Area Forecast Discussion
Issued by NWS Phoenix, AZ

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134
FXUS65 KPSR 241519
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
820 AM MST Tue Jan 24 2017

.SYNOPSIS...
Troughing will remain stretched over the Interior West through the
end of the week, loosing access to the Pacific moisture feed. Enough
moisture will linger over the higher terrain for showers to linger
into today. Skies will return to more clear conditions as a drier,
colder airmass settles over the region midweek. Some of the coldest
low temperatures of this winter season are forecast for Wednesday and
Thursday mornings. Clear skies, a slow temperature warming trend and
breezy conditions will develop by the coming weekend.

&&

.DISCUSSION...

A broad trof and cold airmass was beginning to settle over the
region behind Mondays cold front. While the deeper boundary layer
moisture has pushed east of the state, weather balloon soundings
showed drier conditions but with just enough moisture for occasional
light snow showers above 4000 feet in the higher mountains east of
Phoenix, southern Gila County zone 24. It will be mostly clear
elsewhere in our forecast area from southeast CA to south central AZ
today. 545

Otherwise, the coldest airmass of the season will settle over our
forecast area later today through Thursday. Although this incoming
airmass is not cold enough to warrant freeze warnings, it should
lower Wed and Thu morning deserts temps into the upper 20s to middle
30s (generally 28-37 F).

Current short term forecasts through Thu look ok. No updates planned.



.PREVIOUS DISCUSSION...545 AM MST...

Surface cold front that passed through the forecast area Monday is
now working eastward just into New Mexico this overnight. Shower
activity lingers loosely in a line from Williams down through
northeastern Maricopa County and down south to Tucson this hour.
Very stark boundary on overnight IR imagery and sfc obs w/ pressure
rises also place the front now to our east, with mostly clear skies
spanning westward from central Phoenix across the southwest AZ
deserts and out towards Imperial/JTNP near and along the leeward side
of the Peninsular Ranges. Rain shower activity has mostly wrapped
for the lower desert locales in the wake of the surface front and
lower valley rain/mountain snow showers will persist over the higher
terrain of Southern Gila County and on and off through the day.
Winter Weather Advisory remains out through this morning for
elevations above 6000ft where the greatest snow accumulations are
expected and while snow levels continue to fall, a few flurries and
flakes are possible down as low as 4000ft or so as the main trough
axis cuts a slow path across northern AZ, fully exiting the Four
Corners by Wednesday.

Cold airmass will settle into the area, with afternoon 850mb
temperatures progged only to peak around 0C this and Wednesday
afternoons. Values this cool are near daily climo mins per SPC
Sounding Climatology (using KTWC as a proxy). Below normal high
temperatures will result, with many lower desert locales holding in
the 50s and low 60s for the western forecast zones. This incoming
airmass is the coldest so far this winter season and will be enough
to drop minimum temperatures on the lower deserts into the low to mid
30s, possibly 28 degrees both Wednesday and Thursday mornings.

By the late week and coming weekend, temperatures will slowly recover
to forecast values closer to seasonal averages if not just a little
below average. Another low pressure trough will make a trip through
the Four Corners and Colorado River Valley regions from the north
Friday. Given the overland trajectory of the system and little by way
of moisture advection into the area ahead of it, PoP chances remain
at or close to zero for the weekend. Sensible weather impacts from
this dry trough passage will likely be a prolonged period of
northerly to northeasterly breeziness, with gap/pass winds filtering
through the mountains northeast of Phoenix as well persistent gusty
conditions along and west of the Colorado River Valley. High pressure
will begins to build off the CA coast and could gradually expand into
SE CA and Arizona by early next week. Temperatures trending closer to
late January normals are probable with dry conditions continuing.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA,and KSDL:

Weather system has exited to the east, and showers have pretty much
ended across the greater Phoenix area. Threat for additional precip
today is minimal so no mention will be made in the TAFs. There will
be lingering lower level moisture in the form of CU/SC decks; expect
genly FEW-SCT decks in the 3-5k foot range and the 7-10k foot range.
CIGs are possible in any of these decks but more often than not
expect the decks to be SCT at most. Still, feel we may see a
redevelopment of CIGS around 4-5k feet at KSDL and KIWA this
afternoon - thus the BKN decks in those TAFs. Will be more
optimistic at KPHX and leave out any CIGs today but cannot rule them
out at times today. Otherwise, winds will struggle to turn to the
southeast, especially at KPHX, today in this post frontal regime.
Speed should mostly be below 12kt through the early evening today.

Southeast California/Southwest Arizona including KIPL and KBLH:

A general dry west/northwest flow aloft will spread some mid/high
clouds across the western deserts today with BKN decks possible at
times. May see a few lower SC/CU mainly during the afternoon with
bases genly above 7k feet. Winds to favor the west at KIPL and KBLH
today mostly below 12kt.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Thursday through Monday: Cool dry northwest flow aloft will settle
into the area Thursday for generally sunny skies; high temperatures
will be well below seasonal normals. For Friday into the weekend,
high pressure aloft builds along the west coast allowing a dry
system to dive south across the desert southwest. This will be a dry
system allowing humidity levels to fall but keeping temperatures on
the cool side. Strong gusty north winds will develop over the
western Arizona and far southeast California deserts Friday into
Saturday with strongest gusts along the lower Colorado River Valley.
Although humidities do fall, minimum RH values typically remain 15-
20 percent over the lower deserts through the weekend. High pressure
will remain to the west Sunday into Monday keeping dry north flow
aloft across the area and allowing high temperatures to finally
climb back near seasonal normals with warmer deserts into the low
70s by Monday. Winds will decrease Sunday into Monday but remain
locally breezy out of the north over the western deserts.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

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DISCUSSION...Vasquez/Nolte
AVIATION...CB
FIRE WEATHER...CB



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