Severe Thunderstorm Warning 187, NWS Flagstaff, AZ

Area Forecast Discussion
National Weather Service Flagstaff AZ
308 PM MST Sat Oct 6 2018

.SYNOPSIS...A significant storm system will bring colder and
stormy conditions to the area this weekend. Expect rain showers
and thunderstorms to spread across much of northern Arizona
tonight through Sunday. A few strong or severe storms are possible
and could produce locally heavy rainfall. Cool and unsettled
conditions will persist through Tuesday. Fair weather returns on
Wednesday.

&&

.DISCUSSION...A deepening trough of low pressure over the Great
Basin this afternoon has pushed a cold front into far northwestern
Arizona (Mohave County). The trough will continue to deepen while
pushing the potent cold front southeast through Arizona tonight
into Sunday morning. Plenty of dynamic lift will lead to a band
of moderate to locally heavy rain showers and thunderstorms moving
west to east across the area. Marginal instability will combine
with sufficient deep layer shear of 40-60 kt tonight to produce a
few strong or severe thunderstorms capable of hail and strong
winds. Enhanced low level southerly flow will draw additional
moisture northward ahead of the cold front ensuring efficient
rainfall production. The best combination of moisture and
instability will come together over southern Coconino, eastern
Yavapai, and Gila Counties which are forecast to receive between
1-2 inches of rain, with locally higher amounts possible from
stronger convection. Due to saturated soils in this area from
widespread rain earlier this week, watersheds will see a faster
than normal response to any heavy rain and we have issued a Flash
Flood Watch for tonight through Sunday afternoon.

By daybreak Sunday, the frontal boundary will be moving across
eastern Arizona while the cold post-frontal air mass settles into
northern Arizona. Showers will become more scattered behind the
main frontal passage. Much colder temperatures are in store Sunday
with 40s in the mountains and 50s - low 60s in the valleys. The
first accumulating snow of the season will fall over the highest
mountains above 8,000 - 9,000 feet, with several inches possible
above 10,000 feet. Snow levels will mostly remain at or above
8,000 feet through the event.

The trough will remain over the region through Monday with cold
and unsettled weather persisting. Drier and briefly warmer weather
will ensue midweek before another cold trough moves into the
region. There is still a possibility of this next trough
interacting with the remnants of Hurricane Sergio. Latest
forecasts are trending this moisture further south, but stay
tuned through the coming days for updates.

&&

.AVIATION...For the 00Z package...MVFR/IFR conditions will continue
to move E/SE with an approaching cold front bringing cloud cover and
SHRA/-TSRA. SW sfc winds at 10-15 kts, gusts up to 25-30 kts, remain
through Saturday evening. More widespread -RA/-TSRA will begin this
evening and through the overnight. ISLT -SHRA/-TSRA return on
Sunday. Aviation discussion not updated for TAF amendments.

&&

.Fire Weather...Gusty southwest winds will remain through Saturday
evening ahead of an approaching cold front. This front is producing
showers and thunderstorms across northern Arizona that will last
through Sunday night. Temperatures will drop 10-15 degrees Sunday.
Mountain snow accumulation is expected over the highest peaks. Cold
and unsettled weather will continue Monday.

.Tuesday through Thursday...Chance for showers begins to decrease
Tuesday with possible gusty winds. Drier weather and warmer
temperatures return Wednesday.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...Flash Flood Watch from 11 PM MST
this evening through Sunday afternoon FOR AZZ016-018.

Flash Flood Watch from 8 PM MST this evening through Sunday
afternoon FOR AZZ008-015-037-038.

&&

PUBLIC...MCT
AVIATION...NL
FIRE WEATHER...NL

For Northern Arizona weather information visit
weather.gov/flagstaff