Forecast-Discussion

000
FXUS65 KPSR 300205
AFDPSR

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

.SYNOPSIS...
Somewhat drier and calmer weather will return to the region Friday
though residual moisture may allow a few isolated showers to linger
over mountainous areas. However, the weekend will feature seasonally
warm temperatures and mostly clear skies. Continued dry weather
will persist next week though a sharp cooling trend will arrive
during the middle of the week as a strong area of low pressure
passes by to our north.

&&

.DISCUSSION...
An upper low approaching SE CA/SW AZ was ejecting a vorticity
center and jet max into southern AZ early this evening. 00Z KTWC
sounding data sampled an impressive 40kt 0-6km bulk shear juxtaposed
with MLCape values near 1000 J/kg in a weakly capped environment.
Given the favorable dynamical ascent, strong to locally severe storms
erupted over the SE portion of AZ earlier this evening in a region
of notable deep moisture convergence. Closer towards central AZ,
Cinh has objectively been analyzed much stronger with boundary layer
moisture profiles slightly lower precluding deep convection. With
loss of heating and much of the forecast area just removed from the
better jet level difluence, storm chance appears relegated to only
the southeastern portions of Gila County for the remainder of the
night.

As such, have cut POPs rather abruptly from the Phoenix metro
westward while increasing a sliver of SE Gila County where the
northern edge of the Tucson convective complex will enter this
evening. Objective analysis suggests activity should weaken slowly on
its northward trek into the San Carlos area as it enters a more
hostile environment, though decent dynamics and MUCape still above
500 J/kg may keep embedded storms on the strong side. Otherwise,
made some adjustments to cloud cover where Cu fields have dissipated
with loss of daytime heating.

&&

.PREVIOUS DISCUSSION...
/145 PM MST THU SEP 29 2016/
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.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Very little aviation impacts through Friday evening with timing of
wind shifts the main forecast challenge. Thunderstorm activity
should remain well east of terminals, though distant outflow
boundaries may eventually lead to wind shifting to a more easterly
direction during the late evening hours. Confidence in timing of
wind shifts is only moderate. Few-Sct cloud decks will be common
with only a very remote chance of additional showers within the air
space Friday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation impacts will affect SE CA terminals through Friday
evening under mostly clear skies. A brief period of stronger west
winds at KIPL will weaken overnight. Periods of variable directions
will be common across the area Friday with speeds trending below
12kt.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday...
A drier and more stable airmass will spread into the region under
westerly flow aloft next week. Mostly clear skies with seasonably
warm afternoon temperatures will prevail early in the week before
substantially cooler air filters into the districts during the
middle of the week. Minimum relative humidities will drop
to a 10 to 25 percent range early in the week with more solid single
digits and teens later in the week. Good overnight recovery
will become only fair through the week. Afternoon southwest winds 5
to 15 mph with higher gusts will be common, though stronger winds
approaching critical thresholds may be possible Monday afternoon.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed.

&&

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

&&

$$

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

NWS PSR Office Area Forecast Discussion