Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 222054
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
154 PM MST Tue Aug 22 2017


.SYNOPSIS...
A decrease in monsoon moisture will confine any chance of thunderstorms
to far eastern Gila County today. It is not until Wednesday that
an increase in moisture will allow storm chances to return to the
valley. Another drying trend starts to lower chances Thursday
evening while temperatures gradually warm into the weekend.

&&

.DISCUSSION...

The GOES-16 experimental visible channel shows mostly clear skies
over south-central Arizona with some cumulus building over the
higher terrain in northern and eastern Arizona. Chances for rain
in the lower desert this afternoon and tonight are quite slim.
Arizona is currently sandwiched between an upper low off the
California coast and a high pressure ridge over Chihuahua, both
of which are allowing for dry southwesterly winds to spread over
the area. As a result, the sounding this morning indicated just
1.14 inches of precipitable water, and with additional drying
today, we should see even lower PW values and hardly any CAPE.
Some chance for storms will remain over Gila County and other
higher terrain features where upslope flow will be prevalent.

The best chances for storms overall will occur later Wednesday
and into early Thursday as the aforementioned high migrates a bit
further north allowing a more southerly flow to advect additional
moisture into the region. Meanwhile, the California low also
moves inland which will allow for favorable atmospheric dynamics
to spread over Arizona. The increase in moisture, instability, and
upslope flow will permit for even greater storm initiation over
the higher terrain when compared to Tuesday. However, Phoenix and
other lower desert locations may only see a 10 to 15 percent
chance for storms with hardly any chance over southeast
California. Low storm chances will linger through Thursday.

As the upper level trough moves through, another reinforcing shot
of dry air comes marching in Thursday afternoon and will
effectively shut off all convection in our CWA by late Thursday
evening or early Friday morning. Models advertise that these dry
conditions will largely persist through the weekend as an
expansive ridge of high pressure builds over the west coast. While
large, some of our best verifying guidance suggests highs may
only top out in the upper 100s Saturday and Sunday which is just a
few degrees above normal for this time of year.

Beyond this, the flow becomes more convoluted and various
solutions being to present themselves. The GFS and ECMWF show that
by early next week the ridge may position itself far enough north
to bring a moistening easterly flow to our area. If this comes to
fruition, an uptick in moisture and monsoon activity could be
realized. Given how far out this is, have gone with PoPs
closer to climatology.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:

No aviation concerns expected during the TAF period as drier
southwest flow aloft keeps any thunderstorm chances confined well to
the east of Phoenix. FEW to SCT mid-level cloud decks with bases aoa
10kft. Winds will follow typical diurnal tendencies with speeds
mostly below 10kt.

Southeast California/Southwest Arizona Including KIPL and KBLH:

No aviation concerns during the TAF period as drier southwest flow
aloft dominates the weather producing mostly clear skies. Winds will
be on the light side, favoring the south at KBLH and the southwest
at KIPL during the afternoon. Due to the light nature of the winds,
variable wind reports can be expected from both sites.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday:
A break in the Monsoon continues over the forecast area through
Wednesday with negligible storm chances. Overnight recoveries will
remain fair to good. Storm chances return to the higher terrain of
south-central AZ on Thursday with a gradual upward trend from east
to west through Sunday. Humidities will trend upward in a similar
fashion. Apart from thunderstorms, winds will tend to follow
familiar warm season patterns. In addition, temperatures will be on
the increase through Friday, with highs near 110 degrees across the
lower deserts.
&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

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DISCUSSION...Deemer/Wilson
AVIATION...Hernandez
FIRE WEATHER...Hernandez



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