Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 240959
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
300 AM MST Tue Oct 24 2017

.SYNOPSIS...
Well above normal temperatures will persist Tuesday as strong high
pressure prevails across the western states. This high pressure
system will slowly weaken through the rest of the week, resulting in
a gradual cooling trend, though temperatures will continue to remain
above normal. Dry conditions will also persist with no chance of
rain through the end of the month.

&&

.DISCUSSION...

.Today and Tonight...

A rather balmy morning (for late October) across the region this
morning, under clear skies and very strong ridging aloft. The
combination of strong subsidence aloft and locally gusty northerly
winds mixing warmer air down to the sfc has resulted in some
locations in the Lower Colorado River Valley and at foothill
locations in South-Central AZ seeing temperatures in the 80s at this
hour. More sheltered lower desert locations, in contrast, are seeing
temperatures in the 60s and low 70s. Latest global model output
continues to show 500 mb heights remaining in the 591-594dm range
this morning across the region, which is at near-record values from
this time of year. The combination of this very warm air aloft and
downslope easterly flow near the surface will once again allow
temperatures to rise well up into the 90s at many lower desert
locations, with a few spots perhaps even reaching 100 degrees.
However, the very dry airmass (dewpoints in the 20s and 30s) and
some afternoon breeziness will keep things from feeling quite as hot
as it could be.

.Wednesday through next Tuesday...

Model guidance (both the operational GFS/EURO and their respective
ensemble members) continues to remain in close agreement with the
last few model runs. They all continue to show deep long-wave
trofing becoming established east of the Rockies as the main long-
wave ridge axis shifts a bit westward to a position along/just off
the west coast of North America, as a strong typhoon recurving to
the northeast just of the coast of Japan helps to establish very
deep long-wave trofing over the Bering Sea/north-central Pacific.
This will allow broad northerly flow aloft to develop southward from
western Canada into the northern/central Rockies over the next
several days, which will pull much colder air into the region.
Although the lion`s-share of the cold air being pulled southward
into the central/eastern CONUS will remain well to our west and
north, enough cooling aloft is expected to occur to allow
temperatures aloft to slowly drop over the next several days, with
lower desert highs perhaps finally falling into the upper 80s by
early next week, which is still above normal for this time of year.
Along with the slowly cooling trend, very dry conditions and mainly
clear skies are expected to last into at least the early part of
next week, with virtually no chance for rainfall through the rest of
October.

&&

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

Main aviation impact will be increasing low level northeast winds
across the greater Phoenix area overnight tonight and into the day
Tuesday as surface gradients along with 850/925mb winds increase.
Expect winds around 2k feet to be around 30kt from the northeast by
sunrise Tuesday; much of that will mix to the ground by mid morning
allowing for gusty northeast winds with gusts over 20kt likely at
the TAF sites. Expect east winds to persist pretty much all
afternoon tomorrow with west winds developing very late or not at
all.

For Phoenix; added in low level wind shear from 11z to 16z; the
strength of the low level northeast wind combined with particular
orientation of the surface gradient will likely result in surface
winds at KPHX becoming light southwest or near calm by 10-11Z
resulting in the LLWS. After mixing begins mid morning the shear
will dissipate as gusty northeast winds develop at the surface.

Otherwise skies to be clear at the TAF sites next 24 hours at least.

Southeast California/Southwest Arizona including KIPL and KBLH:

Main issue will be modest north-south surface gradient along with
brisk northerly low level winds over the deserts resulting in
locally breezy north winds down the lower CO river valley next 24
hours. Expect winds to stay genly at or above 10kt at KBLH overnight
into mid morning Tuesday then becoming gusty by late morning/early
afternoon. May see a few gusts overnight but took that out of the
TAF due to lack of occurrence over the past few hours. Less wind
expected at KIPL; wind to favor the west overnight then becoming
north 10-15kt by late morning. Skies to be clear next 24 hours.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Thursday through Monday:

Somewhat cooler temperatures will descend into the districts for the
second half of the week though readings will still remain several
degrees above average. The airmass will remain very dry ensuring
clear skies and no chance of rain. This will also lead to afternoon
minimum humidity levels falling into a 10-15% range following only
fair overnight recovery. After some gusty winds Wednesday, wind
speeds should not pose any threat towards enhanced fire behavior the
remainder of the week.

&&

.CLIMATE...
PHOENIX    NormalRecord
Mon Oct 2386100 in 2003
Tue Oct 248596 in 2014
Wed Oct 25      85          96 in 2014

YUMA     NormalRecord
Mon Oct 23      87          102 in 1959
Tue Oct 24      87          105 in 1959
Wed Oct 25      87          100 in 1959

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

$$

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DISCUSSION...Percha
AVIATION...CB
FIRE WEATHER...MO
CLIMATE...Nolte/MO



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