Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 251502

Area Forecast Discussion
National Weather Service Phoenix AZ
800 AM MST SUN SEP 25 2016

Quiet weather with dry air and warmer temperatures will dominate the
region today. A westward moving upper low will then push increasing
amounts of cloudiness and moisture into the region from the south
from Monday into Thursday, with a chance of showers and
thunderstorms each day. Drier weather will return from Friday onward
into the weekend.


Dry northerly flow at all atmospheric levels will continue today,
resulting in mostly clear skies across our forecast area, southeast
CA to south central AZ. It may a little breezy at times, especially
tonight and Monday morning as a large low pressure system develops
over northwest Mexico tonight, tightening the regional pressure
gradient. Increasing clouds are expected area-wide on Monday,
becoming solid overcast at times, then leading to an increasing
threat of showers Monday night and Tuesday. This covered in current
forecasts. No updates planned.

.Previous Discussion...519 AM MST...


We are still expecting one more day of dry weather across the
region, under brisk northerly the northeasterly flow aloft, as an
upper low becomes pitched off and drops southward across extreme SE
AZ into NW Mexico. These northerly winds will continue to bring more
dry air into the region, keeping skies mainly clear. Temperatures
today are expected to rise further (into the mid-upper 90s across
the lower deserts), as a combination of warming aloft and local low-
level downslope flow helps to warm things up.

Monday through Thursday...

A change to a more unsettled pattern is expected to occur during
this period, as the main upper low continues to sink southward to a
position over central Baja by Monday afternoon, shift back northward
into western AZ on Wednesday, then into UT on Wednesday
night/Thursday. Deep easterly to southeasterly flow on the north
side of the upper low center is expected to pull subtropical
moisture back northward into the region on Monday and Tuesday before
some drying occurs on Wednesday as the flow aloft becomes
southwesterly. Although moisture/instability levels are not expected
to become that great (PWATS in the 1.00-1.25 inch range with a few
hundred joules of CAPE), it still should be enough to trigger
isolated-scattered showers and thunderstorms over the region, with
the best chances for measurable rainfall being on Tuesday afternoon
across south-central AZ as the main trof axis moves across the

Another moisture surge is possible again on Thursday,as the
operational GFS and EURO are showing moisture from a decaying
tropical cyclone moving into the region. However, there is quite a
bit of spread in the movement of this cyclone in the emsemble
members, so only slight chances for showers/thunderstorms are being
put into the grids at this time.  Current WPC QPF guidance for
rainfall for the entire period is actually quite bearish, keeping
total amounts aob 0.50 inch at even the wettest locations north and
east of Phoenix, with most lower desert locations seeing next-to-
nothing. The clouds and rainfall are expected to push temperatures
back down on Monday and Tuesday across south-central AZ, into the
upper 80-low 90 range. Warming is then expected on Wednesday and
Thursday as the upper low lifts off to the north.

Friday through Sunday...

A drying trend is expected during this period, as increasing
westerly flow aloft pushes drier air back into the region.



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

Main aviation weather impact today will be the wind. Somewhat
variable winds early this morning will contain more of a northerly
component before veering to the east-northeast around 15z. Occasional
gusts as high as 25 kt will be possible through mid-afternoon. Winds
will then subside during the early evening. Directions during the
late afternoon and early evening remain more uncertain, though a
return to strong easterly flow is anticipated after 06z.

Southeast California/Southwest Arizona including KIPL and KBLH:

Stronger than normal northerly component to the flow will persist at
KIPL/KBLH. Gusts as high as 25 to 30 kt will be possible at KBLH
during the afternoon, but somewhat lighter winds are expected519 AM
MST at KIPL. Winds will subside during the evening, but remain
stronger than normal.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday:

A low pressure system across southwestern Arizona will again generate breezy
conditions Tuesday, particularly across Arizona. The low will also
bring an increase in moisture that will persist through late week.
Higher than normal rain chances are expected during this period,
however any rain that falls will likely not be heavy. Drier
conditions are anticipated Friday and through the weekend as
southwesterly flow develops ahead of an area of low pressure in the
Pacific Northwest.


Skywarn radio Net activation will not be needed this weekend.




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