Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 211228 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
530 AM MST Mon Aug 21 2017

.UPDATE...Updated aviation and fire weather discussions.


A decrease in monsoon moisture today will limit convection to
mainly the higher terrain of Gila County through Tuesday. It is
not until Wednesday and an increase in moisture will storm chances
return to the valley. Another drying trend lowers chances by
Thursday while temperatures gradually warm into the weekend.



Current radar shows a few showers across western Arizona and
southeast California this morning although little if any of this
rainfall is likely reaching the ground thanks to relatively dry
conditions at the surface. Simulated soundings at KBLH this
morning shows a saturated layer around 12kft with just a bit of
CAPE (~100J/kg) above this level. Because of this, not really
expecting these showers to amount to much nor continue for much

Looking at the large scale pattern, a 500 mb low is off to our
west and is centered just off the coast of Southern California.
Models are in excellent agreement showing this low expanding
eastward turning our easterly flow to the southwest today. This
will advect drier air into our CWA this afternoon further
limiting the already present low grade monsoon conditions.
However, some storms may spark this afternoon over terrain favored
areas but given the lack of CAPE, they should be rather weak.
This SW flow will continue into Tuesday keeping the few storms
that do develop confined to the higher elevations of Gila County.

The trough will continue to gradually advance eastward all while
a high pressure center over northern Mexico will rotate up
through our area on Wednesday. While there are still some
disagreements between models, this will act to turn our flow more
southeasterly and pump a bit more moisture into our area. The GEFS
ensemble shows about a 0.25 inch bump in PWs, a value hard to get
excited about but enough to mention a slight chance of storms in
the valley. Furthermore, with the trough and a PV anomaly scooting
by to our north, weak convection could last well into Thursday
morning given the upper level support.

Behind the trough, drier air looks to punch into our CWA again
limiting convection to the higher terrain of Gila County. All the
while, the GFS and ECMWF agree that a rather large ridge will
begin forming over the west coast starting Friday and lasting at
least through the weekend. At this point, it doesn`t look
unbearably strong as lower desert locations may see temperatures
warm into the upper 100s. Certainly a few degrees above average
but nothing out of the climatological norm.


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

Variable amounts of mid cloud will linger through the morning hours
with SCT to BKN decks mostly aoa 12k feet. A few light showers or
sprinkles could occur but confidence too low to mention in the TAFs.
Expect clearing by late morning to early afternoon and after that,
storm chances for tonight are very meager. Do not expect storms or
outflow winds to affect the TAF sites tonight and cloud decks will
be mostly of the scattered variety. Expect east/southeast winds this
morning to turn to the west after 20Z and then become light variable
or light southeast after about 06Z.

Little or no thunderstorm chances in the Phoenix area Tuesday
afternoon/evening but a weather disturbance will bring elevated
chances on Wednesday night.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect mostly clear skies next 24 hours at KIPL with winds favoring
the southeast today and the southwest after around 03Z tonight. SCT
to BKN mid cloud decks will linger into mid morning at KBLH with
bases mostly aoa 12k feet. Isolated light showers possible as well.
Expect some clearing by afternoon with skies becoming mostly clear
this evening. Winds to favor the south next 24 hours at KBLH mostly
below 12kt.

Aviation discussion not updated for amended TAFs.


Wednesday through Sunday:
A moderate amount of monsoon moisture will persist into Thursday
across the eastern half of Arizona leading to a continued chance
of showers and thunderstorms. A drying trend starting Friday or
Saturday will lead to only slight chances of high terrain storms
through the weekend. Minimum humidities will hover around 15-20%
in the lower deserts while the higher terrain of Gila County will be
much more favorable, around 30-40%. High temperatures will stay near
seasonal normals through Thursday but they will begin to warm Friday
into the weekend as high pressure strengthens overhead. Hotter
western deserts could approach 110 this weekend.


Spotters should follow standard reporting procedures.




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