Flash Flood Warning 66, NWS Flagstaff, AZ

Area Forecast Discussion
National Weather Service Flagstaff AZ
930 AM MST Mon Sep 23 2019

.SYNOPSIS...Showers and thunderstorms will continue through the
afternoon. A few stronger thunderstorms will be possible this
afternoon and evening, mainly over Yavapai and Gila counties.
A chance of showers and thunderstorms are forecast Tuesday, and
will continue each day through the end of the week.

&&

.UPDATE...A line of showers with embedded thunderstorms has set up
generally along a line from Scottsdale to Young, northeast into
Heber-Overgaard, Snowflake-Taylor, into Chambers. West of the
line, isolated to scattered thunderstorms have developed.

Most of the hi-res guidance progs this activity to fill in through
the day as the upper low dives south over the Las Vegas area.
MUCAPE values are progged to increase to around 2000 J/kg over
Yavapai and Gila counties this afternoon, coincident with 0-6 km
bulk shear values around 50 knots. This will lead to an
increasing severe weather threat over Yavapai and Gila counties
this afternoon, with large hail and damaging winds being the main
hazards. Furthermore, heavy rainfall will be possible with any of
these storms, producing a flash flood threat as well. Will
continue to monitor and coordinate with SPC to see if a SVR watch
will be needed, coincident with the current flash flood watch.

The forecast has been updated based on current radar obs and expected
forecast trends through the overnight hours based on hi-res
guidance.

&&

.PREV DISCUSSION /429 AM MST/...Showers and thunderstorms have
initiated across southern portions of the CWA early this morning.
A low pressure system continues to dig into the Southwest and is
currently situated in northwestern Nevada. The primary
precipitation band over northern Arizona currently stretches from
south of Payson and northeast towards Show Low, but will continue
to expand in coverage through this morning.

Dewpoint temperatures have gradually increased this morning,
especially along and south of the Mogollon Rim as a much more
moist airmass was drawn northward overnight. Mesoanalysis of the
850 mb pressure level displays this moist air well and will be a
primary ingredient for expanding coverage of thunderstorms
throughout today. CAPE values of around 500 - 1500 Joules per
kilogram are providing sufficient instability for current
thunderstorms and will likely be higher into the afternoon with
some diurnal heating.

The above described moisture and instability, aided by dynamics
associated with the closed low and trough, will create conditions
favorable for a slight chance of severe thunderstorms this
afternoon. The greatest chances within the Flagstaff CWA are for
Yavapai and Gila counties. These thunderstorms will be capable of
producing quarter size hail and wind gusts of around 60 mph,
besides heavy rainfall. The slight chance for severe thunderstorms
was added into the forecast for a portion of these two counties
this afternoon and early evening. Remember to have a way to
receive the latest weather information and warnings today.

Rainfall rates of up to 1 inch per hour are likely from some of
the stronger thunderstorms, while localized storm total rainfall
of around 3 inches is possible. These moist conditions, especially
seen across eastern Yavapai and western Gila counties, have
warranted a Flash Flood Watch to be issued. Never drive across a
flooded roadway or wash.

The center of the closed low will be over southern California by
this evening and continue to bring rainfall into the overnight
hours for many locations in the CWA. PoPs are similar to earlier
forecasts due to the continued likely nature of thunderstorms
within this early overnight period. As the center of the closed
low continues to move south, a more east to northeasterly mid-
level flow will begin to cut off access to the deeper moisture
that will be over the region today. This will create decreased
chances for precipitation beginning early Tuesday morning and
through Wednesday for areas north of the Mogollon Rim. Scattered
thunderstorms and showers are still forecast within this period.

Current guidance is in strong agreement that more southeasterly
mid- level flow will bring another round of deeper moisture late
Wednesday evening. This is the result of a slight northward shift
in the center of the closed low (previously situated over far
southwestern Arizona). As the highest available moisture is
forecast to be southwest of the Four Corners region, chances of
precipitation on Thursday were decreased for that region.

The core of the low pressure system will move out of northern
Arizona by early Friday morning. This will bring a more west to
southwesterly flow for Friday and Saturday out ahead of another
advancing deep trough. Guidance continues to be in strong
agreement on the presence of this deep Pacific Northwest trough.
At least slight chances of precipitation will continue through
Sunday, mainly in western and southern portions of the CWA.

&&

.AVIATION...For the 18Z package...Areas of showers and
thunderstorm will spread from south to north through the morning
with widespread precipitation developing during the afternoon and
evening hours. Pockets of MVFR/IFR with the stronger and more
persistent precipitation areas. Surface winds remaining light
outside of any thunderstorm gusts. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...Showers and thunderstorms will develop ahead of an
approaching low pressure system today and will continue through
tonight. Some of these storms may be strong and could produce heavy
rainfall. The most likely areas for wetting rains will be along and
south of a line from the Grand Canyon to Show Low. Shower and
thunderstorm chances decrease on Tuesday, with a wind shift to the
northeast.

.Wednesday through Friday...Unsettled weather will continue with
the best chance for additional rainfall from Wednesday night through
Thursday. Look for increasing southwest winds Friday, into next
weekend.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...Flash Flood Watch through late tonight FOR AZZ008-018-037-038.

&&

PUBLIC...TM
AVIATION...TM
FIRE WEATHER...McCollum

For Northern Arizona weather information visit
weather.gov/flagstaff

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
1122 AM MST MON SEP 23 2019

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

1121 AM     HEAVY RAIN       5 NW TAYLOR             34.50N 110.17W
09/23/2019  M1.78 INCH       NAVAJO             AZ   COCORAHS

            1.78 INCHES OF RAIN MEASURED IN SNOWFLAKE.
            GROUND IS SATURATED. SOME MINOR FLOODING.

&&

TPS

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