Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 222045

Area Forecast Discussion
National Weather Service Phoenix AZ
145 PM MST Fri Sep 22 2017


A low pressure system and cold front that is now sweeping through
the southwest will bring substantially cooler temperatures across
the region this weekend. Occasionally breezy conditions will also be
common across the area. Temperatures will only slowly warm back to
near average next week.



...Tonight through Sunday...

The cold upper low center, that has been well-advertised by the
global modal suites over the last several days, is now making its
presence felt across our cwa today. For example, The current (1 pm)
temperature at Sky Harbor has only risen to 85F, which is a full 8
degrees cooler than what was observed at the same time yesterday.
Further upstream, near the upper low center, temperatures have been
cold enough to produce snow across parts of northern NV. Although we
are not expecting those kind of temperature extremes across AZ, the
continued slow progression of this upper low center across the Great
Basin will keep enough cold air in place to keep temperatures well
below normal through the weekend. Current 850mb temperatures in the
13-15C range and 700mb temps in the 2-4C range (which is over 3SD
below normal for this time of year) are expected to be maintained
across the region through Sunday. As a result, lower desert high
temperatures will likely be kept in the 80s at many locations on
Saturday and Sunday. This cool airmass, combined with clear skies
and low dewpoints (20`s and 30`s) will also allow temperatures to
drop smartly each night, with lows in the 50`s likely at many
outlying lower desert locations. Afternoon breeziness can also be
expected each day as the very cool air aloft helps to mix stronger
winds aloft down to the surface during the warmest part of the day.

...Monday through next Thursday...

The overall troughing pattern will be maintained over the SW Conus
through at least the middle of next week as eastern Conus ridging
stubbornly holds while east Pacific ridging becoming amplified
(hence forcing lower heights over the Southwest). Heights will
slowly rise as filling of the circulation center takes place
corresponding to sfc temperatures moderating closer to average.
However, ensemble and operational spread has maintained an
increasing dispersion beyond Wednesday hinging on the lingering
trough becoming either progressive or cutoff and stationary. The 00Z
operational ECMWF and its cutoff solution has some ensemble support
in drawing moisture westward into eastern Arizona during the latter
half of next week, however a majority of ensemble members lean more
towards a GFS/GEM idea of slow progression and persistent dry
weather. With only limited confidence, have stuck with the majority
ensemble output next week advertising continued dry, but
increasingly warmer weather.


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

An upper low center tracking across the Great Basin is expected to
push rather dry air into the region through the taf period, with the
current SCT mid/high cloud layers dropping into the CLR-FEW range by
late tonight/Saturday. Winds to mainly follow typical diurnal
trends, but with westerly winds lingering a bit later than usual
into the late evening hours. Some afternoon gustiness can be
expected as well.

Southeast California/Southwest Arizona including KIPL and KBLH:

A dry airmass to keep skies mainly CLR through the taf period. Winds
to remain mainly from a westerly to northwesterly direction at both
SE CA taf sites as well.

Aviation Discussion not updated for amended TAFs.



Monday through Friday: A low pressure system will linger over the
interior West through Tuesday before beginning to shift eastward.
Temperatures will climb to near or slightly above normal by Thursday
and Friday. Humidities will be on the low side with minimum readings
falling into the 10-25 percent range Monday through Thursday, with
fair-good overnight recoveries. Some increase in humidities is
possible by next Friday. Anticipate north and northwesterly
breeziness Monday and Tuesday over southeast CA and southwest AZ.
Otherwise, light winds.


Spotters should follow standard reporting procedures.





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