Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 251045

Area Forecast Discussion
National Weather Service Phoenix AZ
345 AM MST WED MAY 25 2016

.Update...Aviation and Fire Weather Discussions.


A large region of low pressure will continue over the Western States
through the weekend bringing overall cooler than normal temperatures
and dry conditions. A more compact low pressure system will move
through today bringing gusty winds to much of the area. Building
high pressure for next week will likely bring a warming trend
pushing highs to near 100 degrees for the lower deserts by the
middle of next week.


At 2 am this morning, tranquil weather continued across the lower
deserts of southeast California and southern Arizona as dry
southwest flow aloft persisted over the area. IR imagery showed
clear skies area wide, and the latest area soundings showed just
how dry the airmass was with pwat values down to about one quarter
of an inch at Tucson and Flagstaff. Surface dewpoints over the lower
central AZ deserts were mostly in the 20s although they had risen
into the 40s out west in places such as Blythe and Imperial.

Models, including GEFS ensemble members, remain in good agreement
that dry weather with continued below seasonal normal high
temperatures will continue into the weekend under the influence of
large scale upper troffing that has been persistent across the
western CONUS. Water vapor imagery at 2 am showed an upper closed
low spinning just off the far SE California coast, and progs
continued to call for the low to eject inland into far southern CA
later today, then move northeast and across Arizona overnight
tonight. This will be a moisture starved system with PWAT values
mostly below one half inch, with most of the moisture confined to
mid or high levels. Despite rather strong UVV and mid level Q-
convergence fields associated with the low, it will simply be too
dry to allow for any measurable precipitation, and for the most part
there will not be much in terms of cloudiness. The only real threat
for any precip remains over the higher terrain of Joshua Tree
National Park during the afternoon and evening hours today as the
low ejects inland and moves across far SE California. We will keep
the 10 percent chances for afternoon/evening showers or
thunderstorms in CAZ030 overnight tonight and as the low ejects
northeastward, a much more subsident northwest flow aloft
overspreads the area eliminating any remaining threats for showers.
The low will usher in a cooling trend mainly to south central
Arizona on Thursday, dropping high temperatures into the middle 80s
over the lower deserts; Phoenix is forecast to fall to 86 degrees
which is 12 degrees below seasonal normals for the date.

For Friday into the weekend, upper troffing is forecast to persist
across the western CONUS, although it will not be overly strong or
deep; for the most part a baggy trof sets up over the desert
southwest keeping dry southwest flow going across the area each day
with only a bit of high clouds embedded in the flow for mostly sunny
days and generally clear nights. With 500mb heights not really
falling all that much there will be somewhat of a warming trend with
high temperatures climbing into the low to mid 90s through Sunday;
these temperatures will still be a few degrees below normal. The
normal high on Sunday at Phoenix is 99 degrees, and the forecast is
for 95 degrees; it makes sense for temps to be slightly below normal
given the presence of weak troffing over the desert southwest during
this period.

Early next week, model ensemble spread becomes quite large and
operational runs from both the GFS and ECMWF argue that much of the
low pressure aloft over the western states will shift eastward as
upper ridging forms just off the west coast. As this occurs, heights
rise and high temperatures over the lower deserts climb putting the
warmer deserts into the upper 90s, possibly reaching around 100
degrees by Tuesday. Thus we are calling for a warming trend with
Phoenix set to reach 97 degrees next Tuesday. It will stay dry each
day with generally sunny days and clear nights across the area.


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

Another Pacific low pressure system and associated cold front will
move into southern California today and into northwest Arizona this
evening. Although the airmass is relatively dry, gusty southwest
winds are expected this afternoon and evening. Through 18z Wed,
south to southwest wind 4 to 8 knots. Clear skies. From 18z Wed
through 03z Thu. Clear skies. Increasing southwest winds 15 to 25

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...a series of weak low pressure systems will
continue to move into the western states mainly north of Arizona.
Although dry weather is forecast, the proximity of the low pressure
systems to Arizona will result in breezy afternoons, 15 to 20 mph
each day. Minimum relative humidities will range from 10 to 15
percent. Good overnight recovery is expected. A slow warming trend
is expected with afternoon temperatures approaching seasonal normals
by Tuesday.


Spotter activation is not expected.




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