Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 252125

Area Forecast Discussion
National Weather Service Phoenix AZ
225 PM MST SUN SEP 25 2016

A developing low pressure system over northwest Mexico and the Baja
Peninsula, will slowly move north toward southwest Arizona by later
Monday. As a result, increasing east winds will develop across most
areas late tonight, then becoming mostly cloudy Monday and Tuesday
with a slight chance of showers. Partly cloudy skies are forecast
Wednesday and Thursday with shower threats confined to northern and
eastern Arizona. Afternoon temperatures this week will fall some 5
to 7 degrees below normal. Clear skies and normal temperatures will
return by next weekend.



Tonight through Wednesday...

Dry northerly flow at all atmospheric levels will continue through
late tonight, resulting in mostly clear skies across our forecast
area, southeast CA to south central AZ. Increasing easterly breezes
are expected across most areas later tonight through Monday as a
large low pressure system develops over northwest Mexico, tightening
the regional pressure gradient.

Models are still resolute in moving the developing northwest
Mexico/central Baja CA low pressure system north into the lower
Colorado River Valley by Tuesday afternoon, with remnants of the low
near Las Vegas by Wednesday evening.

As a result, as the Mexican weather system approaches, increasing
clouds are expected area-wide Monday morning, becoming solidly
overcast during the afternoon, and leading to a few sprinkles or
light showers by late in the the day. Some light wetting rain is
possible over the higher elevations of southern Gila County late
Monday afternoon. Ditto Monday night, however the slight chance of
showers will spread to portions of southeast CA and southwest AZ
by late Monday night.

The threat of showers over our forecast area Tuesday is complicated
by a couple of factors. The vertical structure of the upper low, as
it wobbles closer southwest AZ, becomes negatively tilted. In other
words the 500 mb low center by late Tuesday afternoon is forecast to
be centered in Mexico 147 miles southeast of Yuma, but the 300/250
mb centers 30 miles west of Phoenix. Negatively tilted trofs usually
provide little upper level forcing, however at the same time 500 mb
temperatures over the area drop to near minus 13 deg C, certainly an
unstable profile given desert and mountain temps from the lower 80s
to lower 90s. Also, there is still an uncertain amount of boundary
layer moisture expected from Mexico. Current modeled soundings for
late afternoon/early evening on Tuesday vicinity Phoenix and in the
mountains around Globe show a convective trigger temp of 90 deg F
(near aftn highs) and 78 deg F (few deg lower than fcst highs)
respectively. Therefore a slight chance of showers and thunderstorms
will continue across the forecast area Tuesday and Tuesday evening.

On Wednesday afternoon, the upper low finally becomes more
barotrophic near Las Vegas. 850/700 mb winds across back to a
southerly direction, importing more Mexican moisture into eastern AZ.
Relatively warm afternoon temperatures and slightly more difluent
southwesterly flow aloft provide an environment for a continued risk
of afternoon and evening thunderstorms, mainly from Phoenix north
through east, particularly mountains.


Increasing southwesterly flow aloft will advect varying amounts of
mid level moisture into the area for mostly cloudy skies.
A slight chance of afternoon thunderstorms will be confined to the
mountains of southern Gila County east of Phoenix.  Afternoon
temperatures will should approach normal values on the deserts, i.e
mid to upper 90s.

Friday through Sunday...

Mostly clear and seasonably warm weather is expected.



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

Intermittent breezy conditions are expected with northeasterly winds
occasionally gusting to 20 kt. Winds will subside during the early
evening. A return to strong easterly flow is anticipated after 06z.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds up to 25 to 30 kt are possible until 01Z, especially for
KBLH. Winds will subside during the evening, but remain stronger
than normal.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday:

A low pressure system across southwestern Arizona will generate
breezy conditions through Tuesday, particularly across Arizona. The
low will also bring an increase in moisture that will persist
through late week. Slight 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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