Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 182245

Area Forecast Discussion
National Weather Service Tucson AZ
345 PM MST Thu Jan 18 2018

.SYNOPSIS...Dry conditions with unseasonably warm temperatures to
occur Friday. A storm system will then bring scattered to numerous
valley rain and mountain snow showers Saturday into Sunday morning.
Much cooler temperatures this weekend followed by dry conditions
with a warming trend early next week.


.DISCUSSION...Upper level ridge axis extended from southwest Arizona
northeastward into the northern CONUS plains this afternoon.
Meanwhile, a deep upper trough was approaching the Pacific Northwest
southward into northern California. The various 18/12Z numerical
weather prediction solutions remained fairly consistent versus
previous solutions regarding the progressive upper trough into
California by Friday evening, then further eastward into this
forecast area Saturday.

Dry conditions with unseasonably warm daytime temperatures will
prevail Friday. The models depicted a slower onset of measurable
precip within this forecast area versus solutions from 24 hours ago.
Thus, the official forecast continues with precip-free conditions
into Friday night. Thereafter, precip chances were increased
somewhat markedly versus the previous official forecast, especially
for Saturday afternoon.

The 18/12Z GFS/ECMWF/CMC deterministic solutions were fairly similar
with depicting the mid/upper level trough axis by Saturday evening
to extend from western Utah southward into southwest Arizona. A
surface cold front should move eastward across the Tucson metro area
Saturday afternoon. This scenario supports the best chance of
widespread measurable valley rain and mountain snow to occur
Saturday afternoon. Prior to this time, isolated to scattered
showers are expected to develop Saturday morning then become
scattered to numerous valley rain/mountain snow showers Saturday

Precipitation chances will decrease especially Tucson westward
Saturday evening. A chance of snow showers continues mainly across
the mountains of Greenlee, Graham and Cochise Counties, and
particularly the White Mountains, late Saturday night. Expect a few
lingering snow showers across the White Mountains Sunday morning.
Dry conditions will then prevail Sunday afternoon into next Thursday.

Some minor upward adjustments were made to the storm total liquid
amounts and snow accumulations. Appears that a few hundredths of an
inch to around one quarter of an inch liquid amounts *generally* for
the valleys, and 0.10" to 0.50" *generally* for the mountains. Snow
accumulations *generally* of 1-4 inches above 7000 feet for the
Catalina/Rincon Mountains, the Pinaleno Mountains including Mount
Graham and the White Mountains should occur. Expect snow
accumulations only to around one inch for the Chiricahua Mountains
and some peaks south to southeast of Tucson.

Much cooler temperatures will occur Saturday with below normal
daytime temperatures continuing Sunday. Freezing temperatures for
the Tucson metro area appear increasingly likely Saturday night and
Sunday night. A warming trend starts Monday and continues into next
Wednesday with above normal daytime temperatures also next Thursday.


.AVIATION...Valid thru 20/00Z.
FEW-SCT clouds above 20k ft AGL into Friday afternoon. Surface
wind generally ely/sely less than 10 kts into Friday morning, then
swly 5-15 kts Friday afternoon. Aviation discussion not updated for
TAF amendments.


.FIRE WEATHER...Dry conditions through Friday night. There is a
chance of valley rain and mountain snow Saturday into Saturday
evening, then precipitation chances decreasing Saturday night and
Sunday morning. Dry conditions return Sunday afternoon and continue
into next Thursday. Gusty southwest to northwest winds will occur
especially Saturday afternoon and evening. Otherwise, terrain
driven winds mainly less than 15 mph to prevail at other times.






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