Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281245

Area Forecast Discussion
National Weather Service Phoenix AZ
545 AM MST FRI OCT 28 2016

.UPDATE...To Aviation and Fire Weather Discussions...


A large and wet Pacific weather system is forecast to move into
northern Nevada today, and this system will spread considerable
clouds over the region with a slight chance of showers over the
southeast California deserts and a somewhat better chance over the
mountains. Dry weather with mostly clear skies will return this
weekend, with desert temperatures continuing in the lower 90 degree
range. Another massive Pacific weather system will approach the
central California coast by Tuesday, then move into the western
states Wednesday providing mostly cloudy skies and cooler afternoon
temperatures to southeast California and southern Arizona.


Early this morning, a well defined upper low, clearly visible in the
IR and vapor imagery, continued to help pump up a large scale ridge
across the western and central portions of the CONUS. 500mb heights
around 590dm were noted on the latest 00z plot data across the
southern AZ deserts, and despite a decent amount of high cloud
cover, the ridge was strong enough to allow the high temperature at
Phoenix to hit 100 yesterday. This was the latest ever occurrence of
100 degrees in Phoenix by 4 days. IR imagery at 2 am continued to
depict significant amounts of mainly high clouds spreading inland
and across the desert southwest including Arizona. Progs, including
GEFS ensemble members, advertise the strong ridge persisting through
the weekend; it will serve to deflect the short wave energy moving
out of the trof along the coast, having it ride around the periphery
of the high and mostly affect the higher terrain areas of eastern
Riverside County as well as deserts north and northwest of Phoenix.
The most prominent such short wave will pass through our area today
giving a chance of showers to Joshua Tree National Park and just a
slight chance of a light shower to lower deserts areas north and
northwest of Phoenix, mainly over La Paz and far northern Maricopa
Counties. Despite the continue presence of significant high clouds
today, high temperatures will stay in the low 90s over the warmer
lower deserts. In fact we are looking at low 90s across the lower SE
California and Arizona deserts through Sunday as the ridge aloft
remains on the strong side.

Models do indicate that the upper ridge will finally begin to break
down by Monday and continue to do so into the middle of the week as
stronger short wave energy moves into the desert southwest with an
open wave trof moving into Arizona by Wednesday. For the most part
it looks like this wave will be quite dry, maybe bringing a few
light showers to the higher terrain east of Phoenix by Wednesday.
The idea of continued dry weather is supported nicely by the NAEFS
POPs guidance, and we will heavily leaning on this guidance for our
POP forecast next week. Both the operational GFS and ECMWF are also
pretty dry for the lower deserts next week as well and the primary
impact of the increased troffing will be a decrease in temperatures.
We can look forward to high temperatures over the deserts finally
lowering into the 80s starting MOnday and persisting into the middle
of next week. Phoenix is forecast to fall to 88 next Monday and then
down to 85 by next Wednesday. Despite the drop into the 80s, temps
will likely stay above seasonal normals; the normal high in Phoenix
next Wednesday is just 82 degrees.



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

Flow aloft will transition to broad southwesterly headings as a storm
system works on-shore over the central West Coast. Steady feed of
BKN-OVC high level decks, with some FEW-SCT mid-level fields, will
continue to stream over the Phoenix area today. CIGs should stay at
or above 15kft. Winds will follow usual diurnal trends with speeds
8kts or less.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light AM winds will begin to increase into the afternoon hours,
southwesterly for KIPL and southerly for KBLH, as a storm systems
moves into central CA from the Pacific. Steady stream of BKN-OVC high
level decks, with SCT-BKN mid level fields will continue throughout
the day, at times developing down 8-10kft. A slight chance still
remains for portions of southeast CA during the day, but latest hi-
res model solutions indicate light shower activity staying mainly
north of the terminals. As such, left VCSH out of the 12z TAFs.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday...
More noticeable cooling trend will begin after the weekend as a storm
system moves into the region from off the Pacific. Temperatures will
still be above seasonal averages, but not near the record-breaking
levels observed over the end of previous week. Moisture continue
remains on the low side with this storm system, so precip chances are
next to none and the most noticeable impact will be the slight
temperature cool down and some elevated breeziness Sunday afternoon.
Winds will be at their strongest across southeast California, where
from daytime gustiness will be possible up to 25 mph. Daytime
humidities will range 20 to 30 percent and make good overnight


Spotter activation will not be needed this week.




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