Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180412

Area Forecast Discussion
National Weather Service Tucson AZ
910 PM 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 into Saturday before winds diminish Sunday. Dry conditions
with a strong warming trend will then prevail Monday into the middle
of next week. Cooler temperatures will occur next Thursday and
Friday with dry conditions continuing.


.DISCUSSION...The upper pattern is defined by an amplified and
strong trough of low pressure moving into California...a closed low
over Oklahoma and a shortwave ridge in between. Satellite imagery
indicated plenty of mid-high clouds across all of Arizona this
evening...with lower clouds begining to move into the western
portions of the state.

The latest Water Vapor imagery indicated the upper trough still has
the strongest winds still rounding the base of the trough. This
suggests the strongest front/dynamics associated with this feature
will drive to our south thru Sonora Mexico. That doesn`t mean that
Arizona is going to miss this feature will still have a
impact on our neck of the woods.

The last few runs of the GFS and NAM suggest strong low to mid level
southerly winds tracking along and ahead of the front as it passes
through Southeast Arizona tomorrow morning/afternoon. The 18/00Z GFS
indicated southerly winds at 850 mb between 45-50 kts moving through
south central Arizona...with winds at 700 mb between 50-55 kts.
Given the strong dynamics associated with the front and these strong
winds aloft, we will likely see several hours of wind advisory
criteria from Tucson southwest and south. We issued a Wind Advisory
from 14Z-23Z Saturday to reflect this thinking.

Timing of the front with the latest 00Z model runs still on track
with inherited forecast regarding this feature moving into SE
Arizona from the west during the day Saturday, with valley rain/
mountain snow impacting the area Saturday into Sunday. Valleys still
appear to be poised for 0.25-1.00 inch of rain, with storm total
snowfall amounts of 10-16 inches possible above 7500 feet.



The upper trough will continue to dig southeastward adjacent the
California coast tonight, then continue to deepen over the
southwestern CONUS Saturday. There is a slight chance of showers
mainly west of Tucson this evening, then precip chances increase
markedly from west-to-east by Saturday morning. The 17/12Z NAM/GFS/
ECMWF remained quite similar to their respective solutions regarding
precip potential later Saturday into Sunday.

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

Cold mid-level temps combined with some surface heating justifies a
slight chance of thunderstorms Saturday mainly from Tucson westward
into western Pima County. Gusty southeast to southwest winds will
also accompany this system into Saturday. There was consideration
given toward a Wind Advisory issuance. However, at this time, appears
that widespread wind advisory criteria speeds will probably not be
achieved; or, at least any speeds achieving criteria will likely not
be long-lived. However, would not be surprised to ultimately
experience brief wind advisory criteria Saturday for some locales.

Valley rain and mountain snow will be likely Saturday night, then
the coverage of showers will very gradually decrease Sunday as the
mid-level trough axis advances eastward into the southern Rockies. A
more pronounced drying trend will begin Sunday night under
northwesterly flow aloft as the upper trough moves into the central/
southern CONUS. However, there should be enough residual moisture
to warrant 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.

Thus, projected storm total liquid rain amounts at lower elevations
should generally range from one quarter of an inch to one inch,
though locally heavier amounts should occur. Storm total liquid
amounts in the mountains will mainly be from one half of an inch to
1.50". The Winter Storm Warning total snow accumulations are
forecast to generally range from 10-16 inches, though locally
higher amounts on south facing slopes. The Winter Weather Advisory
total snow accumulations are lower, with amounts of 5-9 inches above
7500 feet.

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.
The flow aloft is then progged to become northwesterly next Friday
as the upper trough moves east into the central CONUS. At any rate,
this pattern is forecast to translate into dry conditions.

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 temperatures cooling to a few degrees below seasonal normals
Thursday and Friday.


.AVIATION...Valid through 19/06Z.
ISOLD-SCT -SHRA mainly west of KTUS tonight then numerous valley
-SHRA and higher terrain SHSN Saturday. ISOLD -TSRA may occur
Saturday especially KTUS westward. MVFR and occasional IFR
conditions Saturday particularly near mountains. Otherwise, cloud
decks by daybreak Saturday generally 4k-8k ft AGL. Surface wind
through Saturday afternoon sely to swly with gusts to near 40 kts.
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 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.

    Wind Advisory from 7 AM Saturday Morning to 4 PM Saturday
      Afternoon for AZZ502>504-507-508-512>515



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