Dust Storm in Arizona, August 15, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
208 PM MST MON AUG 15 2016

.SYNOPSIS...
The weather pattern will allow for a transition to drier and more
quiet conditions through early this week as moisture levels continue
to trend downward. With this drying trend, temperatures will also
increase with readings of a few degrees above normal. A return of
modest monsoon moisture looks likely for the latter half of the week
resulting in slight chances for showers and thunderstorms across
southeast California and all of southern Arizona.

&&

.DISCUSSION...
Latest streamline analysis and water vapor imagery shows a dry
anticyclone off the west coast, which is producing a northwesterly
flow across the desert southwest. Morning sounding from Phoenix
registered a PWAT of 1.22 inches, which is below average for this
time of year. A strong subsidence inversion is also evident on this
afternoon's AMDAR soundings just below 500 mb. Ensemble of CAMs
continues to suggest that isolated showers and thunderstorms will
develop this afternoon, but will mainly be relegated to the Mogollon
Rim, with perhaps some propagation southward into extreme
northeastern Maricopa and southern Gila counties. However, mid-level
lapse rates remain poor and impacts are generally expected to be
negligible across the lower deserts.

Main weather concern for today and tomorrow is the heat, particularly
across Southeastern California in the proximity of the anticyclone.
Latest NAEFS shows temperature and height anomalies near 1
standardized deviation above the mean across the Imperial Valley and
an Excessive Heat Warning continues for this evening and Tuesday.
Some relief is expected overnight as temperatures retreat into the
80s, though this is still several degrees above normal.

&&

.PREVIOUS DISCUSSION...
More and more, it appears that far southeast California and southern
Arizona will be in for a very low grade monsoon, not just for the
next couple of days but for the entire week as a drier flow pattern
becomes established across the area.

Over the next couple of days, progs call for the light northeast
steering flow to shift more out of the northwest over the lower
deserts as troffing continues to become established over central
California. Moisture remains suppressed over the deserts, with PWAT
values a bit over one inch over the south central AZ deserts and
below 0.75 inches over portions of eastern Riverside county. 850mb
dewpoints are not overly high, running 8-10C many places. Given the
unfavorable steering flow, no real forcing aloft and only modest
moisture/instability, chances for storms remains below 10 percent
except for the higher terrain areas to the east of Phoenix. High
temperatures over the western deserts remain elevated due to drier
conditions with many locales over 110 and the Imperial Valley
reaching to around 115. South central AZ will stay a few
degrees above seasonal normals through Tuesday.

Beginning Wednesday and continuing through the end of the work week,
previous guidance had suggested a return to more humid conditions as
a decent monsoon moisture corridor became set up over Arizona due to
a baggy low developing along or just off the southern CA coast. Flow
aloft was set to become more southerly as this occurred. Now it
looks much more likely that the upper low will develop as forecast
but the orientation of the flow under the low and into the desert
southwest will be much more westerly in nature and as such drier and
unfavorable for thunderstorm development over the lower deserts. Due
to this shift in thinking, we are only expecting very slight chances
for afternoon/evening thunderstorms Wednesday through Friday mainly
east of the lower Colorado river valley with POPs mostly 10-15
percent except southern Gila county where values reach into the low
end chance category. Temperatures slowly fall off due to increased
moisture but mostly stay near seasonal normals in south central AZ
and remain a bit above normal over western deserts into far SE CA.

Over the weekend, the persistent west/southwest flow into the
deserts continues to slowly erode moisture, with 850mb dewpoints
into the central AZ deserts struggling to reach to around 6C. We
will keep the slight chances for afternoon and early evening storms
in place east of the lower Colorado river valley but POPs in the
greater Phoenix area look to be barely above 10 percent each day and
it is possible that single digit numbers look more realistic if the
forecast steering flow pans out. A look at latest NAEFS POPs
supports the idea of a very low grade monsoon as the numbers over
the southwest and southcentral deserts through the weekend generally
stay below...even well below...20 percent.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL. Southeast
California/Southwest Arizona including KIPL and KBLH:

High pressure and dry westerly flow aloft will keep skies mainly
clear through the TAF period at all of the terminals. At Phoenix
terminals a westerly shift will occur near 21Z before a shift back
to the east this evening near 03Z. At California terminals, winds
will keep a southerly component to them. Speeds at all TAF sites
will remain on the light side aob 10kts.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...limited monsoon moisture will return into
the region during this period. The highest coverage of
thunderstorms(and wetting rains) will be confined to the higher
terrain north and east of Phoenix, with just a few storms making
their way into the lower deserts of south-central AZ each day.
Temperatures are expected to remain near normal across south-central
AZ, and a bit above normal across SW AZ and SE CA. Min RH's to
remain in the 15-30 percent range (lowest across SE CA), with good
overnight recoveries. Outside of thunderstorms, winds to remain
mainly on the light side, following typical diurnal trends.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activity will not be needed today and likely not be need
through the early work week. Spotters are encouraged to report
criteria through appropriate channels.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ033.

&&

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DISCUSSION...Hirsch
PREVIOUS DISCUSSION...CB
AVIATION...Hernandez
FIRE WEATHER...Percha/Hernandez

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PHOENIX AZ
219 PM MST MON AUG 15 2016

AZZ021-152200-
WEST CENTRAL DESERTS AZ-
219 PM MST MON AUG 15 2016

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...
  EAST CENTRAL LA PAZ COUNTY

UNTIL 300 PM MST

AT 218 PM MST...DOPPLER RADAR WAS TRACKING A STRONG THUNDERSTORM 10
MILES SOUTHEAST OF BRENDA...OR 46 MILES SOUTHEAST OF PARKER...MOVING
NORTHEAST AT 14 MPH.

WIND GUSTS OF 50 TO 55 MPH WILL BE POSSIBLE WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...
VICKSBURG AND VICKSBURG JUNCTION.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

TORRENTIAL RAINFALL IS ALSO OCCURRING WITH THIS STORM...AND MAY LEAD
TO FLASH FLOODING. DO NOT DRIVE YOUR VEHICLE THROUGH FLOODED
ROADWAYS.

&&

LAT...LON 3345 11382 3360 11395 3386 11376 3358 11346
TIME...MOT...LOC 2118Z 230DEG 12KT 3358 11382

SAWTELLE