Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 162126 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
225 PM MST Mon Oct 16 2017

.UPDATE...Updated aviation and fire weather discussions.


High pressure will persist across the region through the middle of
the week allowing for continued very warm and dry conditions. High
temperatures for lower desert communities will generally reach
into the middle and upper 90s. Somewhat cooler weather with more
cloud cover will arrive during the end of the week. High pressure
returns quickly by the end of the weekend, sending temperatures
back to above normal readings.



.Tonight through Tuesday Night...

Continued dry conditions and above normal temperatures through this
period, but with the beginnings of a modest cooling trend, as the
very strong ridging that is now in place over the region is
flattened by a series of strong shortwaves that are forecast to
moving into the Pac NW/Northern Rockies. Highs on Tuesday are
expected to be a degree or so cooler than today as 500mb heights
fall to around 588dm.

.Wednesday through Thursday morning...

A bit of a change is expected during this period as a weak trof
moving into the region interacts with moisture streaming northward
from the remains of a tropical disturbance that is currently located
about 900 miles west of the southern tip of Baja California.
Although some decent moisture is forecast by all of the global
models to move into our region (PWATS in the 0.75-.00 inch range),
model soundings are indicating that the vast majority of the
moisture will remain aoa 600mb. This high-level moisture combined
with very warm air aloft (500mb temps in the -8C to -10C range) is
expected to keep impacts limited to just an increase in mid/high
level cloudiness and perhaps some virga/sprinkles. The increase in
cloudiness will likely also lower daytime highs a bit, with lower
desert highs remaining in the mid/upper 90s. The increase in sfc
dewpoints and cloudiness will likely keep overnight lows elevated,
with many urban locations seeing lows in the 70s.

.Thursday night through next Monday...

Heights will continue to lower for the latter half of the week and
into the early weekend as deep troughing expands into much of the
western CONUS by Friday. Model agreements in timing remain with
minor differences in strength. Moisture parameters are fairly
underwhelming, with drier air quickly advecting into the area
during the day Friday. Best pocket for height falls and jet
influenced ascent still remain to our north so PoPs are very low.
Non-zero to account for trough passage but low enough to not
generate any weather. What will be noticeable through the region,
outside of a slight cool down in temperatures, will be increased
winds for Friday and to a lesser extent Saturday afternoons. Gusts
around 20mph will be possible over the lower AZ desert locales
while spots downwind of the Peninsular Ranges in southeast CA
could see gusts nearing 30mph at peak on Friday. While timing is
still a ways out, could not rule near advisory conditions in the
SW corner of Imperial County sometime late Thursday PM into
Friday, as winds through the gap/pass respond well to trough
passages. Temperatures dip closer towards seasonal normals for the
weekend, but the cool down will be short lived.

Deterministic and ensemble members indicate strong ridging will
build back into the region, stronger than the ridge we currently
are under to start this week. Regional 500mb heights head towards
594dm at the end of available model forecasts, which could mean
record warmth would open the last week of October for the
Southwest Deserts.


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

Lingering NE to SW oriented surface gradient along with modest low
level easterly flow will result in winds persisting from the east
through late afternoon at the terminals, with a few gusts into the
teens in the Phoenix area. Winds will have a several hour period
where they attempt to swing to the north or northwest into the early
evening but speeds will be mostly below 8kt. Winds should return to
the east around or shortly after midnight, then stay there thru the
morning on Tuesday. Variable amounts of high clouds expected, mostly
FEW to SCT decks with bases aoa 20k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect rather light winds next 24 hours over the western deserts as
low level gradients relax. Lots of light/variable winds at KBLH
otherwise they will favor the north to northeast below 10kt. Winds
may favor the west overnight at KIPL but otherwise they will be
rather light and variable as well. Some high clouds at times will
move across the terminals, mainly in the form of FEW to SCT decks.
No aviation concerns next 24 hours at least.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:

Weak disturbance moving through generally westerly flow aloft may
produce a few light showers on Thursday, but for the most part high
temperatures will stay well above normal, and minimum RH values will
be on the low side, between 10 and 15 percent. Locally breezy
southwest winds are expected during the afternoon hours. After that,
Friday into the weekend a large area of low pressure will push
across the great basin bringing a cooling trend with the coolest day
likely to be Saturday. Desert highs by Saturday will fall into the
upper 80s to low 90s and minimum RH values will stay mostly in the
teens. It will be breezy again Friday into Saturday, especially in
far southeast California, with winds favoring the west Friday then
turning more to the north on Saturday. Strong high pressure aloft
will build back into the area from the west Sunday into Monday
bringing another warming trend, with the warmest lower deserts
climbing back into the mid 90s by Monday. Drier air will accompany
the warming; by Monday minimum RH values will dive back into the
single digits over the deserts. Less wind is expected both Sunday
and Monday as well.

Extended period forecasts are coming together such that a much warmer
solution is looking more likely with strong ridging returning to
the region. This could result in another stretch of above normal
temperatures, with a shot at triple-digit readings not out of the
question by the end of the forecast period.



Mon Oct 169989101 in 1991
Tue Oct 179788102 in 2009

YUMA     ForecastNormalRecord
Mon Oct 16 9890106 in 1958
Tue Oct 17 100 90 104 in 1959


Spotters should follow standard reporting procedures.





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