Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
242 PM MST SAT SEP 10 2016

.UPDATE...Aviation and Fire Weather Discussions...

&&

.SYNOPSIS...
Low pressure off the California coast is expected to pull monsoon
moisture back northward into central and eastern AZ through Monday.
This will allow the development of isolated- scattered showers and
thunderstorms across south-central AZ each day. A turn to more
westerly flow aloft will then push much drier air back into the
desert southwest from Tuesday onward into the end of next week, with
dry conditions and mostly clear skies returning back to the region.

&&

.DISCUSSION...
Latest satellite imagery shows CU developing along the Mogollon Rim
and the higher terrain of southeastern AZ. Meanwhile, water vapor
imagery shows drier air being transported north and eastward across
southeastern CA ahead of a closed low off the southern CA coast.
Cooler temperatures aloft associated with the low extend eastward
into northern AZ. Further west, surface dewpoints have dropped into
the upper 30s across the southwest deserts while temperatures are
approaching 110 degrees in the Imperial Valley.

Latest CAMs including the HRRR depict isolated convection across the
higher terrain of southern Gila County late this afternoon. However,
CAPE is generally lacking while CIN is predominant, associated with
the relatively dry boundary layer. PoPs for this afternoon and
evening remain near 10-15 percent across these areas.

Conditions will become more favorable for shower and thunderstorm
activity Sunday and Monday, particularly across eastern Arizona as
the aforementioned closed low opens up and migrates east through the
Mojave Desert. Latest operational and ensemble guidance (including
the SREF and GEFS) shows an increase in uncertainty with regard to
the magnitude of the overall moisture, however model consensus
indicates that scattered thunderstorms will develop across eastern
Pinal and southern Maricopa County Sunday afternoon. Deep-layer
shear also increases, suggesting some modest organization to the
cells.

Setup will be similar for Monday with scattered strong storms
again mostly likely across southeastern Arizona, extending back
through Pinal and southern Gila counties. Sharper theta-E gradient
and forcing for ascent ahead of the approaching trough also suggests
an earlier onset to the convection.

&&

.PREVIOUS DISCUSSION...

Tuesday through Friday...

A drying trend is expected during this period as another, stronger
upper low that will be dropping southward over northern CA helps to
kick out the afomentioned upper low/trof to our east, and turn the
flow aloft more westerly over the region. This should end the threat
for convective activity over the region as much drier air is pulled
in from the west, with PWATs falling aob 0.50 inch by Thursday.
High temperatures are expected to hold at near, or slightly below
normal levels through this period, with highs near or slightly below
100 across the lower deserts. Lows will likely end up below normal,
as the very dry airmass allows for very efficient radiational cooling
each night, with lows in the 60s possible at many of the normally
cooler locations.

.AVIATION...

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

Light surface winds with diurnal patters will continue. Some storm
development will likely continue over the higher terrain to the
northeast of Phoenix until 01Z but at this time are not expected to
be strong enough to generate any outflows that would impact the
aviation terminals today.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light southwesterly winds over the area will turn to light and
variable after 01Z. Only a few clouds are expected for the rest of
the day.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Tuesday through Saturday...

A return of Monsoon moisture across Arizona will continue through
the early week period, allowing for scattered thunderstorm
development over the mountains with slight chances for storms to last
or develop over the lower AZ desert elevations. Expansion of moisture
across the area will be held mostly east of the Colorado River Valley
across AZ as dry air off the Pacific is brought into southeast CA.
Minimum humidities will range from driest in the western district
10-15 percent improving towards the east with 20-35 percent into the
Gila County mountains. Overnight recoveries will range from good to
excellent. A turn to more westerly flow during the Tuesday- Friday
period will begin to push the moisture/storms back to the east into
extreme eastern AZ/NM. Minimum humidities will drop to the 10 to 15
percent range from by Friday, with good to excellent overnight
recoveries. Outside of any thunderstorms, winds will remain mainly on
the light side with some afternoon gustiness up to 15 to 20 mph area-
wide on Monday and Tuesday with the strongest gusts along and west of
the Colorado River.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters activation is not expected this weekend.

&&

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

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DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Percha
AVIATION...Waters
FIRE WEATHER...Waters

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PHOENIX AZ
321 PM MST SAT SEP 10 2016

AZZ024-102300-
SOUTHERN GILA/TONTO NF FOOTHILLS AZ-
321 PM MST SAT SEP 10 2016

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...
  GILA...MARICOPA AND PINAL COUNTIES...
     MAINLY FOR LAKE ROOSEVELT AND SURROUNDING AREAS.

UNTIL 400 PM MST

AT 319 PM MST...DOPPLER RADAR WAS TRACKING A STRONG THUNDERSTORM
NEAR TONTO NATIONAL MONUMENT...MOVING NORTHWEST AT 10 MPH.

THUNDERSTORM WINDS BETWEEN 35 AND 45 MPH AND LIGHTNING ARE
POSSIBLE...RESULTING IN A HAZARD TO BOATERS.

LOCATIONS IMPACTED INCLUDE...
ROOSEVELT...ROOSEVELT DAM AND TONTO NATIONAL MONUMENT.

LAT...LON 3378 11109 3376 11100 3364 11092 3366 11089
      3354 11075 3341 11105 3374 11126 3380 11110
TIME...MOT...LOC 2219Z 120DEG 19KT 3362 11102

HV