Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 290323
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
825 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...

Some pretty tall thunderstorms developed over parts of our forecast
area in the mountains of southern Gila County.  A few convective
cloud tops were at 50 thsd ft msl suggesting some support aloft.
After looking at this evenings regional weather balloon soundings we
found a few upper level winds were incorrectly predicted for this
evening.  For example the Phoenix 300 wind was modeled to be
northwesterly at 10 knots, instead the Phoenix sounding detected
northeasterly winds at 10 knots. Another key wind was at Albuquerque.
A modeled northerly wind at 10 knots was forecast at 00z, but instead
it was northwesterly at 10kts.  That might not sound like much but
the wind spread between the two, or weak difluent flow was enough to
weaken the atmosphere and support taller storms today. A few storms
in southern Gila county produced severe gusts, with extremely heavy
rain characteristic of flooding, but the storms were located in
totally remote areas northwest of the city of Globe.

Thunderstorms made a run toward Phoenix from the northeast but too
much convective inhibition (CIN) was in place. For example the from
the morning Phoenix weather balloon sounding the convective trigger
temperature was 114 F but we only managed 110 degrees today. And
this evenings Phoenix sounding showed the convective trigger again
at 114 F. Thunderstorms fizzled quickly as they moved into the
Phoenix valley from the east. Only a isolated thunderstorms were
noted over the distant mountains north of Phoenix at 8 pm mst but
were located just outside our forecast area. A very slight chance of
showers and thunderstorms will continue through the remainder of the
evening.

Models are forecasting a weak Pacific disturbance to move into
northwest AZ early Friday afternoon, the near Phoenix by evening.
This feature will help support afternoon mountain thunderstorms
north through east of Phoenix which might make another run at the
Phoenix valley from the north, since mid level steering currents will
weakly shift from the north late Friday.

Otherwise current forecasts which show a slight increase in
shwr/tstm probabilities for Friday over today look ok. Details for
a convectively active weekend are explained in the previous
forecasts below. The Excessive Heat Warning for the southeast CA
deserts and Colorado River Valley is still posted until 11 pm mst.


.PREVIOUS DISCUSSION...245 PM MST...

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 Meso, 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:

Light and variable winds are forecast through 19z Fri with sct
clouds aoa 12 thsd msl.

Southeast California/Southwest Arizona including KIPL and KBLH:

Persistent south winds 5 to 10 knots are forecast through 19z Fri.

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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