Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 162157

Area Forecast Discussion
National Weather Service Tucson AZ
257 PM MST Sat Dec 16 2017

.SYNOPSIS...Dry conditions will continue across southeast Arizona
into this evening. A storm system will then bring valley rain and
mountain snow showers late tonight into early Monday, along with
colder temperatures. Dry weather and warmer temperatures will return
mid next week before another storm system moves into the area late
next week into weekend.


.DISCUSSION...The latest satellite imagery and model analysis
indicate the developing upper low to impact southeast Arizona
currently moving southward offshore to the west of San Diego attm.
High clouds continue to stream out ahead of this feature across
Arizona. These clouds have keep temperatures about 2-4 degrees below
anticipated levels so far. However, there is still an hour or so of
daytime heating left and with some thinning of the cloud deck noted
over the western half of the state, may still reach those forecast
highs this afternoon.

The upper low will continue to strengthen and close off as it begins
an eastward push across the northern Baja peninsula this evening.
The models indicate decent jet dynamics and favorable ascent/tilt as
the upper low lifts northeast across the forecast area late tonight
and Sunday. It appears the precipitation will start between 06Z and
09Z Sunday along the International border, with the precipitation
then becoming more widespread after 12Z Sunday. The highest pops
will be during the day Sunday, with valley rainfall chances at 60
percent or higher from Tucson east and south and mountain pops at
80-90 percent from Mount Lemmon/Mount Graham south to the Mexican
border and lesser chances to the north in the White Mountains. A
Winter Weather Advisory continues in effect from 6 am to 11 pm
Sunday above 6500 feet for the Sky Island Mountains of southeast
Arizona, including the Galiuro, Pinaleno, Chiricahua, Dragoon, Mule,
Huachuca, Santa Rita, Catalina and Rincon Mountains. Snowfall totals
in the Advisory area will be 3-6 inches in the mountains surrounding
Tucson and 5-10 inches elsewhere to the east/southeast of Tucson.
Some enhancement will be possible with convection Sunday afternoon
and early evening. A few spots above 8000 ft could see 10-12 inches,
but that should be fairly limited in coverage. Further north outside
of the Advisory area in the White Mountains, were only looking at
2-4 inches of snowfall. Storm total QPF values will be in the 0.1
to 0.4 inch range in the valleys and 0.5 to 1 inch in the mountains.

The precipitation will then wind down from the west Sunday night,
although expect lingering showers to hang on across eastern portions
of the forecast area into Monday. The models are consistent with
pushing the main precipitation band into New Mexico between 06Z-12Z
Monday, even though the upper low stays centered over eastern
Arizona until about 18Z Monday. Any lingering precipitation under
the upper low core on Monday is expected to be light and showery.

Behind the system Monday night/Tuesday morning, expect clearing
skies and below freezing temperatures across the valleys to the
south and east of Tucson. This may lead to some frost and/or icing
issues on area roadways in those areas.

Weak ridging is then expected Tuesday through Thursday of next week
with a modest warming trend. The warmest day next week will be
Wednesday when readings top out at 4-7 degrees above seasonal
averages. Another storm system is then expected to impact the region
late next week into next weekend. The models have not been very
consistent with the track of this next system. At this time, expect
cooler temperatures, breezy conditions and slight chance to chance
pops for mainly eastern areas along the New Mexico border.


.AVIATION...Valid thru 18/00Z.
Clouds already increasing across the area and will continue to
thicken and lower through the rest of the evening as low pressure
deepens over the area. Showers will develop overnight with VCSH
possible between 08-12Z, then -SHRA becoming more likely after
daybreak. Cigs should lower to around 6kft although brief reductions
are possible within the strongest convective showers during the day
Sunday. As for winds, breezy southwest winds possible through this
evening, then becoming southeasterly through the day Sunday.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Low pressure will bring increased humidities and
precipitation to the area on Sunday into Monday. Easterly winds on
Sunday will give way to relatively light/diurnally driven winds next
week along with dry weather for most of the week. There is the
potential for a late week system that could bring some breeziness
and possibly some precipitation to the area. Overall no fire weather
concerns through late next week.


Winter Weather Advisory above 6500 ft from 6 AM to 11 PM MST
Sunday for AZZ511>514.



Aviation/Fire Weather...Leins

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