Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 110336

Area Forecast Discussion
National Weather Service Phoenix AZ
835 PM MST Sun Dec 10 2017

High pressure parked across the western United States will result in
dry and quiet weather this week. Temperatures will be well above
average for this time of year with highs in the middle and upper 70s
across the lower deserts. Mostly clear skies will be punctuated at
times with varying amounts of high clouds.


Little change in the weather pattern over the past 24 hours; high
over low blocking pattern remained in place across the western CONUS
keeping very dry conditions in place across southern Arizona while
the upper low off central Baja continued to spread considerable high
clouds inland and into Arizona and far SE CA. PWAT values from the
latest 00z raobs showed just 0.16 inches at Tucson and 0.03 inches
at Flagstaff. Modest NE to SW oriented surface gradient keeping
locally breezy east to northeast winds going over much of south
central AZ which, in combination with high clouds, will keep
overnight lows quite mild again tonight. Forecasts for the rest of
tonight appear to be in good shape and no changes are planned attm.

Well established Rex blocking covers the western Conus this
afternoon as sfc pressure rises continue to be favored over the
Great Basin in association with the highly amplified ridge axis. The
cutoff low portion of the blocking configuration approaching the
Baja was not only pumping copious amounts of high clouds into the
region, but also supporting low pressure over the Gulf of California
and a tighter than usual pressure gradient through the CWA. With
this stagnant pattern, the synoptic setup for Monday morning will be
fairly similar to this morning with an enhanced baroclinic zone and
easterly LLJ emanating over the higher terrain east of Phoenix.
Forecast models and soundings show a 25-30kt LLJ in a 2000-3000ft
layer, part of which will continue to mix overnight yielding stalled
temperatures and breezy conditions for exposed locations. Thickening
cloud cover will also help retard nocturnal cooling, and forecast
lows may even not be aggressive enough on the amount of heat held in

The Rex block will mostly stay intact through the first half of the
week with the low pressure circulation migrating northward into
Sonora as early as Monday. Easterly winds aloft on the north side of
this low will bring increasing mid/upper moisture with cloud decks
lowering and thickening tonight. High temperatures should be
impacted a bit due to the cloud cover, but likely not much as low
temperatures as cloud coverage will start to diminish Monday
afternoon. Very dry air below 15K feet will keep PWATS at or just
below 0.25" over southern Arizona with even lower values to the
north ensuring anything falling from the thicker cloud deck will
evaporate as virga.

High pressure to the north will shift southward by Tuesday, pushing
what moisture does make its way into southern Arizona back into
Mexico for the remainder of the week. The strong upper level ridge
will exert more influence on the region through the rest of the week
with very dry northerly flow returning. For now, models are not
showing any backdoor cold fronts originating in the Rockies, so
temperatures will remain quite mild for December with forecast highs
in the middle 70s over the lower elevation desert communities. Some
model output does show some sort of a trough moving into (or near)
the region from the northwest as early as Friday night, but
operational and ensemble members are hardly unanimous in this
forecast resulting in little confidence of this outcome.


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

Still enough of a NE to SW surface gradient over eastern AZ to keep
the east to northeast breezes going over the greater Phoenix area.
Expect little change in the wind overnight tonight and through the
morning hours on Monday. Gradient should relax slightly allowing
light variable or even light west wind to set up by mid to late
afternoon tomorrow at the TAF sites. Otherwise will see continued
thick mainly high cloud shield with BKN to OVC decks aoa 20k feet
thru Monday afternoon at least.

There is outside chance for some low level wind shear later tonight
and into Monday morning as low level east winds to around 30 knots
combine with lighter east/northeast surface winds. Best chances at
KSDL, assuming that winds decouple enough. May not see lots of
decoupling given the cloudy skies and confidence too low to warrant
putting shear into the TAFs; just something to keep an eye on.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation concerns next 24 hours; winds will stay on the light
side favoring the north/northeast at KBLH and the west to north at
KIPL. Still seeing lots of high clouds aoa 20k feet with BKN to OVC
decks thru Monday afternoon.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday:
High pressure will remain the predominant weather feature through
the week, resulting in above normal temperatures and relatively
dry conditions. Minimum humidity values each day will drop into a
10-15% range with fair overnight recoveries. The winds will remain
fairly light and favor diurnal tendencies with just a few morning
gusts, particularly in the higher terrain east of Phoenix.


Spotters should follow standard reporting procedures.





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