Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 260854

Area Forecast Discussion
National Weather Service Phoenix AZ
154 AM MST Tue Sep 26 2017

Under sunny skies, temperatures will slowly warm over the next few
days though readings will remain slightly below average. An area of
low pressure may pull moisture back into eastern Arizona by the
middle and end of the week supporting some late season showers and
storms mainly in the higher elevations. Warmer temperatures will
materialize by the end of the work week and persist through early
next week &&


A broad upper level trough is currently situated over the west
coast with ridging just downstream centered over the eastern half
of the United States. This is bringing below average temperatures
to many locations on the west coast while just the opposite is
being observed in the eastern US. Only a few cirrus clouds can be
seen on satellite this morning thanks to the extremely dry air
overhead. The latest 00Z sounding from Tucson shows PWATs of only
0.28 inches, just above yesterday`s record minimum of 0.22! With
the lack of water vapor, temperatures have again cooled quickly
with morning lows forecast to be near to slightly above what was
observed Monday morning.

Heading into this afternoon, another below average day is expected
as 850 mb temperature warm just slightly over yesterday.
Subsequently, many locations will only see a degree, maybe two
of warming from Monday. Other than a few more high clouds, a very
similar day is expected.

By midweek, moisture gradually seeps in from the east as energy
shears off from the aforementioned trough and develops into an
upper level low near the CA/AZ/NV border. Some hi-res models
show this more moist air nearing Maricopa County by late
Wednesday, but is positioned farther east than what models were
indicating 24 hours ago. Nonetheless, Gila County looks to have a
shot at some showers and storms come Wednesday afternoon although
activity in Phoenix is looking rather slim (<10%), especially
given the southwest steering flow that will quickly push off any
storms to the north and east. Locations west of the Phoenix metro
have virtually no chance of precipitation.

Isolated afternoon storms in eastern Gila County look to persist
through at least the end of the week until drier air moves in over
the weekend. All the while, temperatures will gradually warm and
approach the century mark in the lower deserts come Saturday as
H5 heights gradually recover from the exiting upper level low.
By Sunday, models show a passing trough to our north that may
bringing subtle variations to our temperatures although nothing
that deviates far from climatology.


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

Light diurnal winds through the next 24-36 hours with FEW-SCT
clouds AOA 20kft will cause little impact to area terminals.

Southeast California/Southwest Arizona including KIPL and KBLH:

Northwesterly flow will continue to weaken this afternoon/evening
with more typical diurnal (weak) flow developing for the overnight
hours. Any clouds will remain AOA 20kft.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday: A large area of low pressure aloft will
dominate the weather pattern over the desert southwest for the rest
of the work week, keeping temperatures below seasonal normals, and
keeping drier air in place over southeast California and the
southwest Arizona deserts. Expect minimum RH values from the single
digits to low teens over the central deserts Wednesday, slowly
climbing mostly into the teens by the end of the work week.
Southerly flow ahead of the upper low will gradually advect moisture
into south central Arizona by Wednesday, bringing a chance for
afternoon and evening thunderstorms to most areas east of Phoenix.
There will initially be a slight chance for storms into the greater
Phoenix area Wednesday but afterward moisture will retreat slightly
to the east, focusing any chance for storms into the higher terrain
areas east of Phoenix Thursday into the weekend. Minimum RH values
will stay mostly in the mid to upper teens Friday into the weekend,
with values over 30 percent in the higher terrain. As the upper low
weakens this weekend, high temperature climb to around seasonal
normals with warmer deserts into the upper 90s. No strong winds are
expected during the five day period, just some minor and typical
afternoon gustiness favoring north to upslope.


Spotters should follow standard reporting procedures.




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