Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 170316

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

.SYNOPSIS...A storm system will then bring valley rain and
mountain snow showers into the region starting late tonight and
continuing 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...Latest WV/IR imagery depicts the upper low across
northern Baja and moving towards the east-southeast. Mostly cloudy
skies with dry conditions across Southeast Arizona at this time
under southerly flow out ahead of the upper low. While the
atmosphere is still fairly dry in the low levels as surface
dewpoints range from the mid teens to around 30 degrees, moisture is
lurking across Sonora and will push northward into the area
overnight. While the radar is still fairly quiet, we`re seeing a few
light showers southeast of Nogales. Meanwhile, favorable jet
dynamics and deep layer ascent will increase overnight into Sunday
morning across the forecast area. Latest high resolution guidance is
showing the first wave of showers moving northward from the
international border around midnight, then across the forecast area
overnight. Additional rounds of showers and even some embedded
thunder is possible during the day Sunday as the upper low gradually
pushes across far northern Sonora adjacent to south/central Pima
County. Have opted to go ahead and move up the start time of the
Winter Weather Advisory to 2 am MST given the increased shower
activity overnight. Also made a few minor tweaks to increase PoPs
slightly on Sunday, otherwise, the rest of the forecast is generally
unchanged from the afternoon package. See previous discussion below
for additional information.


.AVIATION...Valid thru 18/06Z.
Clouds will continue to thicken and lower through the night as low
pressure deepens over the area. Showers will develop overnight with
VCSH possible between 06-08Z, then -SHRA becoming more likely
thereafter through the day with Isold -TSRA possible. Cigs should
lower to around 4-6kft although brief reductions are possible within
the strongest convective showers during the day Sunday. Winds mainly
southeasterly at around 10 kts 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.


.PREV 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.


Winter Weather Advisory above 6500 ft from 2 AM to 11 PM MST Sunday
for AZZ511>514.



Previous Discussion...Zell

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