Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 242131

Area Forecast Discussion
National Weather Service Phoenix AZ
231 PM MST MON OCT 24 2016

A low pressure system northwest of the region continues to draw a
considerable amount of subtropical moisture into the Desert
Southwest. This will lead to a threat of isolated showers and
thunderstorms through tonight. Drier and warmer conditions will
return by the middle of the week.


Latest MRMS shows isolated showers and thunderstorms across northern
Arizona struggling to backbuild southwestward into the deeper
moisture (PWATs > 1.0 inch). Meanwhile, an anomalous jet streak (95th
percentile) across the Baja Peninsula precedes a southern-stream
short-wave trough further west in the Eastern Pacific. Mostly cloudy
skies have persisted throughout the day across much of the Desert
Southwest which has limited diabatic heating and subsequent

Conditions will become more favorable for precipitation to develop
this afternoon and this evening as the low pressure system tracks
eastward. Best chance for showers and even a thunderstorm will be
later this afternoon and this evening across La Paz, northern
Maricopa and southern Gila County where favorable upslope flow from
the approaching jet and vorticity forced ascent ahead of the trough
are maximized. Hi-res WRFs including the latest runs of the HRRR
indicate that any rain that develops will be quite light. Low-
reflectivity depiction in the models also suggests sprinkles are
certainly possible as well. With instability lacking, the chance for
a thunderstorm will be slim, however given the cooler temperatures
aloft, negative LIs suggest the potential for an isolated in-cloud



Much drier air aloft will overtake the region from west to east
starting late this evening as the jet max shifts into New Mexico.
Low level moisture will remain over most of Arizona into Tuesday
which may give rise to a few showers or maybe a thunderstorm over the
higher terrain of east-central Arizona. Flow aloft turns out of the
northwest as upper level ridging pushes northward out of Mexico.
500mb heights rebound quickly with 585dm heights already over the
area by Tuesday afternoon. This will lead to another warming trend
for the middle of this week. Highs may approach record levels in some
areas with readings likely topping out into the 90-95 degree range
starting Wednesday. This upper level ridge will get displaced
slightly to the east late Thursday into Friday as a strong upper
level low moves east northeastward toward the central California
coast. Much of the moisture with this system will remain over
California with only slight chances of showers over southeast
California. The displaced upper level ridge is forecast to remain
just to our east through the coming weekend keeping temperatures well
above normal with little to no chance of any rain.


.AVIATION...South-Central Arizona including KPHX, KIWA, and KSDL:
Southeast California/Southwest Arizona including KIPL and KBLH:

Moisture from a pacific weather system that is passing to the west
and north of the region to keep mid and high level cloud decks over
the terminals through this evening, with skies clearing later
tonight and Tuesday, as drier air begins to move back into the
region from the west and north. Along with the cloudiness this
evening, a few showers and thunderstorms are also expected to
develop, especially over south-central AZ, but coverage/confidence
is too low at this time to include anything more than VCSH in the
PHX area tafs at this time. Winds to remain mainly on the light
side, 8 kts or less, following typical diurnal trends, with just a
remote chance of brief stronger winds from shower/thunderstorm

Aviation discussion not updated for amended TAFs.


Thursday through Monday...

High pressure redeveloping over the Desert Southwest will bring dry
conditions and well-above normal temperatures to the region on
Thursday. Another weather system brushes the Desert Southwest from
Friday into early Saturday for an increase in humidities and a
slight chance of showers over SE CA. Warmer and drier weather is
expected to return for next Sunday. Yet another pacific weather
system brushing by to the west and north will bring cooler
temperatures and an increase in winds/humidities from Sunday night
into next Monday. Minimum humidities to remain mainly in the 15 to
25 percent range through the entire period, with overnight
recoveries in the good range.


Spotters should follow standard reporting procedures.




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