FXUS65 KTWC 172035

Area Forecast Discussion
National Weather Service Tucson AZ
135 PM MST Sun Dec 17 2017

.SYNOPSIS...Valley rain and mountain snow showers associated with
slow moving low pressure south of Arizona will gradually dissipate
tonight as the low moves east. Wraparound moisture will allow for
some lingering mountain snow showers east of Tucson on Monday, but
drier air will move into the area through mid week. A trough will
sweep through the Four Corners late in the week bringing increased
cloud cover and cooler temperatures, but at this time it doesn't
look to bring much in the way of precipitation. Cooler temperatures
will linger through next weekend.


Scattered rain and snow showers continue across the area this
afternoon, although areal coverage is substantially less than what
was seen earlier this morning. Upper low has moved little today
although water vapor imagery clearly shows that a lobe of shortwave
energy has already moved through southeast Arizona and is pushing
eastward into New Mexico. Regional radar imagery is showing another
round of precip forming south of Tucson as of 20Z and this fits well
with the past few runs of the HRRR. After 00Z the HRRR isn't very
enthused with precip chances for the rest of the night, only
depicting widely scattered showers across the area. I'll leave the
Winter Wx Advisory in place as is to account for the precip that's
about to move through the area as it'll easily produce a few inches
of snow above 6000ft. If things quiet down after sunset the Advisory
can always be cancelled a few hours early.

Precip chances don't go away completely as the upper low and its
associated moisture will slowly shift into New Mexico on Monday.
Wraparound moisture could result in scattered valley rain/mountain
snow showers along the AZ/NM border tomorrow although any snow
accumulations would be light and it doesn't appear they'd be enough
to warrant the extension of the aforementioned Advisory. The low
will finally exit the area Monday night with drier air moving into
the area. Dry weather with below normal temps will prevail through
mid week.

The potential for a mid/late week storm still exists, but today's
deterministic 12Z runs keep the storm track well north of the area
and only depict an increase in clouds and wind coincident with the
passage of the trough. GEFS plumes aren't enthused about precip
chances either, same story with the EC ensemble mean QPF (a few
hundredths of an inch in the White Mtns, that's about it). Very low
(single digit) precip chances later this week look to be all that
are warranted at this point. The trough will usher in much cooler
air to the area with widespread freezing or near freezing temps
possible to close out the week into the weekend. If the current
forecast verifies we may need to look at freeze warnings for the
deserts before all is said and done.

Looking forward to Saturday/Sunday - differences remain between the
GFS/EC deterministic guidance with respect to how a trough evolves
as it moves through the Pac NW into the northern Great Plains and
if/how another trough develops offshore. GFS and associated
ensembles suggest Arizona sees the influence of the offshore low
with warmer temperatures across the region. EC and ensembles depict
a much cooler system with Canadian cold air moving into the area.
Neither solution depict precipitation across southeast Arizona but
temperatures are up for debate. Will stick with a colder than normal
solution for now per the NBM.


.AVIATION...Valid thru 19/00Z.
Cloud decks generally around 2-5k ft AGL with layers above thru
about 18/05Z, then increasing to 4-7k ft AGL from Tucson westward
with gradual clearing overnight. Areas east of KTUS will remain BKN-
OVC at 3-6k ft AGL into Monday. SCT valley SHRA and mountain SHSN
will continue into this evening, then gradually taper off overnight.
Some lingering valley SHRA and mountains SHSN is possible Monday
east of Tucson. MVFR cigs/vsbys will occasionally accompany the SHRA
especially near terrain and east of Tucson into this evening. Winds
mainly southeasterly at around 8 to 12 kts, though occasional gusts
to 25 kts may occur in/near stronger SHRA this afternoon, then
mainly less variable at 10 kts or less through Monday. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Low pressure will bring much higher relative
humidity and scattered precipitation to the area through Monday.
East to Southeast winds this afternoon will give way to relatively
light/diurnally driven winds during through midweek. Once the
current weather system moves to the east, expect dry weather the
remainder of the week. As a weather system approaches Thursday,
expect some increased breezes. The bulk of this system will pass to
our north, thus dry and cool weather to end the work week and into
next weekend.


Winter Weather Advisory until 11 PM MST this evening for




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