Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
950 PM MST FRI JUL 29 2016

.SYNOPSIS...
A much more humid airmass will move into the Desert Southwest this
weekend, causing a modest drop in temperatures. There will be a
marked increase in thunderstorm activity continuing into next week.
All typical hazards associated with the monsoon season including
strong damaging winds, blowing dust, lightning, and localized
flooding will be in play.

&&

.DISCUSSION...
Strong to severe storms materialized over much of south-central
Arizona this evening in a near-classic monsoon setup. Strong low-
level convergence between two main outflow boundaries generated
rapidly-forming cells across the Phoenix area. Some damage has been
reported along with widespread winds generally in a 40 to 55 mph
range and areas of blowing dust. Wind gusts reached 70 mph at Sky
Harbor Airport and KIWA radar indicated winds as high as 90 mph just
above the surface in the West Valley. Fast-moving nature of the
storms has limited the flooding potential, however there have been
some isolated reports of rainfall around an inch, particularly across
portions of Scottsdale.

Latest runs of the HRRR indicate that storms will continue to
decrease in coverage as the main MCS migrates westward further into
the lower deserts of western Arizona. However, any storms that form
still have the potential to produce strong to severe winds and heavy
rain.

&&

.PREVIOUS DISCUSSION...

Activity Saturday may become highly dependent on evolution of
outflow and convection tonight; and much of the forecast model output
indicates very delayed build up of instability Saturday afternoon. In
fact, forecast soundings show the typical convectively overturned
environment during the morning, then a subsident capping
inversion/elevated mixed layer developing near 700mb during the
afternoon. Ensemble mean probabilities and high resolution output
correspondingly are rather muted in convective response into the
early evening, though show a far more robust response into the late
evening overnight. A vast majority of model output still latches onto
a well defined inverted trough/shortwave Saturday night while at the
same time surges an anomalously high theta-e airmass northward
towards central Arizona. POPs remain quite high during the Sunday
morning time frame with the potential for locally heavy rain.
Consequently, residual cloud cover through the daylight hours Sunday
may very well keep temperatures over central Arizona well retarded
while shifting convective activity westward.

Monday through Thursday...
1000-700 mb mean mixing ratios remain in the 10-12 g/kg through at
least Tuesday, keeping above-climo rain chances in the forecast.
GFS trends toward a slow decline of moisture beginning Wednesday and
especially Thursday (GEM starts drying on Monday). This is reflected
in a gradual eastward retreat PoPs from NAEFS and ESRL output. ECMWF
however holds on to moisture longer as it stalls out an inverted
trough over Arizona. Thus held on to PoPs over south- central AZ
through Thursday. Temperatures during the week will trend slowly
upward.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Westerly winds to prevail through the late afternoon hours with
stronger winds (sustained speeds as high as 12 kts) during the mid-
late afternoon hours today.

Latest high-res computer models are now in much better agreement on
the idea that thunderstorms developing over the higher terrain east
of Phoenix, at least sending strong outflow winds  this evening into
the Greater Phoenix Area during the 01Z-02Z hours. This will likely
affecting all of the Phx area taf sites with even a decent chance for
the storms themselves affecting one or more of the taf sites. Wind
speeds of at least 25 kts, with gusts as high as 35 kts look likely,
with even stronger speeds possible along with blowing dust reducing
visibilities. Heavy downpours and even small hail are possible if
thunderstorms do actually reach the terminal(s). Winds will become
light easterly later this evening, once the storms move through.

Southeast California/Southwest Arizona including KIPL and KBLH:
Should be another quiet taf period at KIPL, with little chance of
any isolated showers or storms, and winds remaining from a
southeasterly direction.

Winds will continue to favor southerly directions and wind speeds at
KBLH through this evening, with stronger winds during the late
afternoon/evening hours. There is fairly high confidence that
outflows from thunderstorms over South-Central AZ will affect KBLH
for a time later this evening, with winds becoming gusty from an
easterly direction along with some blowing dust possible.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday...
Deep southerly to southeasterly flow aloft is expected to push
copious amounts of monsoon moisture into the region, resulting in
scattered to numerous thunderstorms Monday through Wednesday with
good to excellent chances for at least some wetting rains and locally
gusty winds. The combination of considerable cloudiness and rainfall
will also keep temperatures below normal during this period, and
humidities above normal. A shift to more westerly winds aloft is
then expected to gradually reduce moisture levels, and chances for
wetting rains during the Thursday-Friday period, while allowing
temperatures to return to near-normal levels. Minimum humidities in
the 25-45 percent range in the Monday-Wednesday period to drop into
the 20-35 percent range by Friday, with good-excellent overnight
recoveries. Outside of thunderstorms, winds to mainly follow typical
diurnal trends.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is possible this evening and Saturday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Blowing Dust Advisory until 10 PM MST this evening for AZZ026.

CA...None.
&&

Visit us on Facebook...Twitter...and at weather.gov/phoenix

DISCUSSION...HIRSCH
PREVIOUS DISCUSSION...MO/AJ
AVIATION...Percha
FIRE WEATHER...Percha

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1155 PM MST FRI JUL 29 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1154 PM     DUST STORM       7 W FORTUNA FOOTHILLS   32.64N 114.52W
07/29/2016                   YUMA               AZ   TRAINED SPOTTER

            TRAINED SPOTTER REPORTED REPORTED VISIBILITY
            DOWN TO 1/4 MILE. .

&&

AD

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1157 PM MST FRI JUL 29 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1156 PM     DUST STORM       2 NNW PARKER            34.15N 114.29W
07/29/2016                   LA PAZ             AZ   TRAINED SPOTTER

            SPOTTER REPORTED NEAR ZERO VISIBILITY. .

&&

AD

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1212 AM MST SAT JUL 30 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1211 AM     DUST STORM       EAST BLYTHE             33.62N 114.58W
07/30/2016                   RIVERSIDE          CA   TRAINED SPOTTER

            TRAINED SPOTTER REPORTS VISIBILITY TO 1/4
            MILE OR GREATER. .

&&

AD

PRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE PHOENIX AZ
1215 AM MST SAT JUL 30 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1211 AM     DUST STORM       EAST BLYTHE             33.62N 114.58W
07/30/2016                   RIVERSIDE          CA   TRAINED SPOTTER

            TRAINED SPOTTER REPORTS VISIBILITY DOWN TO
            1/4 MILE.

&&

AD

PRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE PHOENIX AZ
1224 AM MST SAT JUL 30 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0730 PM     LIGHTNING        1 SSE NORTH SCOTTSDALE  33.61N 111.89W
07/29/2016                   MARICOPA           AZ   BROADCAST MEDIA

            FIRE CREWS RESPONDED TO A HOUSE FIRE CAUSED
            BY A LIGHTNING STRIKE. NO DEATHS OR INJURIES
            BUT HOUSE SEVERELY DAMAGED. TIME ESTIMATED
            FROM RADAR. .

&&

AD

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