Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 190540 AAA

Area Forecast Discussion
National Weather Service Phoenix AZ
1040 PM MST Wed Oct 18 2017

.UPDATE...Updated Aviation Discussion...

A slight chance for very light showers over south central Arizona
will exist Thursday morning as a weak disturbance moves through
the region, but will quickly move out of our area by late
Thursday afternoon. A cooldown will occur into this weekend,
before high pressure rebuilds over the region leading to well
above normal temperatures by early next week.


Mid-high level cloud cover associated with a weak disturbance over
southern California is spreading eastward across Arizona this
afternoon. A modest increase in moisture aloft has occurred, with
precipitable water values near about 0.75 inch, but low-level
moisture is still lacking. Latest high-resolution model guidance
continues to show some weak showers developing mainly north and
east of Phoenix overnight, but may primarily be in the form of
virga across the lower deserts given the dry air near the ground.
A marginally better chance for measurable rain will be possible in
higher elevations.

Midlevel heights will remain relatively suppressed into the
weekend as a secondary and stronger upper trough moves across the
western CONUS, with most forcing for ascent remain focused to the
north of the region. The biggest effect on our weather will be a
slight cooling trend into Saturday, closer to seasonal averages.
West winds will also become breezy area-wide on Friday, while
remaining gusty out of the north across southeast California on
Saturday. This system will remain dry.

Unfortunately, the cooldown will be short-lived, as a prominent
ridge of high pressure builds over the Southwest. Latest NAEFS
mean 500-mb heights suggest magnitudes will be near the 99th
percentile. It currently appears that Monday and Tuesday will be
the hottest days of the week, with temperatures climbing into the
upper 90s across the lower deserts, which will be near or above
daily temperature records at many spots. The heat may abate by a
couple degrees by the middle of the next week, but above-normal
temperatures look likely to continue into the foreseeable future
as a blocking pattern becomes established across North America.



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

A weak weather disturbance moving eastward across the region will
continue to push SCT-BKN mid/high cloud decks across the region
through Thursday morning with some clearing expected Thursday
afternoon/evening as the disturbance moves off to the east. There is
also a remote chance for a light sprinkle/showers as well
tonight/early Thursday, with showers more likely over the higher
terrain to the north and east. Weak sfc pressure gradients to keep
winds on the light side through midday on Thursday with somewhat
stronger westerly winds developing Thursday afternoon. Wind
directions to mainly follow typical diurnal trends through Thursday
afternoon, but with westerly winds persisting later than usual into
Thursday night.

Southeast California/Southwest Arizona including KIPL and KBLH:

SCT/BKN mid/high cloud layers to persist through tonight before
gradually clearing on Thursday as drier air moves in from the west.
Winds to mainly follow typical diurnal trends through Thursday
afternoon, then increase Thursday afternoon/evening (from a westerly
to southwesterly direction) as a Pacific weather system begins to
approach from the west.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday:
Temperatures hovering near average over the weekend will rebound
back close to 10F above normal on Monday and Tuesday. High
pressure and dry air will become locked over the districts
ensuring no chance of rain. Afternoon minimum humidity levels in a
10-20% range over the weekend will fall into the single digits
next week while overnight recovery deteriorates from good down to
poor to fair levels. Breezy westerly winds are possible area-wide
on Saturday, and gusty northerly winds developing on Sunday across
southeast California, but speeds will remain well below critical


Spotters should follow standard reporting procedures.




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