224 
FXUS65 KPSR 161110
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
410 AM MST FRI SEP 16 2016

...Updated Aviation and Fire Weather...

.SYNOPSIS...
Dry air will be in place across the region through Sunday for clear
skies and no chance of rain. Highs will be two to three degrees below
normal today before warming two to three degrees above normal by
Sunday. Next week, clouds and humidity will increase, leading to a
return of shower and thunderstorm chances, along with cooler
temperatures.

&&

.DISCUSSION...
Today through Sunday...
Temperatures early this morning are running in the 60s to low 70s at
most lower desert locations. With weak positive tilt troughing
remaining over the interior west and southern California today, highs
will once again be 2-3 degrees below normal. Models continue to show
troughing within the southern branch of a split in the westerlies
which goes on to become a cutoff low centered near the west coast of
northern Baja this weekend. Meanwhile, high pressure centered over
Texas will slowly expand westward. Thus a warming trend is in store
with highs climbing a few degrees above normal by Sunday. Otherwise,
conditions remain dry through the weekend.  

Monday through Thursday... 
Models have come into better agreement on the scenario of a moisture
return next week. This is because of southerly flow developing over
us due to the positioning of a cutoff low centered to our southwest
and an anticyclone centered to our southeast. Also, there may be a
tropical disturbance getting into the mix as well. Of course, there
are considerable differences amongst the global models on when,
where, and how much moisture there will be. This is driven by
differences on how they handle the cutoff low, the anticyclone, and
the developing tropical system (currently off the southwest coast of
Mexico). For instance, the GFS is slower and weaker with the tropical
feature and dissipates it off the west coast of southern Baja.
Conversely, the ECMWF has a stronger feature and is faster with the
northwestward movement. A key reason for this difference appears to
be how the GFS develops a separate anticyclone center over northern
Mexico which shunts the tropical feature. Whereas the ECMWF keeps one
main center over/near Texas. This also has implications for the
cutoff low wherein the GFS opens up the low (in response to a larger
upstream trough) and tracks it across southern California and
Arizona Tuesday and Tuesday night (shearing out in the process). The
ECMWF dissipates the cutoff low and the tropical system takes its
place with the upper portion eventually moving inland Wed night-Thu
(in a greatly weakened form). However, the lower portion of the
storm dissipates offshore. Meanwhile, the Canadian-GEM has a mix of
the GFS and ECMWF solutions. These operational runs have support for
their solutions from their respective ensemble means...more so for
the GFS. Some similarities amongst the solutions include the
depiction of moisture increasing from above before the richer lower
level moisture arrives. Speaking of the lower level moisture, the
ECMWF tracks the initial push further west than before and even has
something of a drier patch for a time over portions of Arizona and is
thus less generous with the moisture than previous runs. Another
point of agreement is that due to the cloud cover, CAPE suffers. This
ties in to another similarity wherein there isn't tremendous QPF
being depicted by any of the models. A lot of uncertainty remains
but not expecting a major precip event at this point. PoPs remain in
the slight chance category for most areas starting Monday then
tapering off from west to east Wednesday (lingering slight chances
Thursday higher terrain of south-central AZ). As for temperatures,
anticipate highs dipping at least a few degrees below normal Tue-Wed
due to clouds/humidity. 

&&

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

No aviation concerns through Saturday morning under clear skies.
Light easterly winds in the Phoenix area may holding longer into the
afternoon than previous days before turning NW. Given the weak
pressure gradient, wide variability in directions may be common
though with speeds below 10kt. Wind trends over SE CA will be
almost identical to Thursday. 

Aviation Discussion not updated for amended TAFs.

&& 

.FIRE WEATHER...
Sunday through Thursday... 
Humidity levels will increase rapidly during the beginning of next
week with single digits to lower teen minimum values on Sunday
afternoon increasing to a far more moist 25-40 percent on Tuesday.
Poor to fair overnight recovery will quickly become good to
excellent. Confidence continues to grow that better rainfall chances
will arrive by Monday and possibly persist into Wednesday. Wind
speeds may increase somewhat next week as a weather disturbance
begins to approach the region, though should not become unusually
strong. 

&&

.SPOTTER INFORMATION STATEMENT... 
Spotter reports will not be needed through at least the weekend.

&&

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

&&

$$

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

DISCUSSION...AJ
AVIATION...MO
FIRE WEATHER...MO