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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 131116 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
415 AM MST Fri Dec 13 2019

.UPDATE...Updated aviation and fire weather discussions.


High pressure will result in a slight warming trend through
Saturday. Skies will be mostly sunny with few periods of high clouds.
A mostly dry weather system will arrive Sunday bringing cooler
temperatures across the area with chilly morning low temperatures
Monday and Tuesday. Temperatures will rebound slowly through the
first half of next week.


Ridging across the southwestern CONUS is evident in the latest water
vapor imagery while broad troughing persists downstream across the
rest of the country. The 13/00Z upper air analysis shows H5 heights
were near 582 dm across central Arizona placing near the 90th
percentile of climatology for the date. Upper level moisture will
bring a few high clouds while daytime temperatures will approach 70
degrees again.

Saturday morning and afternoon will not differ much than Friday but
an increase of upper level moisture ahead of a developing Pacific
trough will lead to thicker cloud cover beginning late Saturday
night. The trough axis will pass over the Four Corners area fairly
quickly Sunday and moving off to the east by Monday. Most moisture
with this system will remain over Northern Arizona although a few
showers may develop over the higher terrain east of Phoenix. The
most notable impact from this trough will be much cooler
temperatures with Monday and Tuesday forecast to be the coldest days
in at least two weeks, if not so far this winter. Overnight lows for
KPHX, typically one of the warmest locations in the area, is
forecast to drop to near 40 degrees during that time (coldest so far
this year is 42). Daytime highs will also be a few degrees below
seasonal normals. Otherwise, breezy conditions will develop across
southeast California Sunday but wind gusts will remain mostly below
30 miles per hour.

Dry conditions will solidify with ridging rebuilding in the wake of
the aforementioned trough. Temperatures recover close to seasonal
normals by midweek but model solutions indicate another trough
developing over the Pacific towards the end of next week.
The location of trough development is more favorable to ingest
deeper moisture but the GFS and ECMWF differ on the depth of the
trough axis and moisture content. Still, both the GEFS and EPS
ensembles provide indications of rain for the desert, but as of now,
amounts and aerial coverage do not appear significant with plenty of
uncertainty. As of now, low 5-10% PoPs are in the forecast for


.AVIATION...Updated at 1115Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT;
Southeast California/Southwest Arizona including KIPL and KBLH:

Weak high pressure aloft will persist across the area for the next
24 hours at least; some high clouds will move through the lower
deserts but decks likely to be FEW to SCT with bases aoa 20k feet.
Winds will be very light, weakly favoring typical diurnal tendencies
in the greater Phoenix area. Lots of light/variable observations
expected especially across the western TAF sites with speed mostly
5kt or less. No aviation concerns of any kind for at least the next
24 hours.


Sunday through Thursday: A weak low pressure system affects the
region Sunday into Monday mainly ushering in cooler conditions.
Moisture levels with the late weekend system should be rather dry,
hindering precipitation chances although there is a chance of rain
or high elevation snow on Sunday over high terrain east of Phoenix.
Cooler conditions develop Sunday with highs falling below normal by
Monday and into the lower 60s over the cooler lower deserts. Minimal
warming returns returns Tuesday into Thursday as highs approach
seasonal normal levels.Minimum humidity levels initially should
stick around 25-35% much of the period, lowering into early next
week with desert readings falling into the teens before humidity
levels increase again later in the period. Breezy conditions move in
for Sunday and Monday, mainly over deserts west of Phoenix. Light
winds return again by Tuesday as weak high pressure aloft returns to
the area.


Spotters should follow standard reporting procedures.





Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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