FXUS65 KTWC 091031

Area Forecast Discussion
National Weather Service Tucson AZ
330 AM MST Mon Dec 9 2019

.SYNOPSIS...Expect periods of rain, higher elevation snow, and
isolated thunderstorms into this afternoon. Precipitation ending
east of Tucson this evening, then dry conditions Tuesday into next
weekend. Much cooler temperatures today and cold temperatures
tonight before a strong warming trend occurs later this week.


.DISCUSSION...Scattered showers and embedded thunderstorms were
occurring mainly across eastern Pima County, eastern Santa Cruz
County and about the western half of Cochise County at this time.
Otherwise, cloudy skies were noted across southeast Arizona early
this morning. The precipitation is ahead of a closed upper low
centered over southern California. The upper low will move
southeastward and be generally centered near the International
border and just southwest of Nogales AZ late this afternoon.

Expect periods of rain into this afternoon as the upper low
approaches the area. For this forecast issuance, the Storm
Prediction Center Convective Outlook was used to delineate where
isolated thunderstorms should prevail into this afternoon. The area
essentially excludes portions of western Pima County and south
central Pinal County from thunderstorm potential. Any thunderstorms
late this afternoon and early this evening should then occur
basically east of a Clifton to Sierra Vista line. Otherwise,
isolated to scattered showers from Tucson eastward this evening,
then any measurable precipitation should be east of this forecast by
midnight tonight.

For this forecast issuance, WPC guidance was heavily used for liquid
amounts into this evening. Based on these amounts, which match
various high resolution models such as the HRRR and the Univ of AZ
WRF-NAM and WRF-GFS quite, the heaviest liquid amounts should favor
a band from near Kitt Peak northeastward across the Tucson metro
area, and further northeast into southern Graham County. As such,
the heaviest liquid amounts favor the Catalina/Rincon Mountains near
Tucson, and the Pinaleno Mountains including Mount Graham. Based on
camera reports and RAWS temperatures, snow levels as of this writing
generally range from about 8000-8500 feet. Snow levels will
gradually fall during the next several hours, and are forecast to be
around 7000-7500 feet this afternoon.

Given this analysis, have opted to issue a Winter Weather Advisory
until 5 pm MST this afternoon above 7000 feet for the Galiuro and
Pinaleno Mountains (including Mount Graham), and the Catalina and
Rincon Mountains near Tucson. Expect snow accumulations mostly
ranging from 6-11 inches, although higher amounts may ultimately
occur at the highest peaks.

The mid/upper trough axis is progged by early Tuesday morning to
extend from the Texas Panhandle southwestward into central Chihuahua
Mexico. With the expectation of only very gradually clearing skies,
fog formation should be somewhat limited. Dry conditions will then
prevail Tuesday into next weekend as high pressure aloft builds west
of the southern California/northern Baja California region. Although
the models depict a shortwave trough will move eastward across the
Four Corners region Wednesday night, any measurable precip
associated with this system will occur well north of this area. The
09/00Z models generally agree that the aforementioned upper high
will be shunted southward this weekend as another low amplitude
trough moves across the Four Corners region. Any measurable precip
with this system will also remain north of this forecast area.

Much cooler temperatures will occur today given the expected clouds
and precipitation. A hard freeze should occur tonight in colder
eastern and southern valleys. A brief freeze is possible in outlying
areas around the Tucson metro area and foothills locations as well.
A warming trend will occur Tuesday into Wednesday followed by minor
cooling on Thursday. Warmer daytime temperatures will resume Friday
into Saturday followed by a few degrees of cooling next Sunday.


.AVIATION...Valid through 10/12Z.
BKN-OVC clouds 6k-10k ft MSL through this evening then clouds will
slowly start to thin out. Expect periods of light to moderate rain
into this afternoon before moving east of the area this evening.
Some -TSRA will be possible into the afternoon at all terminals. In
rain MVFR ceilings and visbys will be likely. Winds will be variable
less than 8 kts through the morning becoming southwesterly to
northwesterly at 7-12 kts this afternoon and evening. Near showers
or thunderstorms winds could gust to 30 kts. Aviation discussion not
updated for TAF amendments.


.FIRE WEATHER...Minimal fire weather concerns. A weather system is
expected to impact the area this morning through tonight with a
chance of valley rain and mountain snow, with the heaviest amounts
across the higher elevations. Dry conditions return Tuesday and
continue into the weekend. 20-ft winds will generally remain less
than 15 mph through the forecast period.


Winter Weather Advisory until 5 PM MST this afternoon for AZZ511-514
above 7000 feet.




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NWS TWC Office Area Forecast Discussion