Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 171655

Area Forecast Discussion
National Weather Service Tucson AZ
955 AM MST Fri Feb 17 2017

.SYNOPSIS...A strong Pacific storm system will bring valley rain,
mountain snow and cooler temperatures this weekend. Expect gusty
winds this afternoon into Saturday. Dry conditions with a strong
warming trend will then prevail Monday into the middle of next week.


.DISCUSSION...IR/visible satellite imagery and surface observations
indicate partly cloudy to cloudy skies across southeast Arizona at
this time. Dewpoints at lower elevations valid 16Z were mainly in
the lower-mid 30s, and these temps were 2-10 degs higher versus 24
hours ago. Surface temps valid 16Z were also about 2-10 degs warmer
versus this time Thursday.

17/12Z KTWC sounding total precip water value of 0.51 inch was 0.32
inch higher versus 12 hours ago. The sounding exhibited a dry
surface-700 mb layer, but had moistened markedly in the 650-300 mb
layer versus last evening. 17/12Z upper air plots low pressure
systems centered over west-central Texas and west of the California
coast near 38N/130W. Meanwhile, a ridge axis extended from northern
Sonora Mexico northeastward across this forecast into the Four
Corners region. Light to moderate wly/nwly flow above 700 mb
prevailed across southeast Arizona.

The upper ridge axis will move eastward into the southern Rockies
this afternoon. The flow aloft will become increasingly swly in
response to the eastward moving upper ridge, and the digging upper
low pressure system approaching the California coast. Dry conditions
will continue today with high temps this afternoon averaging just a
few degs above normal.

A slight chance of showers mainly west of Tucson begins this evening,
then precip chances increase markedly from west-to-east by Saturday
morning. The 17/12Z NAM/GFS and 17/00Z ECMWF remained quite similar
to their respective solutions regarding precip potential later
Saturday into Sunday.

Thus, the inherited Winter Storm Watch was upgraded to a Winter
Storm Warning for most mountains above 7500 feet from Tucson
eastward to the New Mexico border. The exception was the Chiricahua
Mountains, where the Winter Storm Watch was changed to a Winter
Weather Advisory for snow above 7500 feet. The Winter Storm Warning
and Winter Weather Advisory is in effect from 3 PM Saturday to 5 PM
MST Sunday.

Cold mid-level temps combined with some surface heating justifies a
slight chance of thunderstorms Saturday mainly from Tucson westward
into western Pima County. Valley rain and mountain snow will be
likely Saturday night, then the coverage of showers will very
gradually decrease Sunday. The upper level trough will move well
east of this forecast area Sunday night before an upper ridge moves
across the area Monday. There should be enough residual moisture to
maintain at least a slight chance of valley rain/mountain snow
showers most sections Sunday night, then dry conditions are forecast
to occur area-wide Monday.

The upper ridge is progged to flatten and move east in the central
CONUS Tuesday. Southwesterly flow aloft Tuesday will transition to
more westerly flow aloft Wednesday into next Thursday as an upper
trough moves from the Pacific Northwest into the northern Rockies.
Dry conditions should prevail most sections except for a slight
chance of snow showers across the White Mountains Wednesday night
into Thursday.

High temps Saturday will generally be about 5-10 degs cooler versus
this afternoon, then another 3-8 degs of cooling is on tap Sunday. A
strong warming trend will prevail Monday through Wednesday followed
by cooler temperatures next Thursday.


.AVIATION...Valid through 18/18Z.
Mainly SCT clouds at 10k-15k ft AGL and BKN clouds above 20k ft AGL
into this afternoon. Cloud decks lowering to around 7k-11k ft AGL by
Saturday morning as a storm system approaches from the west. ISOLD
-SHRA west of KTUS this evening, then SCT-NMRS -SHRA and higher
terrain -SHSN toward the end of the period. Surface wind sely/sly 10-
20 kts with gusts mainly up to 30 kts this afternoon into Saturday
morning. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...A strong Pacific storm system will bring widespread
valley rain and mountain snow this weekend. Gusty mainly south winds
will also occur into Saturday, then winds will diminish Sunday. Dry
conditions with a strong warming trend will occur Monday into the
middle of next week.


    Winter Storm Warning from 3 PM Saturday to 5 PM MST Sunday for
      AZZ510-511-513-514 above 7500 feet.

    Winter Weather Advisory from 3 PM Saturday to 5 PM MST Sunday
      for AZZ512 above 7500 feet.




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