Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 242138

Area Forecast Discussion
National Weather Service Phoenix AZ
238 PM MST Sun Sep 24 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


An anomalously strong low pressure system across UT is producing
a cool and dry northwesterly flow across the Desert Southwest.
Temperatures across the area are generally a degree or two warmer
than those observed 24 hours ago, while dewpoints are running a
few degrees lower. Below normal temperatures are again expected
overnight under clear skies with a few spots outside of Phoenix
again flirting with lows below 50 degrees.

The aforementioned low pressure system will weaken overnight while
lifting northeastward. Another weak vort max will dive through the
broad western CONUS trough Monday, but will only result in some
high clouds during the day. Despite the presence of the vort max,
latest guidance indicates that there will be a net gain in mid-
level heights, translating into a few degrees of warming.


Model trends continue to indicate that a jet streak topping an
amplifying east Pacific ridge will dive into the mean western
Conus trough and induce a strengthening vorticity maximum and
deepening area of low pressure across the Desert Southwest
Wednesday and Thursday. This evolution will promote lower heights
cutting off from the northern stream and a temporary rex block
across the Rockies. As a result, operational models and their
corresponding ensembles still exhibit a large amount of
variability with respect to the placement of the low and
subsequent moisture transport.

Given the apparent source region of the moisture from the east side
of the continental divide and current expanse of extremely dry air
residing in the southwest, skepticism is rather high that deep
quality moisture supporting storms outside of mountain areas will
truly come to fruition. More robust outflows may be needed to mix
moisture through the boundary layer, and current forecast soundings
are not terribly optimistic about this outcome. Regardless, the
overall pattern is supportive for convection during the latter half
of the week though could not get too aggressive with POPs given all
these uncertainties. While both the 00Z operational GFS and ECMWF
eject the closed low eastward by the weekend, ensemble members
exhibit a much larger spread in outcomes. Historically, blocking
patterns are not modeled very well so confidence is somewhat low
towards the end of the forecast period though it does certainly look
like much warmer temperatures (a return to 100F) are on the horizon
for the weekend and beyond.


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

A very dry airmass to keep skies mainly CLR through the entire taf
period at both the Phx area and SE CA terminals. Sfc high pressure
over the Great Basin to keep gusty northerly winds going at KBLH
through Monday. Winds at KIPL to remain mainly out of a westerly
to northwesterly direction through Monday as well. Winds to mainly
follow typical diurnal trends at the Phx area terminals.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday:
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. Daytime humidities will
increase back into double digit readings by Wednesday, and north
of 15 percent for most locales by Thursday. 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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