Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 301256
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
556 AM MST FRI SEP 30 2016

.UPDATE...
Updated Aviation and Fire Weather sections.

&&

.SYNOPSIS...
Somewhat drier and calmer weather will return to the region today
though residual moisture may allow a few isolated showers or
afternoon thunderstorms to linger over mountainous areas. However,
the weekend will feature seasonally warm temperatures and mostly
clear skies. Continued dry weather will persist next week though a
sharp cooling trend will arrive during the middle of the week as a
strong area of low pressure passes by to our north.

&&

.DISCUSSION...
Early this morning, convection across Arizona had pretty much ended
or shifted east into New Mexico, and IR imagery indicated generally
clear skies over south-central Arizona with partly cloudy skies over
the western deserts and into far SE California. Most of the
shortwave energy that had been sitting to the southwest of AZ and
along the Baja coast has lifted northeastward and passed through our
area thus limiting any dynamical forcing for the rest of the morning
and through the day today. A deep Pacific upper trof continued to
develop along the Pacific Northwest and northern CA coast early this
morning and this feature will continue to drive a brisk southwest
flow aloft into the area today through the weekend. This will lead
to a drying trend and by Saturday chances of rain or storms will
pretty much be over for some time to come for areas from Phoenix
westward.

Despite the southwest flow aloft, lower level moisture remains hard
to really scour out, and at 2 am surface dewpoints over the south
central deserts were mostly in the 50s, running as high as 60 at
Phoenix. 850mb dewpoints from Phoenix east remain at or above 8c an
will stay elevated through the evening today. Thus we still expect
to see isolated showers or afternoon thunderstorms across portions
of south central Arizona today with best chances for storms over the
higher terrain areas to the east of Phoenix. We raised POPs slightly
today, putting the greater Phoenix area under low end slight chance
numbers just above 10 percent...with values reaching around 30
percent over portions of southern Gila County mainly east of Globe.
During the evening today, and following sunset and lack of heating,
rain chances will end in the Phoenix area and diminish to around 10-
15 percent over the high terrain from Globe eastward and we can
expect clear to mostly clear skies across the deserts from Phoenix
westward.

Operational runs from the GFS and ECMWF as well as GEFS ensemble
members all call for the deep Pacific trof to drop southeastward and
inland over the west coast during the course of the weekend, keeping
drier southwest flow in place over the area and resulting in sunny
days and clear nights. Both GFS and ECMWF also suggest that as the
low center moves inland, southerly lower level flow ahead of the
main low will tap a bit of deeper tropical moisture and spread it
into portions of southern Gila County. CAPE appears to be minimal
but there appears to be sufficient moisture and instability to
justify adding a slight chance of thunderstorms to southern Gila
County for Sunday afternoon and evening, mainly over areas from
Globe eastward. POPs will stay low and just above 10 percent in that
area. By early Monday, stronger west/southwest flow ahead of the
approaching deep low will push moisture off to the east and end any
threat of precipitation over our area.

For Monday into Tuesday, guidance is consistent in calling for the
deep upper trof to move progressively inland and pass through the
desert southwest with the main upper low center to stay well north
of Phoenix as it passes through Utah. We will still see significant
height and thickness falls with the trof passage, and as such high
temperatures will fall to well below seasonal normal levels both
days. Desert highs will drop into the 80s with many of the lower
deserts expected to see highs in the low to mid 80s at times. The
coolest day in Phoenix will likely be Tuesday when the high falls to
just 84 degrees; the normal high for the date is 93 degrees. In
addition to the cooling, the trof passage will lead to breezy or
windy conditions Monday during the afternoon hours; winds should
remain below wind advisory levels but may approach critical fire
weather thresholds in some locations.

For Wednesday into Thursday, behind the exiting upper low a drier
northwest flow aloft will spread across the area bringing sunny
days, clear nights along with a warming trend. High temperatures
will stay below normal levels but climb back into the 90s over the
lower deserts by Thursday.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Expect light surface winds today following familiar diurnal
patterns. Enough leftover moisture for a stray thunderstorm this
afternoon (as early as 20Z) over southwest Maricopa County. If that
materializes, southwesterly steering flow would move it toward the
metro before dissipating. Could possibly even see some weak outflow
that makes it into south and east portions of the metro. Expect more
numerous storms over southeast and northern AZ. Otherwise, anticipate
some increasing mid clouds after 16Z with bases AOA FL100 before
clouds dissipate after sunset.

Southeast California/Southwest Arizona including KIPL and KBLH:
Expect light surface winds today following familiar diurnal
patterns (except for very modest "sundowner" winds over and west of
Imperial Valley this evening). Otherwise, anticipate scattered to
locally broken mid clouds with bases AOA FL120.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday...
A cold front will push through southern California later Sunday and
across Arizona on Monday. This will lead to breezy to windy
conditions (especially west of the Colorado River Valley). Drier air
will be swept into the area just behind the front which will lead to
a significant uptick in fire danger. In fact, some locales will
approach critical thresholds in the afternoons on Sunday and Monday.
Before the drier air arrives, there will be a slight chance of
thunderstorms over the higher terrain of south-central Arizona on
Monday. Significantly lighter winds can be expected Tuesday through
Thursday. The cold front will also drop the temperatures to well
below normal levels with highs in the mid 80s on the lower deserts
Monday and Tuesday before slowly warming Wednesday and Friday.
Minimum humidities will be low with fair overnight recovery.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed.

&&

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

$$

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DISCUSSION...CB
AVIATION...AJ
FIRE WEATHER...AJ



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