Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 230220
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
720 PM MST Sat Apr 22 2017

.SYNOPSIS...
High pressure will remain situated over the region through early
next week keeping unsettled weather well north of Arizona. Much
above average temperatures will affect the region over the weekend
as highs flirt with the 100 degree threshold in many desert
communities on Sunday. A gradual cooling trend will begin Monday
lasting through the middle to end of next week.

&&

.DISCUSSION...
Midtropospheric ridging has built over the SW Conus early this
evening as troughing deepens over the Gulf of Alaska/NE Pacific and
amplifies the downstream pattern. KTWC 00Z sounding data sampled H5
heights at 585dm with a -9C temperature corresponding to around the
95th percentile. And not surprisingly, afternoon highs around the
region were nearly 10F above normal. Otherwise, nothing looks
unusually abnormal for the remainder of the night with near picture
perfect late April weather. Any adjustments to the forecast were
minimal to none.

&&

.PREVIOUS DISCUSSION...
/140 PM MST Sat Apr 22 2017/
Tonight through Sunday...
No real change in the forecast thinking for this period. Latest
model output are very similar to the last few runs. They all
continue to forecast well-above normal temperatures through the
period, with heights remaining up in the 579-582dm range and 500mb
temps near -10C. These mid-level temps are aoa the 99th percentile
for this time of year. However, lower-level temps (850mb) continue
to show a bit less of a positive anomaly (90 percentile). Given such
warm air aloft and low-level southerly flow, still expecting mainly
lower desert locations to approach or even reach 100 degrees on
Sunday. As far as the forecast high at Sky Harbor is concerned,
still calling for a high just short of the century mark (99
degrees) given the fact that almost all of the various model suites
are still keeping the forecast high aob 99 degrees. Still, a pop to
100 degrees is not out of the question if there is enough of a
southerly wind on Sunday afternoon. Most locations over SE CA will
likely also have a tough time reaching 100 degrees on Sunday as
well, as an approaching trof begins to flatten the ridge and cool
mid-level temps a bit over that region. This afomentioned trof is
also expected to increase winds over the region, especially over the
extreme SW portion of Imperial County which will likely see winds
approach or hit advisory criteria.

Monday through Thursday...
Operational model guidance including the GFS and ECMWF as well as
GEFS ensemble spaghetti guidance remains consistent in calling for a
gradual cooling trend next week as a large upper trof gradually
settles in over much of the western and central CONUS. There is
uncertainty among a number of the GEFS members with smaller scale
details such as various impulses moving through the deep upper trof,
and GFS and ECMWF operational runs also have their differences but
overall the key ideas remain as such: cooling trend with time,
increasingly breezy/windy conditions each day and dry weather over
the lower deserts with just a slight chance for showers developing
over the higher terrain east of Phoenix starting Wednesday night and
continuing into the end of the work week. If Phoenix does not see
100 on Sunday, it will take quite a few days before the next chance
to reach that mark, probably sometime after the end of the extended
forecast period. As the trof deepens and the airmass cools, high
temps over the central deserts will lower into the low 90s by next
Tuesday and then into the mid to upper 80s by Thursday. A series of
disturbances will move through the deep upper trof and they will
bring periods of high clouds; most of the moisture should stay above
the 500mb level and result in largely thin areas of cirrus allowing
for partly to mostly sunny days for the most part.

Friday through next Saturday...
The global models are now in basic agreement that another upper trof
will likely drop southward into the 4-corners region/central Rockies
during this period. However, a considerable amount of model spread
is seen during this forecast period. For example, the operation GFS
has a very deep upper low digging/developing southward into the 4-
corners region on Friday, then into central NM on Saturday, but the
GFS ensemble mean, the operational EURO, and the GEM all are
depicting a much weaker system. At this point, just keeping temps in
the mid-80-low 90s range across the lower deserts and keeping POPs
mainly aob 10 percent for now.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Briefly gusty winds and timing wind shifts will be the main aviation
concerns through Sunday afternoon under scattered thin high cirrus.
With a weak pressure gradient, the shift back to an easterly wind
should occur at least an hour or two earlier tonight versus
recently.

A couple models indicate an easterly LLJ developing near sunrise
over the Phoenix vicinity with a wind speed of 25-30kt around 1500
ft AGL above a light sfc wind creating some measure of LLWS. Feel
the speeds won`t be quite that strong, but could lead to some sfc
wind gusts immediately after mid/late morning mixing; and a few
hours of gusts near 20kt will be possible. Given the uncertainty and
timing into Sunday morning, have not added these mentions into the
TAF package, but may need a TEMPO mention in future updates.

Southeast California/Southwest Arizona Including KIPL and KBLH:
Increasing winds Sunday will be the primary aviation concern for SE
California terminals under scattered thin high cirrus. A weak
pressure gradient tonight will lead to periods of variable winds,
however a predominant W/SW component will develop Sunday. Gusts near
or over 25kt will be common during the mid/late afternoon.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Temperatures will continue to hold above seasonal averages, but not
at readings as warm as the weekend. After Wednesday, a gradual
cooling trend will settle into the region as mostly dry storm
systems sweep through the Great Basin and down through the
Southwest. Outside of a remote chance at some light showers over
high terrain of Gila County Thursday, widespread dry weather will
cover the districts. Dry afternoons with 5-15% will be common with
overnight humidities varying between mostly fair to good. Active
storm track through the West will keep afternoons breezy for the
district, with upslope west winds and occasional gusts possible
through week`s end. While widespread areas of fire danger due to the
low humidities and winds look unlikely, some localized pockets of
elevated fire danger may be realized Thursday.

&&

.CLIMATE...
First Hundred Degree Day Facts for Phoenix and Yuma:

PHOENIX
Average First 100F (30 yr Normal Period 1981-2010): May 2nd
Earliest (Period of Record 1895-2016): March 26, 1988
Latest first occurrence (1895-2016): June 18, 1913

YUMA
Average First 100F (30 yr Normal Period 1981-2010): April 29th
Earliest (Period of Record 1876-2016): March 12, 1916
Latest first occurrence (1878-2016): June 13, 1971

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed through the middle of next
week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory from 5 PM to 11 PM PDT Sunday for CAZ562.

&&

$$

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


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