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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 142049

Area Forecast Discussion
National Weather Service Phoenix AZ
149 PM MST Thu Dec 14 2017

Windy conditions will continue across southeast California and
southwest Arizona into this evening, before winds begin to drop
off most areas. Quiet weather is expected Friday and most of
Saturday before the next weather system moves in. This one is
looking a bit stronger now, with chances for rain and mountain
snow now in the forecast for the eastern half of Arizona Sunday
and into early Monday. Temperatures will also be quite cool with
this system, likely bringing the coolest temperatures of the
winter so far. Tuesday and beyond will see a return of dry and
gradually warming weather.


Short wave trough moving through the Southwest today is bringing
some cooler temperatures to Central Arizona and an increase of
winds to Southeast California and Southwest Arizona. Our GOES-16
came back this morning (after it moved to a more permanent
location at 75W) and just in time to show us a few plumes of
blowing dust, primarily emanating from the Imperial Sand Dunes
and Ford Dry Lake. This may cause some impacts, especially along
I-8 and I-10 through this evening. Pressure gradients will begin
to decrease this evening and with surface decoupling, allow the
winds to subside some. Around Phoenix, some areas, especially
foothill areas to the north/east, may still see some decent
breeziness overnight which would also hold up temperatures there.

Transitory short wave ridging will take place Friday but by
Saturday afternoon we`ll start to see the influence of the next
series of short waves. Seeing better deterministic/ensemble
agreement with the pattern evolution this weekend. As the West
Coast ridge axis shifts westward, this will allow two short waves
to drop into the Southwest. They will constructively interfere
with each other by Sunday, causing an upper level closed low to
form. There does still remain plenty of variability in just how
deep the low will be, how far west/east it will be once it forms,
etc., which will play into the specific of the forecast. For now,
increased precipitation chances accordingly (along with FGZ and
TWC) for Maricopa County and areas east. Likewise, this will
result in much cooler temperatures. Some lower desert locations
Sunday afternoon may struggle to break into the 60s, which would
be a marked difference from the last several weeks.

Depending on how strong the system is, dynamical lift and mid-
level cooling may yield some positive instability. Seeing some
more robust convective showers, include a few rumbles of thunder
and (where it gets cool enough) some graupel, wouldn`t be outside
of the realm of possibilities.

Clouds, a few showers, and cooler temperatures will linger into
Monday before upper level ridging starts nosing back in Tuesday.
One thing to watch for in the Week 2 range is a fairly bullish
forecast by the GEFS and ECens of a pronounced and high latitude
ridge over northwest North America. This pattern would promote
below average temperatures and an increased probability for
weather systems to come down the West Coast. This is well
reflected in CPC`s current Week 2 outlook.


South-Central Arizona including KPHX, KIWA, and KSDL:
East to northeast winds will persist throughout the day at all
area terminals, with a few gusts to 15-20 kt possible. Some
concern exists about the development of low-level wind shear given
strengthening flow above 1000 feet or so, while surface winds are
expected to weaken early Friday morning. Have included this
mention overnight for all area terminals. Winds should remain out
of the east for most of Friday as well, with high pressure
persisting to the northeast. Skies will be clear.

Southeast California/Southwest Arizona including KIPL and KBLH:
Windy conditions are developing across the lower Colorado River
Valley and southeast California this afternoon. Gusts over 30 kt
will be possible, especially at KBLH, and should persist out of
the north into the early evening. Not quite as gusty at KIPL with
gusts anticipated to only reach around 20 kt. Winds will weaken
area-wide overnight, with clear skies expected.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday...
A fast moving upper trough will cut southward through SoCal and
dive off the western Baja coast Saturday into Sunday, developing
another round of elevated northerly winds over southwest AZ, the
Lower Colorado River Valley and portions of southeast CA. With
cooler high temperatures in the forecast daytime humidities range
10-20 percent over the weekend with mostly fair overnight
recoveries. Current fire weather forecast elements support very
sparse and spotty red flag conditions for Sunday west of the
Colorado River for an hour or two, but nothing at this time
warranting more than mention here. Some recent changes to model
forecasts now have some moisture in play to the south across
northern Mexico. Some increased in humidities regionally and even
remote chance for light precip (mainly eastern-southeast AZ) could
develop. Quasi-zonal flow will settle over the region by early
next week, keeping temperatures on the more mild side but still
slightly above seasonal normals.


Spotters should follow standard reporting procedures.


AZ...Red Flag Warning until 5 PM MST this afternoon for AZZ131.

CA...Red Flag Warning until 4 PM PST this afternoon for CAZ230>232.

     Wind Advisory until 4 PM PST this afternoon for CAZ560-564-565-



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