Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210407

Area Forecast Discussion
National Weather Service Phoenix AZ
907 PM MST Sun Aug 20 2017

An increase in monsoon moisture over the next couple days will
lead to rain chances moving back into the area, especially across
southeast and south-central Arizona. Rain chances will last into
the middle of the week before another drying trend lowers chances
by Thursday or Friday. Temperatures over the next week will
remain generally near normal.


A weak gravity wave and/or outflow boundary is quickly advancing
eastward across the lower deserts of south-central Arizona this
evening, generated by convection over Gila County. Spotty light
showers have developed in association with these features. The 03Z
Phoenix sounding displayed minimal CAPE, along with a weak
inversion just above 500 mb, which should limit the potential for
thunderstorms across the deserts this evening. Rainfall amounts
will also be limited given the very dry/inverted-V type boundary
layer observed. Cloudiness will continue to linger overnight, and
scattered mid-level clouds could persist into the late morning.


A thick cloud deck, associated with the vort max over western New
Mexico, pushed through much of Maricopa County allowing for some
very light showers to move across the Phoenix area late this
morning. These showers formed along a weak moisture gradient that
sliced north and south from near Bryce, Utah through Arizona and
into Sonora. The showers and clouds are clearing slowly, mostly in
the east over the Rim country, with thicker clouds still slowly
progressing through the Phoenix area. Elsewhere, stronger
thunderstorms are occurring throughout portions of Northern
Arizona where the upper atmospheric dynamics are more supportive
for storms. In Phoenix, enough cloud cover has hung around to keep
temperatures lower with a forecasted high now at 101 degrees.

The 12Z upper air analysis depicts an upper low churning off the
coast of Southern California. Models suggest this feature will
continue to strengthen as it pushes eastward towards Arizona later
this week. This feature, combined with the ridge to our east, is
advecting some moisture back into the area. The increase in
moisture will allow for thunderstorm chances to enter the forecast
with the greatest chance most likely for Wednesday evening. The
GOES-16 satellite infrared channels show the deepest moisture
situated over far eastern Arizona and New Mexico so today may be
likened as a low grade monsoon day. The CAM consensus shows storm
initiation beginning this afternoon over the Rim, which is
already occurring, but with convection weakening as it approaches
the Phoenix area. The lingering cloud cover this morning will do
well to limit some surface instability over the lower desert, but
there are perturbations revolving through the upper levels of the
atmosphere that may provide enough dynamics to support storms
late this evening. All things considered, 10 to 15 percent chance
for rain in the lower desert seems fair for this afternoon.

Over the next few days, through about Tuesday, the low to our
west will continue to strengthen; southerly flow ahead of it will
import increasing monsoon moisture north and west leading to
mainly slight chances for afternoon and evening desert showers and
thunderstorms. For the most part we are looking at low grade
monsoon conditions as moisture is not forecast to become
excessive; 850mb dewpoints generally stay below 10C and PWAT
values do not rise much above one inch. CAPE values also remain
modest each day, mostly below 300 j/kg. Vort spokes rotating
around the upper low will add occasional dynamics/lift to the
equation but they are problematical to time, and most of the
difluence aloft is forecast to stay north of our area. With high
temperatures expected to stay generally near seasonal normals each
day, we won`t have the benefit of excessive solar heating during
the afternoon to spur significant convection.

Operational runs from the GFS and ECMWF as well as GEFS ensemble
output continue to call for the low to our west to open up and move
inland Wednesday into Thursday. As this occurs, wind fields become
elevated over the central deserts and although steering flow becomes
southwesterly, divergence aloft as well as QG forcing will combine
with sufficient moisture and instability to force scattered
thunderstorms over south-central Arizona starting Wednesday
afternoon and running through the evening hours. POPs have been
raised into the 20-30 percent ballpark for the greater Phoenix area
Wednesday night, which looks to be the most active portion of the
entire 7 day forecast period. By Thursday evening, drier air will
have overspread much of our western deserts, pretty much eliminating
any threat for storms for areas west of Maricopa County, and largely
confining storm chances to the higher terrain east of Phoenix. This
will be the case into the weekend as POPs lower into the single
digits across most of the lower deserts.

High temperatures Thursday will fall a few degrees and below
seasonal normals under the influence of the low moving into the
area. However Friday into the weekend we expect a warming trend as
high pressure aloft again takes hold over the desert southwest; as
the ridge builds aloft 500mb heights climb above 592dm and we can
expect high temps to climb several degrees above normal. By
Saturday, the Phoenix area should see highs near 107 with readings
approaching 110 over the hotter western deserts.


South-Central Arizona including KPHX, KIWA, and KSDL:
A weak outflow boundary/gravity wave approaching from the east
should result in winds shifting out of the east across the Phoenix
area earlier than typical this evening, by 05Z. Weak showers may
also develop, but should remain rather spotty and light in nature.
Broken cloudiness above 12 thousand feet could linger overnight.
Otherwise, diurnal wind patterns will continue Monday with only a
slight chance of thunderstorms area-wide during the afternoon and

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will remain southerly at KBLH through the TAF period.  For
KIPL, winds mostly favor an east to southeast direction, but
westerly winds are expected during the evening and overnight.

Aviation discussion not updated for amended TAFs.


Wednesday through Sunday:
A moderate amount of monsoon moisture will persist into Thursday
across the eastern half of Arizona leading to a continued chance
of showers and thunderstorms. A drying trend starting Friday or
Saturday will lead to only slight chances of high terrain storms
through the weekend. Minimum humidities will hover around 15-20%
in the lower deserts while the higher terrain of Gila County will
be much more favorable, around 30-40%.


Spotters should follow standard reporting procedures.




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