Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 300947

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

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.


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

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

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.


South-Central Arizona Including KPHX, KIWA, and KSDL:
Very little aviation impacts through Friday evening with timing of
wind shifts the main forecast challenge. Thunderstorm activity
should remain well east of terminals, though distant outflow
boundaries may eventually lead to wind shifting to a more easterly
direction during the late evening hours. Confidence in timing of
wind shifts is only moderate. Few-Sct cloud decks will be common
with only a very remote chance of additional showers within the air
space Friday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation impacts will affect SE CA terminals through Friday
evening under mostly clear skies. A brief period of stronger west
winds at KIPL will weaken overnight. Periods of variable directions
will be common across the area Friday with speeds trending below

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday...
A drier and more stable airmass will spread into the region under
westerly flow aloft next week. Mostly clear skies with seasonably
warm afternoon temperatures will prevail early in the week before
substantially cooler air filters into the districts during the
middle of the week. Minimum relative humidities will drop
to a 10 to 25 percent range early in the week with more solid single
digits and teens later in the week. Good overnight recovery
will become only fair through the week. Afternoon southwest winds 5
to 15 mph with higher gusts will be common, though stronger winds
approaching critical thresholds may be possible Monday afternoon.


Spotter activation will not be needed.





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