Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 162108

Area Forecast Discussion
National Weather Service Phoenix AZ
208 PM MST Sat Dec 16 2017

Quiet weather is expected today before the next weather system moves
in. This one still looks to bring chances for rain and mountain snow
to the eastern half of Arizona Sunday and into early Monday.
Temperatures will also be cool with this system, potentially
bringing the coolest temperatures of the winter so far. Tuesday and
beyond will see a return of dry and gradually warming weather before
another cooling trend sets in for the latter part of next week.


It seems models are now in good agreement with the track of a
deepening positively tilted trough currently digging south
southeastward along the California coast. Current water vapor
imagery shows the upper level low center just off the coast of San
Diego with modest moisture advection to the east of the low. Mid
and high clouds have become more scattered since earlier today
with a decent amount of sunshine currently seen across the western
half of Arizona.

The trough will continue to dig southeastward through this
evening before making a turn to the east tonight into Sunday. The
upper level low is set to slowly track to our south early Sunday
and then swing a bit northeastward into southeast Arizona for the
latter part of Sunday. This southern track will limit rain chances
across a good portion of the area with the best chances remaining
across southeast Arizona. Low level moisture will begin to get
drawn up out of Mexico tonight, but by midday Sunday, low level
flow becomes easterly, cutting off our moisture feed. PWATS are
forecast to increase to between 0.50-0.75", definitely sufficient
for light to moderate rainfall, but upper level dynamics will
mostly be lacking across all about southeast Arizona. The bulk of
the rain shower activity should occur from Phoenix south and east
between 15-00Z on Sunday when upslope low level flow is
maximized. Rainfall amounts will be limited with south-central
Arizona lower desert areas likely only seeing a few hundredths of
an inch to maybe a tenth or two in the eastern Arizona high
terrain. The low center will move into southeast Arizona starting
Sunday evening shifting the rain shower activity eastward into far
east Arizona. Lingering rain showers and snow showers above
6000ft are likely across Gila County Sunday evening, but
diminishing considerably after midnight.

After high temperatures in the upper 60s to around 70 today,
temperatures will dip even further on Sunday with highs mostly in
the middle 60s, likely a bit lower across eastern Arizona under
mostly cloudy skies. Gusty winds will also be an issue across
southeast California after midnight tonight through Sunday
afternoon. The strongest northerly winds will be focused across
central Riverside County, including JTNP, where Advisory level
winds are expected.

On Monday, as the low moves off to our east, some wrap around
moisture will linger over the higher terrain of southern Gila
County with slight chances of showers through the morning. By
Monday afternoon we expect mostly sunny or sunny skies along with
desert highs in the mid to upper 60s.

Upper ridging will build into the area Tuesday into Wednesday for
mostly sunny days, clear nights and a modest warming trend allowing
the warmer deserts to reach into the lower 70s by Wednesday

For the latter portion of the extended, Thursday into Friday, latest
guidance has become more insistent that another large and rather
cold upper trof will drop into the area from the north and gradually
develop with a closed low eventually forming somewhere over southern
Arizona or northwest Mexico. GFS and ECMWF differ on details and
moisture, but we have sufficient confidence to add some slight
chances to the forecast mainly over southern Gila County on Friday.
This deepening trof will usher in another bout of much cooler air so
a marked cooling trend has been introduced, with desert highs
falling into the low to middle 60s over the deserts by Friday.
Phoenix is now expected to see a below normal high of just 63 next
Friday under mostly sunny skies.


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

A cold front and associated upper-level low center moving into the
region will gradually increase and thicken mid/high cloudiness over
the region over the next 24 hours. Lower clouds (cigs in the 5-6K
foot range) and a chance for light showers is expected to move into
the Greater Phoenix area on Sunday morning, with these lower
cigs/shower chances persisting through the day on Sunday. Since
confidence in the timing of the shower activity is low at this time,
only VCSH is mentioned in the Phoenix area tafs at this point. Winds
to remain mainly easterly through the taf period, with the strongest
winds (up into the 10-12kt range), and even a bit gustiness during
the mid/late morning hours on Sunday.

Southeast California/Southwest Arizona including KIPL and KBLH:

A cold front and associated upper-level low center moving into the
region will gradually increase and thicken mid/high cloudiness over
the SE CA region over the next 24 hours, with SCT layers as low as
6K feet) possible during the day on Sunday. Light southeasterly
winds this evening to give way to gusty northerly to northwesterly
winds later tonight and on Sunday. Strongest winds will be over area
ridgetops to the west of the terminals, but elevated gusts are
possible for KBLH (more so than KIPL) near 20-30kts through the day

Aviation Discussion not updated for amended TAFs.


Monday through Friday...
Upper low pressure feature that moved into the region on Sunday
will be slow to exit, however end most precip chances across the
eastern Districts. In the wake of the exiting system, winds will
be light and variable with subtle trends toward drainage slopes
and diurnal patterns. Dry northwest flow will linger for Tuesday
and Wednesday before another colder and more potent storm system
sweeps in from the northwest. Breezy west-northwest winds will
develop overnight Wednesday and into Thursday, impacting area
ridgetops first then mixing into the lower desert elevations
during the daytime. There could be some precipitation that
develops over Arizona with this next system, however forecast
model differences are really noticeable after Thursday with how
deep the system is and how long it sticks around the area. Daytime
humidities will be range higher than the last few weeks, 15 to 20
percent for the lower elevations and 30 percent or greater for the
higher terrain.


Spotters should follow standard reporting procedures.


CA...Wind Advisory from 11 PM this evening to noon PST Sunday for



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