Area Forecast Discussion
Issued by NWS Flagstaff, AZ

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FXUS65 KFGZ 202225

Area Forecast Discussion
National Weather Service Flagstaff AZ
325 PM MST Sun Aug 20 2017

.SYNOPSIS...Monsoonal moisture has returned to the region. Expect
mainly afternoon and evening showers and thunderstorms through at
least Thursday of this week. Storm intensity and coverage may
decrease by this coming weekend.


.DISCUSSION...A low pressure center moving north through New
Mexico is giving us a range of storm motions today, with
thunderstorms along the Mogollon Rim having a tendency to
move/develop west-southwestward while storms near Grand Canyon
and the Kaibab Plateau are moving more in a northward direction.
Lower moisture values than those we`ve observed previously this
season have kept rain rates somewhat subdued for the most part.
Relatively cold temperatures aloft have also supported a hail
threat this afternoon, with hail up to one half inch in diameter
observed so far. Expect scattered rain and perhaps isolated
lightning activity to continue well into this evening.

Through Wednesday, northern Arizona will be located between an
area of high pressure over Texas/northeast Mexico and a low
pressure trough off the southern California coast. Such a pattern
will maintain moisture flow into the region, as well as chances of
mainly afternoon and evening showers and thunderstorms. We may
observe a slight decrease in thunderstorm activity on Tuesday, but
expect shower/thunderstorm coverage to pick back up again by
Wednesday night into Thursday as the trough off the California
coast moves eastward toward Arizona.

From Friday onward, ensemble models indicate that we`ll be in for
another period of drying and warming with a high pressure center
setting up over northern AZ/southern UT by Sunday or so. The GFS
ensemble is faster/more aggressive with the drying than is the
ECMWF ensemble, but the general trend with both is the same.
Expect temperatures near to slightly above seasonable averages
this coming weekend.


.AVIATION...For the 00Z package...SCT to numerous SHRA/TSRA will
continue until around 03Z with ISOLD-SCT -SHRA then continuing
through much of the night. Another round of storms is expected to
develop after 17Z Monday. Generally VFR conditions are expected with
ISOLD MVFR to IFR conditions in the stronger storms. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Deep moisture will remain over northern
Arizona through Monday with showers and thunderstorms continuing.
A slow drying trend will begin again on Tuesday.

Wednesday through Friday...Thunderstorm chances will slowly
decrease...but not end through the period, with generally light
winds outside of storm activity.






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