Weather Service Report

FXUS65 KPSR 281518

Area Forecast Discussion
National Weather Service Phoenix AZ
818 AM MST WED SEP 28 2016

A low pressure system over northwest Mexico will continue to slowly
move north through the Desert southwest the next couple of days.
This will lead to variable cloudiness, cooler temperatures, and a
threat of showers and thunderstorms through Thursday. Best chances
for precipitation look to be on Thursday. Dry and stable weather
will return Friday through early next week.



Weather balloon soundings showed the northern edge of the monsoon
boundary or deeper moisture from Mexico was along a line from Tucson
to San Diego. Increasing low level south winds today will move
much of this deeper moisture north into central AZ.

At the same time, a more convergent flow aloft in the wake of a
passing shortwave this morning (moving from south to north through
south central AZ), will provide some stability aloft over our
forecast area. Therefore any shwr/tstm threats today will be diurnal
and thermodynamically driven, particularly over the mountains. A
slight chance of shwrs is forecast on the central deserts vicinity
Phoenix and Casa Grande. For example, 

An approaching Pacific shortwave from the southwest direction
Thursday should increase precip probabilities over much of southwest
and south central AZ.

Current forecasts look ok. no updates planned.

.Previous Discussion...526 AM MST...

Upper level low that had been centered well to the south of Arizona
has started to lift north and the center has moved to a position
near Yuma as seen in the latest vapor imagery at 2 am. A series of
weak vort spokes/disturbances continued to rotate around the low,
sparking occasional areas of showers or even weak thunderstorms such
as an area of convection now moving north and into mainly southern
Gila County. Latest guidance is consistent in calling for the low to
continue shifting north today with the center moving into far
southeast California and just northeast of Joshua Tree NP by late
this afternoon. As the low moves north a slightly drier southwest
flow underneath the low starts to affect far SE California and
southwest Arizona, and dynamics become rather weak area wide as
evidenced by meager Q-convergence fields shown by the NAM and GFS.
As such POPs today will fall off slightly, running from single
digits to low end slight chance over the lower deserts and around 30
percent over portions of southern Gila County. The somewhat subdued
pattern will continue overnight keeping rain chances mostly below 10
percent over the deserts, and in the slight chance ballpark over the
higher terrain north and east of Phoenix.

It appears that the weather will become a bit more interesting
Thursday into Thursday night before a drier southwest flow starts to
scour out the moisture from the west and lead to a drying trend that
will begin on Friday and continue into next week. Current satellite
imagery shows a weak tropical system lurking off the southern tip of
Baja CA, and a second weak area of low pressure is situated to the
southwest of the main upper low. On Thursday the weak low off
central Baja will combine with the decaying remnants of the tropical
system and push northeast into Arizona, leading to an increasing
chance of showers and thunderstorms which will mainly affect south
central Arizona. Increasing QG forcing such as PVA and Q-convergence
will enhance lift as vort maxes push northeast and across the
deserts and showers/storms will increase especially across higher
terrain areas east of Phoenix. POPs across southern Gila county will
climb in excess of 50 percent by Thursday afternoon with numbers
running around 20 percent across much of the central and southwest
deserts. Slight chances are even expected into southeast California.
The increasing clouds and showers will lead to below seasonal normal
high temperatures on Thursday with Phoenix expected to see readings
around 90 degrees.

Operational runs of the GFS and ECMWF both agree that a much deeper
Pacific low pressure system will develop just off the northern CA
and Pacific Northwest coast on Friday and this will set up a
stronger and drier southwest flow aloft which will begin the process
of scouring out the low and mid level moisture from the Arizona
deserts. By Friday afternoon, precip chances should be confined to
higher terrain areas east of Phoenix with POPs falling into the
single digits over the lower deserts. The drier air combined with
mostly sunny skies will allow high temperatures to climb a few
degrees and mostly into the middle 90s over the deserts.

GFS/ECMWF guidance as well as the GEFS ensemble members agree that
this drying southwest flow aloft will persist through the weekend
leading to sunny days and clear nights with virtually no chance of
showers even across the high terrain east of Phoenix. The deep
Pacific upper trof will dive southeastward and into the southwestern
CONUS, leading to locally breezy conditions during the afternoon
area wide. High temperatures will start to fall off on Sunday as
heights fall and despite the dry air and sunshine, highs will lower
mostly into the lower 90s over the warmer lower deserts.

Early next week, GEFS ensemble members show a much more significant
spread between the various members and although the GFS operational
run suggests a continued very dry west flow aloft, the ECMWF calls
for a series of disturbances to move through the large scale trof
potentially bringing a threat of showers or storms back to the area,
especially south-central Arizona on Monday night. NAEFS POPs still
remain quite low early next week and our forecasts will reflect
that, but we do have some single digit POPs across the area
reflecting some uncertainty, with numbers approaching but staying
just below 10 percent over the high terrain east of Phoenix Monday
night. High temperatures will fall off early next week into the mid
to upper 80s due to the pronounced upper height falls moving into
the area.



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

Weak disturbances embedded within an upper low centered near Yuma
early this morning will lead to occasional isolated light showers
this morning. Lingering dust trapped under the inversion may lead to
decreased slantwise visibility this morning until about 16Z.
Thunderstorm development later today will primarily remain outside
of metro Phoenix but will be over Gila, Yavapai, and eastern Pinal
Counties. Could see a stray storm over eastern portions of the metro
after 23Z. Overall, less active over the southern half of Arizona
than yesterday (Tuesday). Thus, not anticipating a dust storm
affecting metro Phoenix, but some modest outflow from distant storms
could push into northern and eastern portions of Phoenix area after
23Z. Surface winds will favor east and southeasterly directions this
morning before trending southerly by mid afternoon possibly becoming
southwesterly. Anticipate sct-bkn layers with bases AOA FL100
throughout the day.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light and variable winds will trend toward southeasterly by mid
morning before becoming southwesterly after 23Z. Anticipate sct cloud
layers (locally bkn near higher terrain) AOA FL100. Isolated
thunderstorms possible later today - mainly vicinity Joshua Tree Nat.
Park and northern La Paz County. 

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...
There will be a lingering slight chance of thunderstorms Friday over
the higher terrain of south-central Arizona. Otherwise, dry
southwest and westerly flow aloft will overspread the region this
weekend and next week. Conditions will be breezy at times (mainly
Sunday and Monday) favoring southwesterly directions. Minimum
humidities on the lower deserts will decline from 20-25 percent on
Friday down to 10-15 percent by Sunday. Overnight recovery will
decline as well but remain fair.


Skywarn radio Net activation will not be needed.




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