Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 280418

Area Forecast Discussion
National Weather Service Phoenix AZ
920 PM MST THU OCT 27 2016

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


After a record-setting day, which saw Phoenix Sky Harbor set a new
record for the latest 100-degree day on record, temperatures have
dropped to much-more-comfortable levels at this hour, with Sky
Harbor now down to 84. Still, with a very warm air mass still in
place over the region and considerable high cloudiness that has
spread over our cwa ahead of a storm system that is now moving into
central/southern CA, a very warm night (for this time of year)
appears to be in the cards, with most lower desert locations only
falling into the mid-60 to mid-70 range tonight. The forecast low of
74 at Shy Harbor (if it proves to be correct) would set not only a
record warm min for the date, but also be the warmest low
temperature for so late in the fall by a substantial margin. For the
very short term, outside of some minor adjustments to the hourly
temp/dewpoint grids, inherited forecast look good.



Tonight through Friday...

A strong high pressure system over the area will weaken tonight as a
fairly potent Pacific storm approaches the central and southern CA
coast. Increasing high clouds this afternoon and especially tonight
are fore-runners of this massive weather system. Unfortunately the
Pacific storm and its precipitation is forecast to track into
central CA tonight, then into northern Nevada Friday. This will
leave most of our forecast area, southeast CA to south central AZ
cloudy but dry, with the exception of the southeast CA mountains and
higher deserts where dense cloud spill-over will generate a slight
chance of showers Friday. The tail end of a cold front will move
through southeast CA around 9 am Friday, and with increasing low
level west wind, rain shadow type, it should limit shower activity
to sprinkles or an occasional light shower especially over the
mountains of Joshua Tree N.P.

Saturday through Monday...

Dry west to southwest flow aloft with occasional high clouds are
forecast Saturday through Monday. Unseasonably warm temperatures
will continue, i.e. lower 90s on the deserts due to a fairly
vertically thick atmosphere as the storm track remains well north of
AZ.  However on Monday, afternoon temperatures will likely decrease
as another very large Pacific low pressure system edges toward the
west coast and closer to AZ.

Tuesday through Thursday...

This period is somewhat uncertain with regards to how far south the
massive Pacific weather system will move, and its affects on
southern AZ. All models forecast this system to take a deep
southward turn into southern CA and northern AZ.  However a very
progressive flow pattern across the Pacific should continue, and
quite possibly eject this approaching Pacific weather system farther
north than forecast, just like all the earlier weather systems.
Confidence is low this period, and all offices decided to take a
wait and see position regarding storm magnitude this far south,
and precip if any.


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

High pressure at the sfc to keep winds quite light (mainly aob 8
kts) through the taf period. A pacific weather system moving off
well to the west and north to continue to push sct-bkn mid and high
cloud decks over the region through the taf period, with cigs mainly
aoa 10k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds into Friday morning then increasing from the south. High
clouds will thicken and gradually lower, approaching 10kft late
Friday morning. Thick mid/high clouds persist late Friday morning
through the afternoon. Although there is also a slight chance for
some light sprinkles/showers at KIPL Friday afternoon as a pacific
storm system passes off to the west and north, confidence is too low
to include them in the taf at this point.

Aviation discussion not updated for amended TAFs.


Saturday through Wednesday...

High temperatures this weekend will remain well above normal, then a
gradual cool down is expected through the middle of next week as a
a low pressure system moves across southern California Tuesday night
and into Arizona Wednesday. As of now, little moisture is expected to
accompany this system, and the greatest impacts will be the slight
cool down in temperatures.

Minimum humidities will remain near the 20 to 30 percent range each
day with good overnight recoveries. Winds will remain light and
follow normal diurnal headings through the entire period, except for
some periods of afternoon breeziness up to 20 to 25 mph in southeast
California Sunday.


Spotter activation will not be needed this week.




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