FXUS65 KTWC 232256

Area Forecast Discussion
National Weather Service Tucson AZ
355 PM MST Fri Feb 23 2018

.SYNOPSIS...Expect gusty winds this evening with a few showers
northeast of Tucson tonight. A brief warming trend early next week
will be followed by a stronger winter storm system around the middle
of the week, with a decent chance of valley rain and mountain snow
late Tuesday into Wednesday. Dry conditions with a warming trend
next Thursday and Friday.


.DISCUSSION...Visible satellite imagery and surface observations
depict clear skies across much of southeast Arizona at mid-afternoon.
The exception was across western Pima County where scattered to
broken cumuloform clouds were noted. These clouds were ahead of a
mid/upper level trough axis that extended from eastern Montana
southwestward into southern California. Gusty generally westerly
surface winds were also ongoing, with the strongest speeds southeast
to south of Tucson. However, wind speeds were remaining below
advisory criteria.

The mid/upper level trough axis will move rapidly eastward across
the area tonight, and will extend from central Colorado southward
into southwest New Mexico around daybreak Saturday. The 23/12Z NAM12
depicted a few hundredths of an inch liquid rain amounts to occur as
far southwest later this evening (between 24/03Z - 24/06Z) as the
eastern portion of the Tucson metro area. The 23/12Z Univ of AZ WRF-
NAM depicted measurable precip to occur somewhat further northeast,
or as far southwest as the Galiuro Mountains in southwest Graham.
The 23/12Z GFS/ECMWF were even further north with measurable precip
tonight, or mostly north of this forecast area.

At any rate, have maintained a slight chance of showers from the
Catalina Mountains northeast to the White Mountains this evening,
then a slight chance of snow showers across the White Mountains late
tonight. Expect winds to decrease particularly Tucson westward later
this evening, but wind speeds may remain elevated late tonight
across eastern sections. Dry conditions with less wind will be the
rule Saturday into Monday.

A 538-540 dm closed upper low is progged via the GFS/ECMWF to be
centered near the central California coast Monday evening.
Thereafter, there were fairly pronounced differences between the
GFS/ECMWF regarding the eastward progression of this system into and
across the southwest CONUS Tuesday into Wednesday.

In essence, the GFS is about 12 hours faster versus the ECMWF
regarding the potential/probable onset/ending precip times. This
separation in timing is actually a wider spread by about 6 hours
versus these solutions from 24 hours ago. Given these differences,
have opted to make only very minor adjustments to the inherited
gridded data PoP values.

Thus, there remains a slight chance of showers as early as Tuesday
morning from Tucson westward with dry conditions continuing across
most eastern sections. A chance of valley rain/mountain snow showers
exists Tuesday night, then precip chances gradually decrease from
west-to-east Wednesday. If the GFS is reality, then most precip
would end around daybreak Wednesday. At any rate, a fairly decent
potential for accumulating snowfall at elevations perhaps ultimately
as low as 3500-4000 feet, should occur. Dry conditions with a
warming trend is on tap next Thursday into Friday.


.AVIATION...Valid thru 25/00Z.
Isolated -SHRA and higher terrain -SHSN mainly KSAD vicinity and NE
into the White Mtns tonight. Otherwise, scattered to broken clouds
at 5k-8k ft AGL west of KTUS early this evening spreading eastward
later this evening. Generally clear skies south of KTUS. Clouds
decreasing rapidly from west to east late tonight followed by clear
skies to a few clouds around 10k ft AGL Saturday. Surface wind wly/
nwly 12-25 kts with gusts to 35 kts this evening then diminishing
late tonight. Surface wind Saturday wly/nwly mainly less than 12
kts. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...There is a slight chance of showers across the
higher terrain northeast of Tucson, and especially near the White
Mountains tonight. Dry conditions will prevail Saturday through
Monday night. A stronger storm system will then bring scattered
valley rain and mountain snow showers Tuesday into Wednesday. Dry
conditions will return next Thursday and Friday. Expect gusty
southwest to northwest winds this evening and again Tuesday. 20-foot
winds will generally be terrain driven at less than 15 mph at other
times through next Friday.





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NWS TWC Office Area Forecast Discussion