storm nearby.

With precipitation coverage increasing in the coming days, high
temperatures are expected to come down. Forecast highs Monday
through Wednesday range from near normal for this time of year, to
below normal. High temperatures will depend on your elevation and
local convective activity. Winds away from showers, thunderstorms,
and outflow boundaries will remain light. Keep your eyes to the
sky and stay tuned to the forecast.

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.AVIATION...For the 00Z TAF Package...Outside of showers and
thunderstorms, VFR conditions are expected through the 24 hour TAF
period. Scattered showers and thunderstorms this evening will wind
down after sunset. Greatest chances for storms (over 50%) will
remain along and south of a KCMR-KFLG-KINW-KRQE line. Activity will
pick back up in the morning, with showers and storms becoming
widespread (over 80%) after noon. Some storms may produce frequent
lightning, heavy rain, and gusty, erratic winds. Away from shower
and thunderstorm activity, light west-southwest winds of 5-15 kts
are expected. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Sunday and Monday...Monsoon moisture deepens with
the most activity so far this year expected. Widespread showers and
thunderstorms are forecast each day, with greatest activity over the
high terrain and in the afternoon and evening. Some storms may
produce frequent lightning, heavy rain (up to 1 inch per hour), and
gusty, erratic outflow winds (in excess of 30 mph). Temperatures
finally cool to near average while southwest winds outside of
showers, thunderstorms, and outflows remain light at 5 to 15 mph.

Tuesday through Thursday...Temperatures remain near normal for this
time of year. Widespread showers and thunderstorms on Tuesday become
more scattered on Wednesday, before turning isolated on Thursday.
Southwest winds remain light away from storms and outflows.

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.FGZ WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM MST Sunday for AZZ006.

&&

PUBLIC...Benji
AVIATION...Benji
FIRE WEATHER...Benji

For Northern Arizona weather information visit
weather.gov/flagstaff