Area Forecast Discussion
Issued by NWS Phoenix, AZ

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935
FXUS65 KPSR 030342
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
840 PM MST Fri Dec 2 2016

.SYNOPSIS...

Strong surface pressure gradients between high pressure over the
Great Basin and and low pressure over northern Mexico will continue
to produce strong and gusty northerly winds across a large part of
southeast California and the Colorado River Valley tonight and
Saturday. Dry and warmer weather will return Sunday and Monday.
Another large Pacific low pressure system is forecast to move into
the western states next Tuesday and Wednesday, however this system
will track farther north across the central Rockies into the central
Plains. Thus, dry, breezy, and slightly below normal temperatures
are forecast for the middle of next week.

&&

.DISCUSSION...
At 8 pm, a vigorous closed upper low continued to dive south with
the low center situated over the far northern gulf of California. On
the back side of the low, strong DVV/subsidence was overspreading
the western and central deserts associated with a prominent mid-
level Q-divergence bullseye. The subsidence was resulting in
substantial thinning of the previously thick mid/high cloud cover
that was over the area during the afternoon hours. IR imagery at 830
pm showed clear skies from western Maricopa county all the way into
far SE California, with partly cloudy skies from central Phoenix
west. Cloud bases have been relatively high, mostly at or above 10k
feet and this has kept chances for precipitation very limited, even
across higher terrain areas to the east of Phoenix. The primary
impact with this weather system has been gusty north winds on the
back side of the low, especially down the lower Colorado River
Valley. North-south surface gradient has been tightening over the
western deserts and the increasing subsidence was helping transport
strong northerly low/mid level winds towards the surface. A Wind
Advisory remains in effect from the lower Colorado River valley
westward through Saturday evening, and despite winds currently
mostly being below thresholds, the advisory will remain in effect.
For the rest of tonight expect generally clear skies west and partly
cloudy skies from Phoenix east. Gusty north winds will continue
overnight out west especially down the CO river valley. Forecasts
look to be in good shape and no updates are needed attm.

.PREVIOUS DISCUSSION...

.Tonight through Saturday...

The strong upper low that moved rapidly southward down the Lower
Colorado River Valley last night and early this morning has moved
into extreme northern Sonora, Mexico. Behind this system, strong
northerly winds have developed, as expected, over SE CA and the
Lower Colorado River Valley. Gusts as high as 40 mph have been
reported at some locations, along with localized areas of blowing
dust. Although the upper low center will be moving rapidly away from
the region tonight and early Saturday, these winds are expected to
continue into at least late morning on Saturday as a 120kt northerly
jet maintains itself over the region, with good CAA and a rather
tight N-S sfc pressure gradient allowing from better-than-average
mixing of these strong winds aloft down to the sfc. NAEFS
Standardized Anomaly tables are currently showing 850mb winds 2-3 SD
above the mean and Climatological Percentiles in the 79.5-99 percent
range. along with the winds, considerable mid-level cloudiness, and
even some virga has developed over south-central AZ. Although a dry
subcloud layer is expected to keep precip from reaching the ground
at most locations, there is still a slight chance for a few showers
over the higher terrain of southern Gila County through Sat morning,
as seen on latest NAM4km high-res model output. Clearing skies and
diminishing winds area-wide are expected Saturday afternoon as the
upper low pressure center moves away and sfc pressure gradients
begins to relax.

Sunday and Monday...

Dry northwest flow aloft will develop over the region this period.
Nearly cloudless skies are expected with afternoon temperatures
approaching seasonal normals.

Tuesday and Wednesday...

Another deep cold Pacific trof is forecast to move into the western
states Tue and part of Wed. This system will track north of our
forecast area, southeast CA to south central AZ, basically across
the 4 corners region. Dry and breezy conditions can be expected with
afternoon temperatures near or slightly below normal.

Thursday and Friday...

Dry northwest flow aloft will develop over the region this period.
Nearly cloudless skies are expected with afternoon temperatures
near seasonal normals.

&&

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

No aviation concerns through this TAF period. Winds will stay out of
the northwest through around 03Z, then switch over to the northeast.
Speeds will remain light aob 6 kts. Skies will be partly to mostly
clear with few-sct clouds near 10 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Breezy winds and localized blowing dust will be the two main
aviation impacts today. At KIPL, breezy northerly winds gusting up
to 30 kts will persist through early this evening, with lighter
winds aob 15 kts expected between the 00Z to 05Z time frame.
Thereafter, winds will pick up in speed again, remaining out of the
north-northwest and gusting up to 30 kts. At KBLH, sustained north-
northwesterly winds near 15 to 25 kts with gusts up to 25 to 35 kts
will be persist through the entire TAF period. Skies over both TAF
sites will remain mostly clear with few-sct clouds aoa 10kft.


Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:

There is a slightly elevated fire danger potential on Sunday for
Southeast California and parts of Western Arizona. The main concerns
will be windy conditions and relative humidities below 15 percent.
However, the duration of these conditions is short lived and the
overnight recovery for relative humidity is fair. In addition,
breezy conditions on the order of 10 to 20 mph are likely for much of
South-Central Arizona and Southeast California beginning Tuesday and
lasting through Wednesday. Minimum RH will stay in the 12 to 30
percent range with decent overnight recoveries.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters are encouraged to report according to standard criteria.
Spotter reports of wind and dust may be needed Friday across
southeast CA and southwest/west AZ.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 9 PM MST Saturday for
     AZZ020-025.

CA...Wind Advisory until 8 PM PST Saturday for
     CAZ030>033.

&&

$$

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DISCUSSION...CB
PREVIOUS DISCUSSION...Percha/Vasquez
AVIATION...Hernandez
FIRE WEATHER...Deemer



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