Special Weather Statement, NWS Flagstaff, AZ

Area Forecast Discussion
National Weather Service Flagstaff AZ
258 PM MST Tue Oct 5 2021

.SYNOPSIS...Widespread rain showers and scattered thunderstorms
are expected across much of the region this afternoon and evening.
Localized flash flooding and severe thunderstorms remain the
key threats. Activity is anticipated to diminish later tonight
and early Wednesday morning. Drier weather is likely Wednesday
and Thursday, with breezy winds and another chance of rain for
Friday.

&&

.DISCUSSION...

Main update this forecast cycle:

*The Storm Prediction Center (SPC) has issued a Severe Thunderstorm
 Watch for much of Arizona from now until 8 PM MST this evening.

Remainder of Today...Current water vapor satellite imagery shows
the dynamic, upper-level low pressure system moving into southern
California and southwestern Arizona with moisture surging
northeastward. Radar is indicating a few isolated storms beginning
to initiate in southern Gila and eastern Yavapai counties as the
leading edge of the second wave of moisture begins to push its
way northward. Expect shower and storm coverage to increase within
the next few hours from a southwest to northeast direction.

The ingredients are there for the severe thunderstorm potential
focused on Yavapai, Gila, Coconino, and Navajo counties this
afternoon and evening. Moisture availability, steep lapse rates
with cooling aloft, sufficient CAPE, and low-level
directional/speed shear are all accompanying the dynamic,
negatively-tilted trough. Add in some daytime heating from earlier
this afternoon thanks to a lull in the activity from this morning
and break in the cloud cover as well. With all of these elements
present at this time, forecaster confidence has increased in the
severe potential resulting in SPC issuing a Severe Thunderstorm
Watch for much of northern Arizona through this evening. Large
hail (~1.00"), strong wind gusts (>60 mph), and a few isolated
funnel clouds or weak tornadoes are all on the table in any storms
that do develop today.

Flash flooding is also likely during this time across the region
with emphasis on the Navajo Nation area where muddy, impassable
roads could result. WPC along with the latest run of the HREF do
suggest rainfall rates of up to 1" per hour in any storms, however
thanks to the faster storm motions (~20-25 mph) with upper-level
jet support, flash flooding should be limited to more localized
coverage, highlighting any low water crossings and slot canyons
throughout the region.

Activity is expected to taper later this evening with some
leftover showers for the Navajo Nation area early Wednesday
morning as the low pressure trough moves northeastward into the
Rockies.

Wednesday through Friday...Drier conditions are forecast for
Wednesday and Thursday with cooler, below average daytime
temperatures anticipated. Mostly cloudy skies return for Thursday
with breezy to gusty winds and chances for showers on Friday as a
moderate, low pressure trough pushes its way into the Intermountain
West. Thunderstorm activity is not forecast for Friday at this
time.

Long Term...For Saturday and into next week, the main story is
chilly high and low temperatures along with the potential for
area-wide gusty winds and precipitation as long range guidance
continues to show an active, fall pattern for much of the West
during this time. There currently is good agreement between the
GEFS, EPS, and WPC Clusters Days 6-7 in regards to a deep,
longwave trough setting up over the western half of the country
from Monday into Wednesday of next week. Confidence is a little
shaky in terms of how deep this trough gets and moisture
availability, however much colder temperatures with possible
widespread freezes at night are looking better. The majority of
the ECMWF Ens members also show southwest wind gusts in the 35-45
mph range for Monday and Tuesday for the Flagstaff area, just
something to keep an eye on at this time. Depending on how deep
and cold this potential trough gets, snow showers could also be
possible as well in the high country, however confidence is low at
this time.

&&

.AVIATION...For the 00Z package...Showers and thunderstorms will
continue through roughly 12Z Wednesday. Areas of MVFR/IFR ceilings
and visibilities are possible with the stronger storms along with
brief, heavy rainfall and hail. Surface winds generally 5-15 kts
outside of storms with gusty outflows possible. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A passing weather system will continue to deliver
showers and thunderstorms to northern Arizona into Wednesday
morning. Strong gusty winds and hail will be possible with stronger
thunderstorms. Shower activity will diminish by late Wednesday
morning, with dry weather forecast to continue on Thursday.

Friday through Sunday...Rain shower chances return for Friday with
breezy to localized gusty winds possible. Cooler temperatures with
seasonal winds are forecast for Saturday and Sunday.

&&

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

&&

PUBLIC...LaGuardia
AVIATION...RR
FIRE WEATHER...RR

For Northern Arizona weather information visit
weather.gov/flagstaff

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