Freeze Warning 1, NWS Tucson, AZ

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

.SYNOPSIS...A cold front moving east 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.

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.DISCUSSION...Sunny and very warm Friday with highs ranging from
the mid 60s to the upper 70s. Tucson International airport just
missed out on being the nations high temperature spot by one
degree as the honor went to Imperial California.

Satellite imagery this evening showed a clear to mostly clear sky
as thin high level cirrus cloud start to increase a bit.

Latest surface analysis has the approaching cold front extending
from southern Nevada to just east of San Diego with isolated
showers. The marine layer, per 00z San Diego sounding, was not
overly deep and not much has breached the northern Baja mountains
this evening. Layer PW imagery showed PW values mostly less than
1" along the front so not overly impressive.

Low to mid level cloud cover will be increasing overnight west of
Tucson as cold front approaches with a few showers possible west
of Sells between 5 am and 7 am. As the cold front pushes east on
Saturday, with passage thru Tucson early to mid afternoon, shower
activity will be on the increase.

Not much change in thinking on snow levels and liquid/frozen
amounts so the following is from the afternoon discussion.
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.

Much colder air mass behind cold front that will lead to very cold
mornings Sunday and Monday. The changes I did make to the evening
forecast package was to hoist a Freeze Watch for south-central
Pinal county and central/eastern Pima county for both Sunday and
Monday mornings. Incoming model data coming in a smidge colder
than current forecast.

&&

.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
times.

&&

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
 Freeze Watch from late Saturday night through Sunday morning for
  AZZ502-504-505.

 Freeze Watch from late Sunday night through Monday morning for
  AZZ502-504-505.

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