Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 211637

Area Forecast Discussion
National Weather Service Phoenix AZ
937 AM MST Tue Feb 21 2017

Drier and more stable conditions will develop through Wednesday
as a ridge of high pressure system builds over the region. This
will result in additional warming today with little change
Wednesday. A dry cold front will move through the region
Wednesday night and Thursday, with temperatures dropping below
normal. Another system will affect the region over the weekend.



Mostly clear skies across our forecast area last night allowed some
dew to form this morning along with some light mist and reduced
visibilities (5-9 miles) in SE California.  Satellite imagery shows
mid-level clouds beginning to move over SE California to NW Arizona
which are riding over a low amplitude ridge. This ridge will slowly
weaken today and allow the mid-level clouds to spill into western
portions of our CWA throughout the day. Despite this, temperatures
in SW CA are expected to warm into the mid 70s for today which is
near to slightly above normal for this time of year.

Closer to home, the Phoenix metro should just see a few passing high
clouds with weak westerlies forming this afternoon from the typical
mountain/valley breeze.  The forecast looks on track for today and
no major changes we made.

Wednesday through Friday...
The ridge gets pushed eastward as the Pacific trough advances
inland on Wednesday. This will lead to some cooling over southeast
CA and southwest AZ but little change over south-central AZ.
Breeziness will pick up in the afternoon (locally windy over
portions of southeast CA). A cold front associated with a short
wave within the main trough will move through the forecast area
Wednesday night/Thursday morning. This will usher in a cooler air
with highs Thursday in the 60s. Breezy conditions can be expected
as well - especially over southeast CA and the Lower Colorado
River Valley. Not much moisture with the system though so PoPs
stay north of our forecast area. Meanwhile offshore, models depict
a rex pattern over the northeast Pacific and a transition to
zonal flow over the Desert Southwest on Friday for slightly warmer

Friday through Monday...
Global deterministic models have had considerable changes in their
solutions for this period compared to some recent runs. A key
factor appears to be how they are handling both the short waves
sliding down the east side of the Pacific ridge/upper anticyclone
and the lows/waves undercutting the ridge. Of note...both the GFS
and ECMWF show more interplay between the waves in the northern
branch of the Westerlies and those in the southern
branch/undercutting flow. In short, they are depicting a slower
arrival of Pacific troughing (with at least two main short waves
involved). There are some differences though with the ECMWF
showing a stronger initial short wave and better PoPs for Sunday.
Latest NAEFS (and GFS) has very little for our area before 00Z
Monday. Taking a blended approach, PoPs start up over Joshua Tree
National Park Saturday night and expand eastward across the
forecast area Sunday with at least slight chance numbers going
through Monday. It`s not looking like a particularly cold
situation and thus temps don`t plummet and snow levels generally
stay above 6000 feet. There will likely be more
changes/adjustments to come in subsequent shifts as models will
likely change their solutions.


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

Light and typical diurnal winds are expected the next 24 hrs at
all area terminals. The potential for patchy fog in outlying areas
remains the primary short-term concern this morning. Current
observations show surface temperatures cooling to near saturation.
While no fog is evident on satellite image products thus far, will
still maintain vicinity fog mention at KIWA through about 16Z in
the event some does develop. Otherwise, high clouds are forecast
to increase later today from the west, becoming BKN around 25k ft
after 20Z.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light and occasionally variable winds are forecast to occur at
both KIPL and KBLH through mid-afternoon. Westerly flow above 1km
agl should become realized at the sfc after 2Z, around 10 kt or so
in magnitude. Otherwise, high clouds are forecast to increase from
the west this afternoon, becoming BKN around 25k ft after 18Z.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday...Dry conditions are expected for most
areas through Sunday. Above normal temperatures will occur Tuesday
and Wednesday. A dry cold front moving through the region
Wednesday night and Thursday will lead to cooler temperatures and
locally breezy conditions. Anticipate slight warming Friday and
Saturday. Minimum humidities will trend downward through the
workweek (especially on Thursday) with most lower elevations in
the 15-25% range by Thursday. Another storm system will affect
the region late this weekend into early next week. The main effect
will be breezy west winds on Sunday and especially Monday.
Chances for rain also increase on Monday over the entire forecast


Spotters should follow standard reporting procedures.




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