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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 211934

Area Forecast Discussion
National Weather Service Phoenix AZ
1234 PM MST Mon Aug 21 2017

A decrease in monsoon moisture today will limit showers and
thunderstorms to mainly the higher terrain of Gila County through
Tuesday. 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 already starting Thursday night while
temperatures gradually warm into the weekend.


The cut-off low which is centered just off the southern California
coast continues to bring south to southwesterly flow across
southeast California and southern Arizona. Precipitable water
values (PWATs) across the area vary between 0.9" across southeast
California and far east-central Arizona to just over 1.3" in a
swath between Yuma and Phoenix up through Las Vegas. This moist
swath in combination with modest dynamical forcing continues to
provide support for scattered showers and thunderstorms across
Yuma and La Paz county. This activity may linger some into the
evening hours, but the overnight hours should be dry in most areas
as further drying occurs. Weaker support aloft and PWATs closer
to one inch on Tuesday should only allow for a few isolated
showers and thunderstorms across far eastern 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:

Minimal aviation impacts through Tuesday morning as thunderstorm
activity will remain well removed from the terminal airspace and
clouds stay above 10K ft. There is a slight chance an outflow from
the Rim briefly kicks up a northerly 25 kt gust late this evening,
but chances and temporal duration preclude any mention in this TAF

Southeast California/Southwest Arizona Including KIPL and KBLH:

A few isold SHRA/TSRA will linger around KBLH this afternoon, but
lift into NW Arizona by this evening leaving little aviation
concerns. FEW-SCT Accas decks will be possible overnight, and sfc
winds should favor a S/SW direction into Tuesday morning.

Aviation discussion not updated for amended TAFs.


Thursday through Monday:
Thunderstorm activity will wane through the end of the week with
any lingering storms focused only over the higher terrain of Gila
County. More expansive thunderstorms may be possible after Sunday
next week. As the atmosphere dries and warms, afternoon minimum
humidity levels will hover in a 15-25% range with fair to good
overnight recovery. Wind will maintain a typically weak summer
speed with nothing unusual expected through the weekend.


Spotters should follow standard reporting procedures.




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Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
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Some may differ only in time posted.)

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