Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 202159

Area Forecast Discussion
National Weather Service Phoenix AZ
259 PM MST Wed Sep 20 2017

A region of low pressure will overspread the Western states the
latter half of this week. The end result will be a significant
cooling trend Thursday through Saturday along with breezy to windy
conditions...mainly on Thursday. Precipitation chances will be
slight and limited to the higher terrain of Arizona Thursday night
through Saturday. Temperatures will slowly warm next week.


Rest of today through Sunday...
Skies are clear over the Desert Southwest with the exception of
some cumulus over southeast Arizona. Temperatures early this
afternoon are running very close to what they were 24 hours ago
and thus very close to normal for the date. Looking at the bigger
picture, there is a large region of low pressure aloft centered
over southwest Canada. Downstream is a ridge with undercutting
flow over the eastern CONUS. Upstream of the low pressure region
is a ridge being amplified by an upstream low centered near the
western Aleutian islands.

Over the next couple days, the low pressure region will change
from an east-west orientation to more of a north-south orientation
in response to the advancing upstream ridge. In the process,
troughing will develop over the southwestern CONUS and a cold
front will move through the region Thursday night and Friday. For
our area, the system will largely be moisture starved and thus the
main impacts will be wind and cooler temperatures. Winds get
strong ahead of the front over southeast California Thursday and
thus we have issued a Wind Advisory for portions of Imperial and
eastern Riverside Counties (see WWA section below). As for
temperatures, highs on the lower deserts will drop to the mid 80s
to near 90 on Friday. A follow-on short wave adds a little more
cooling Saturday. Due to the lack of moisture, there will only be
low end slight chance PoPs for our easternmost areas Friday into
Saturday (east of Globe). The trough axis begins shifting to the
Rockies on Sunday with little change in temperatures.

Monday through Wednesday...
Troughing will be slow to leave leave the interior West early next
week but model consensus shows a high amplitude ridge building
into the Western U.S by Wednesday which will bring lower desert
high temperatures back well into the 90s. Of note, the GFS wants
to develop a new closed low but this is somewhat of an outlier
(though not totally isolated from all GEFS members). The ECMWF
does not depict this.


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

Clear skies will continue with generally light and diurnal winds.
A few gusts around 15-20 kt are possible this afternoon out of the
southwest, before diminishing after sunset. Otherwise, aviation
concerns are expected to remain minimal.

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday:
Cooler temperatures associated with a seasonably strong low
pressure area over the Great Basin will persist throughout the
weekend, about 10-15 degrees below seasonal normals. A slight
chance of thunderstorms is possible across parts of southern Gila
County, with the remainder of the forecast period dry. Humidities
will fall mainly into the 15-30 percent range with good overnight


Spotters should follow standard reporting procedures.


CA...Wind Advisory from 11 AM Thursday to 1 AM PDT Friday for



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