Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 292042

Area Forecast Discussion
National Weather Service Phoenix AZ
145 PM MST THU SEP 29 2016


Low pressure and tropical moisture moving northward over the desert
southwest will continue to produce showers and thunderstorms over
the region through tonight. Dry and calmer weather will then return
Friday through the weekend with seasonably warm temperatures.
Continued dry weather with a sharp cooling trend is expected early
next week as a strong area of low pressure passes by to our north.




Another in a series of shortwaves embedded in broad southerly flow
over the region managed to trigger a band of showers and
thunderstorms this morning over south-central and SW AZ as it moved
northward across the region. At this hour, we are in a break between
systems, with most of the significant convective active now moving
off well to our north, into parts of NE AZ. It appears one more
round of convective activity will be affecting our cwa later this
afternoon/evening as a combination of daytime heating, ample low-
level moisture (sfc dewpoints well up into the 60s), and increasing
mid-level shear (bulk shear values in the 30-35 kt range) ahead of
the next shortwave is already beginning to generate new showers and
thunderstorms across SE AZ. Given the high bulk shear values and
observed MLCAPES across SE and Eastern AZ, as well as mid-level
lapse rates in the 6.0-7.0 C/KM range, SPC has placed parts of our
cwa in a marginal risk area for severe thunderstorm development,
mainly for locally severe winds and small hail. Both SPC and the
last several HRRR high-res model runs favors Southern  Gila County
and extreme eastern Pinal county for any severe thunderstorm
development, with much lesser chances across the remainder of south-
central AZ. Along with the shower/thunderstorm chances, there is
once again the risk for blowing dust across the lower elevations of
Maricopa/Pinal Counties as outflows from distant thunderstorms to
the south will likely move into the region.

Gradual drying from the southwest later tonight is expected under
the influence of persistent southwest flow aloft, and dynamics will
gradually dissipate as much of the shortwave energy embedded in the
upper flow will lift to the north and northeast of central Phoenix.
Still, there will be enough lingering moisture and weak UVV fields
to keep scattered showers and possibly isolated thunderstorms going
mostly to the east of Phoenix and across the higher terrain. CAPE
values will be unimpressive later tonight, falling mainly below 300
j/kg so any storms that form should not be overly strong.

Friday through Wednesday...

Expect continued drying from the west Friday, as southwest flow
aloft strengthens as a deep Pacific upper trof continues to develop
along the nrn California and Pacific Northwest coast. Low level
moisture across south central AZ will be somewhat hard to scour out,
despite the increasing west/southwest flow aloft so there will be a
lingering slight chance of showers or storms from Phoenix east on
Friday and this is supported by the NAEFS POPs. High temperatures
will stay near seasonal normals Friday in the central deserts, and
will rise into the upper 90s out west where drier air and ample
sunshine are present.

Over the weekend, no significant changes to the weather pattern are
expected; the deep trof to our west will slowly push toward the
coast and eventually move inland over the west coast by Sunday
afternoon. Southwest flow aloft will persist area-wide keeping sunny
days and clear nights in play from Phoenix west, and high
temperatures will edge downward slightly but remain near seasonal
normals. Low level moisture will still linger in far eastern
Arizona, and the ECMWF suggests that an isolated shower/storm could
be possible east of Globe over the higher terrain, but we will keep
POPs in the single digits east of Phoenix, more along the lines of
the NAEFS numbers.

Monday into Tuesday...both the GFS and the ECMWF agree that the deep
upper trof will push east through the western CONUS with the main
center moving well to our north and across Utah. Strong westerly
flow aloft will affect southern Arizona, keeping conditions dry and
breezy and as heights fall with the trof passage, high temperatures
will fall substantially. Highs over the deserts will drop into the
80s both days with the coolest day in Phoenix expected to be Tuesday
with a high of just 84 degrees for Phoenix Sky Harbor. Normal high
in Phoenix on Tuesday is 93 degrees. Widespread lows well down into
the 50s are likely across the lower deserts, with a few lower 60s
likely in the Phoenix urban core.

Dry subsident northwest flow aloft is expected Wednesday for sunny
days and clear nights and continued below seasonal normal high
temperatures as the deserts remain in the 80s.



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

Shower activity from this morning will continue to dissipate.
However, redevelopment of showers and thunderstorms can be expected
over the eastern third of AZ (mainly east of a TUS-PAN line) and
thus there is some potential for outflow from distant storms with an
easterly component. Couldn`t rule out a stray thunderstorm moving
through the metro late this afternoon and evening but likelihood too
low to reflect in the TAFs. Winds will remain light with a primarily
north-northeasterly component to them, with a very brief period of
north-northwesterly winds after 00Z. Expect scattered to broken
ceilings to remain above 7kft through early this evening, with a
gradual decrease in coverage and increase in ceiling height. Vsbys
to remain in VFR territory, however some patchy blowing dust from
outflow winds is possible this afternoon and evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Isolated to scattered showers will continue to decrease in coverage
and clear out by 21Z/22Z. Expect ceilings to remain AOA 10kft. Winds
will remain light and favor south and southeasterly directions with
a brief period of westerlies after 05Z.

Aviation Discussion not updated for amended TAFs.



Saturday through Wednesday...

A drier and more stable airmass will spread into the region under
westerly flow aloft. Mostly clear skies are forecast with seasonably
warm afternoon temperatures. Minimum relative humidities will drop
to around 15 percent in southeast California and around 20 to 30
percent in southwest and south-central Arizona on Saturday to 10 to
15 percent areawide on Wednesday. Afternoon southwest winds 5 to 15
mph are expected each day with some afternoon breeziness.
Overnight recoveries will be excellent.


Skywarn radio Net activation will not be needed.




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