Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 232158

Area Forecast Discussion
National Weather Service Phoenix AZ
258 PM MST Sat Sep 23 2017

A gradual warming trend is expected over the next few days, though
temperatures will remain below normal. An area of low pressure
may pull moisture back into eastern Arizona by the middle and end
of next supporting some late season showers and storms.


An anomalously cool and dry air mass continues to overspread the
Desert Southwest in the wake of a frontal boundary that moved
through this morning. Surface dewpoints in the 20s are prevalent
across much of the area with temperatures struggling to reach the
mid 80s in the lower deserts. The broad trough responsible for the
cool and dry conditions will linger across the intermountain West
the next few days. Although height rises will translate into a
warming trend, temperatures will remain below normal through

Models trends continue to indicate that a jet streak between the
aforementioned trough and a ridge across the eastern Pacific will
induce a vort max and a deepening area of low pressure across the
Desert Southwest Wednesday. However, operational models and their
corresponding ensembles are exhibiting a large amount of
variability with respect to the placement of the low and
subsequent moisture transport. Consequently, the models indicate
that predictability is low as it often is in these situations.
Models also suggest that if a closed low does form, it will likely
linger for several days as a Rex Block becomes established across
the West.

Forecast was weighted heavily towards the National Blend, which
paints the highest PoPs across the higher terrain north and east
of Phoenix. However, due to the inherent uncertainty, PoPs really
only peak around 30 percent Wednesday afternoon. It`s also worth
mentioning that if the low pressure system does materialize,
we`re entering a period which is climatologically favored for
organized thunderstorms, including damaging winds and small hail.
The evolution beyond Thursday remains highly uncertain but bears


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

Drier air moving in from the west is expected to pretty much clear
skies completely out by late tonight, with mainly clear skies to
persist through Sunday at all of the terminals. Winds to mainly
follow typical diurnal trends at the Phoenix area terminals, except
for westerly winds persisting longer than usual into tonight. Winds
at KIPL and KBLH to remain from a northwesterly to northerly
direction through the taf period.


Monday through Friday:
Troughing will persist over much of the west, receiving
reinforcing shots of jet wind and energy from the north. One batch
of energy will eventually cut off over southern UT/northern AZ,
turning low level flow south-southeasterly by the early week. Warm
air advection will warm sfc temperatures while also drawing up
moisture into the region. Humidities will be on the low side with
minimum readings falling into the 10-25 percent range with fair-
good overnight recoveries through midweek. Some increase in
humidities is likely beginning Wednesday. Anticipate north and
northwesterly breeziness Monday and Tuesday over southeast CA and
southwest AZ. More noticeable easterly winds will develop over the
eastern AZ districts midweek and beyond as the upper low remains
over the region. Slight chances for precipitation return from
Maricopa County eastward for Wednesday and continue for much of
eastern Arizona through week`s end.


Spotters should follow standard reporting procedures.




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