Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 282146
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
245 PM MST THU JUL 28 2016

.SYNOPSIS...
Very hot weather will continue into Friday before a much more humid
airmass moves into the Desert Southwest this weekend, causing a drop
in temperatures. There will be a marked increase in thunderstorm
activity beginning Friday and continuing into next week. Saturday
evening bears close watching for strong to severe thunderstorms,
mainly across southern Arizona.

&&

.DISCUSSION...
Appears we are on the cusp of finally seeing widespread thunderstorm
activity across Arizona.

Currently, convection continues to develop over the White Mountains
of eastern Arizona and the San Francisco Peaks/Grand Canyon area.
Cloud tops are showing modest cooling and there is a decent amount of
lightning activity, but nothing too vigorous at the moment. Per
latest SPC RAP-based MesoOA, MLCAPE across southern Arizona is around
500-100 J/kg with significant CIN at lower elevations. Per past
several runs of the operational HRRR, NCAR Ensemble, and SPC SSEO
output, expectation is that this convection continues to progress WSW
on 700-500 mb winds and Corfidi Vectors of 20-25 kt then crumple as
it hits the capped environment over Maricopa/Pinal counties. Some
chance convection continues across Pinal county this evening. Will
watch for potential strong/severe storms in Gila County/Globe area.

Friday could be a tricky day. The continued northeast flow in the
midlevels may yield enough cooling, and if the capping inversion is
weakened, to push instability values up enough to support convection
propagating into the lower deserts from the surrounding mountains.
With a deep boundary layer still, strong outflows will be well
supported.

As we head into Saturday, changes will be underway. A large inverted
trough now sits over central Mexico and it will help maintain a moist
southeasterly flow into the Sierra Madre Occidental, allowing very
large nightly complexes of storms to develop. Outflow winds from
these thunderstorm masses will initiate a "gulf surge" Friday into
Saturday. Soundings along the Gulf of California measured PWAT of
2-2.5" this AM, which is supported by nearby GPS-derived PWAT values.
That is what will move our way. By Saturday afternoon, boundary layer
mixing ratio values be in the 10-12 g/kg range, maybe higher, from
the Colorado River east which will be 90th percentile or better.
Hard to find any model guidance that doesn`t product CAPE values well
in excess of 1000 J/kg (some exceeding 2000). Question is if that can
be released. Typically in this scenario, the boundary layer is
exceptionally moist but is also cool with a strong capping inversion.
Both the NAM and the GFS exhibit that behavior. Saturday evening
though, the GFS erodes the cap while the NAM hangs on to it. I could
see a scenario where the erosion takes places, especially if mid-
level convective debris from the northeast migrates in and cools the
cap. If that happens, everything will break loose that night.
Damaging winds, torrential rainfall, prolific lightning - all the
typical threats of a significant thunderstorm outbreak. The
overnight hours will be interesting with continued moisture advection
up the GoC it could be a busy night.

With the expectation that Saturday night is busy, Sunday should be a
cooler, more humid, albeit quieter day. With a convectively worked
over atmosphere, may be difficult to generate much instability that
is not severely capped. Again, predicated on Saturday night`s events.

Boundary-layer moisture in the 10-12 g/kg range remains in place
Monday through at least Wednesday, keeping above-climo rain chances
in the forecast. Wind fields gradually weaken with each passing day,
which may gradually raise the threat of slower moving storms and
localized flash flooding. Some indication toward the end of the week
that a little more southwest flow throughout the atmospheric column
will develop and shunt the monsoon moisture plume further to the
north/east.

&&

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

Winds will generally be light to occasionally breezy and favor a
westerly component along with a slight chance of thunderstorms
during the late afternoon and early evening. However, the confidence
of having storms at the terminals was too low to include them in the
TAFS. Outflow boundaries from distant thunderstorms to the NE, E or
SE of the valley will be weakened somewhat by the time they reach
the terminals but may produce brief easterly wind gusts up to 20
kts., or just light erratic wind shifts during the early evening
along with a slight chance of reduced visibilities in lofted dust.
Otherwise skies will be mostly clear to partly cloudy with periodic
mid to high cloud cigs.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect mainly light to occasionally breezy southerly and westerly
component winds along with typical sequencing of wind shifts.
Otherwise skies will be clear to partly cloudy with few to sct mid
and high clouds.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday...

A series of disturbances from the south and east will maintain
demonstrable Monsoon moisture flow over the region through most of
the period. As a result, significant chances of scattered wetting
rains from thunderstorms from the lower deserts to southern Gila
county can be expected. Below normal maximum temperatures are
expected through Monday, and then near normal beginning Tuesday.
Minimum humidities will range from 20 to 32 percent in the lower
deserts to 32 to 45 percent from the Tonto foothills to the high
country. Seasonably breezy south to southwest winds in the late
afternoons are also likely. Overnight recoveries will be good to
excellent.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is possible this weekend, particularly Saturday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 11 PM MST this evening for AZZ020.

CA...Excessive Heat Warning until 11 PM PDT this evening for
     CAZ031>033.

&&

$$

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DISCUSSION...Iniguez
AVIATION...Sawtelle
FIRE WEATHER...Sawtelle



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