Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 192157

Area Forecast Discussion
National Weather Service Tucson AZ
255 PM MST Fri Jan 19 2018

.SYNOPSIS...A cold front moving eastward across the area will bring
valley rain and mountain snow especially Saturday afternoon and
evening. Showers will end east of Tucson later Saturday night and
Sunday morning. Much cooler temperatures will occur this weekend,
with a widespread freeze including Tucson likely Saturday night and
Sunday night. Dry conditions with warmer temperatures next week.


.DISCUSSION...Sunny skies prevail across southeast Arizona this
afternoon except for a few cirriform clouds mainly near the New
Mexico border. Meanwhile, IR satellite imagery depicted colder-
topped clouds across much of Nevada extending southwestward into
southern California ahead of a deepening upper trough approaching
the California coast.

As noted in the previous Forecast Discussion, the 19/12Z NAM12
depicted markedly lower precip chances associated with this
progressive upper trough and associated cold front versus previous
solutions. However, the 19/12Z GFS/ECMWF were fairly consistent
regarding precip potential versus solutions from 12 hours ago. Per
coordination with neighboring WFO`s, only slight modifications were
made to the official forecast scenario of precip, cooler temps, and
gusty winds on Saturday.

Expect dry conditions to prevail tonight and until around or shortly
after daybreak Saturday. Thereafter, clouds will increase with
isolated to scattered showers developing west-to-north of Tucson
later Saturday morning. Appeared that the 19/12Z Univ of AZ WRF-GFS
had a good depiction of precip associated with the eastward moving
cold frontal boundary Saturday afternoon and evening. Thus,
scattered to perhaps numerous showers should yet occur Saturday
afternoon. For reference, the period for the best chance of
measurable precip to occur for the Tucson metro area appears to be
from about 1 PM to 7 PM MST Saturday.

Scattered to numerous showers to continue east of Tucson Saturday
evening, then isolated to scattered showers mainly near the New
Mexico border late Saturday night. Have continued with a slight
chance of snow showers White Mountains Sunday morning, then dry
conditions area-wide Sunday afternoon.

Snow levels around noon Saturday projected to be around 6500-7500
feet falling to near 3500-4500 feet late Saturday night. Still
appears that valley liquid amounts will *generally* ranged from a few
hundredths of an inch to around one quarter of an inch, and liquid
amounts for mountains will *generally* range from 0.10" to 0.50".
Expect total snow accumulations of about 2-5 inches above 7000 feet,
and perhaps 1-2 inches at 5000-7000 feet.

Dry conditions will prevail Sunday night into next Friday. 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.
Cooler temperatures are on tap next Friday as the next deep upper
trough moves eastward across the southwest CONUS.


.AVIATION...Valid thru 21/00Z.
FEW-SCT clouds above 20k ft AGL into tonight, then SCT-OVC clouds
generally 5k-10k ft AGL Saturday as a cold front moves west to east
across the area. Expect scattered -SHRA and higher terrain -SHSN in
vicinity of the frontal boundary. Best timing of lower ceilings and
vsbys KTUS/KOLS terminals 19Z-23Z Saturday, and -SHRA reaching KDUG
late Saturday afternoon/evening. Improving conditions KTUS vicinity
westward Saturday evening. MVFR conditions especially higher terrain
and with frontal boundary. Sfc wind sely-swly 5-15 kts into Saturday
morning, then surface wind swly-nwly 10-22 kts and gusts to 35 kts
Saturday afternoon. Highest speeds likely at KDUG. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions through tonight. Expect scattered
showers developing west of Tucson Saturday morning, then valley rain
and mountain snow showers with a cold front moving eastward across
the area Saturday afternoon and evening. Precipitation ending east
of Tucson Saturday night and Sunday morning, then dry conditions
will prevail Sunday afternoon into next Friday. Gusty southwest to
northwest 20-foot winds blowing at 20 mph with gusts up to 30 mph
Saturday afternoon and evening. Gusty northwest winds will favor the
Upper Gila River Valley including Safford on Sunday. Otherwise, 20-
foot winds will be terrain driven mainly less than 15 mph at other






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