Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 190350

Area Forecast Discussion
National Weather Service Tucson AZ
850 PM MST Sat Feb 18 2017

.SYNOPSIS...A good chance of valley rain and mountain snow continues
through Sunday afternoon, then showers will end Sunday night. Dry
conditions will prevail across much of the area Monday into next
Saturday. However, a few snow showers may occur across the White
Mountains Wednesday night and Thursday. Expect much warmer
temperatures by mid week before cooler temperatures return late in
the upcoming week.


.DISCUSSION...IR satellite imagery and surface observations depict
cloudy skies across much of southeast Arizona at this time, although
some breaks in the overcast were noted. KEMX WSR-88D detected 30-35
dBZ echoes limited to portions of Pinal County as well as southern
Graham/northeast Cochise Counties. Thus, the bulk of heavier
precipitation that occurred earlier today has ended for the time
being. Surface winds valid 8 PM MST have also diminished to less
than 15 mph.

The upper low pressure system responsible for the precipitation as
per Water Vapor/IR satellite imagery and the 19/00Z upper air plots
was centered near Yuma, AZ. This low pressure system is progged to
move northeastward into central Arizona by daybreak Sunday, then
fill while moving northeastward into southern Colorado by Sunday

The 19/01Z and 19/02Z HRRR solutions and to some extent the 19/00Z
NAM were quite similar with depicting a downward trend in precip
coverage throughout much of the rest of tonight. The coverage of
precip is then depicted to increase Sunday morning. Based on
satellite/radar trends, these solutions seem to be reasonable.

However, am inclined to make only very minimal adjustments to the
inherited gridded data PoP values. Further, the NAM12 and a quick
inspection of the 19/00Z GFS suggests that an additional 6 or so
inches of snow should occur Sunday across the Catalina Mountains and
Mount Graham, and up to an additional 12 inches across the White
Mountains. Snow levels should remain around 6000-6500 feet which
coincides with the present levels. Markedly less wind will occur
into Sunday morning, then some gusts from the southwest to northwest
will prevail Sunday afternoon. However, speeds will be well below
Wind Advisory criteria.


.AVIATION...Valid through 20/06Z.
Scattered to numerous valley -SHRA and mountain SHSN will occur into
Sunday afternoon, then precipitation coverage decreases markedly
Sunday evening. Otherwise, cloud decks will generally be 4k-8k ft
AGL, but occasional MVFR ceilings/visibilities will also prevail.
Surface wind mainly sly 5-15 kts with gusts to 20 kts into Sunday
morning, then surface wind swly to nwly 10-18 kts with gusts to 25
kts Sunday afternoon. Aviation discussion not updated for TAF


.FIRE WEATHER...A weakening low pressure system will bring a good
chance of valley rain and mountain snow through Sunday. A slight
chance of showers exists Sunday night, then dry conditions will
prevail across much of the area Monday into next Saturday. The
exception may be a few snow showers across the White Mountains
Wednesday night and Thursday. Some gusty west or northwest winds
should occur Sunday and Thursday. Otherwise, 20-foot winds will
mainly be less than 15 mph.


.PREV DISCUSSION /221 PM MST/...By Sunday morning model consensus
has the upper low near Casa Grande and slowly shifting to the east
as it opens up. This will leave our area in a slightly unstable
regime with breaks of sunshine and scattered showers, more numerous
over the mountains and with lower snow levels should see snow
showers down to around 6,000 ft with accumulations mostly limited to
above 7,000 ft. Current forecast of 10-16 inches for storm totals
today though Sunday evening above 7,500 ft may be a touch on the
high side but in the ballpark so won`t tweak significantly at this

If current timing holds, the trough axis will be moving east of
the New Mexico border Sunday afternoon, so would expect a general
decrease in the showers in the afternoon from west to east with a
rapid decrease after sunset with the loss of heating and a short
wave ridge quickly building in from the west. With some clouds,
scattered showers and cool air mass in place high temperatures
should peak a few degrees cooler than average for February 19.

As mentioned, drying will kick in Sunday night with dry weather
expected through the coming week with the possible exception of a
stray shower northern areas as a system passes by to the north
Wednesday night. It will warm quickly Monday through Wednesday
with high temps around or a bit above 80 from Tucson westward
Tuesday and Wednesday. Temps will be forced back down to more
normal levels Thursday and Friday thanks to the previously
mentioned system passing by Wednesday night. There is some
suggestion of another system passing by or through the area late
next weekend. Time will tell how that plays out.


    Winter Storm Warning until 5 PM MST Sunday for AZZ510-511-
      513-514 above 7500 feet.

    Winter Weather Advisory until 5 PM MST Sunday for AZZ512
      above 7500 feet.




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