Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 302057

Area Forecast Discussion
National Weather Service Phoenix AZ
155 PM MST MON MAY 30 2016

A slow moving low pressure system across the desert southwest will
persist through Tuesday yielding near normal temperatures and mostly
dry conditions. Thereafter, strong high pressure will build over the
region resulting in a sharp warming trend. By the weekend, excessive
heat will prevail with many lower desert locations reaching the 110
degree mark.


An upper low has finally propagated into central AZ this afternoon
with objective analysis and 12Z KTWC sounding data indicating just
enough moisture around the H8-H7 layer to support congestive CU and
sct showers along the New Mexico border and up the Mogollon Rim.
Elsewhere, better deformation/divergence was noted in satellite
imagery and analysis throughout north central AZ where more
organized ascent was forcing somewhat more organized deep
convection. Other than a muted and diffuse outflow boundary, little
influence from these features will be experienced in the forecast
area through Tuesday morning.

Still a rather complicated and convoluted forecast Tuesday as the
upper low shifts towards SE AZ/N Sonora, placing much of central AZ
in the more favored jet divergence region. High resolution models
still insist on some measure of a modified surge of shallow Gulf
moisture into lower elevations Tuesday morning, though the magnitude
is somewhat dampened versus previous model iterations. This may
combine with residual midlevel moisture and stronger lift over
higher terrain north of Phoenix to produce showers/storms with NE
steering flow tending to accelerate outflows towards lower
elevations. Given the deep inverted-v type soundings, actual
rainfall reaching the ground is not likely; however more structured
outflow boundaries with accompanying gusty winds and blowing dust
would be possible. Confidence is not particularly high regarding
widespread gusty winds and dust given lower high resolution/global
model ensemble support, but have kept some mention valid given the
overall pattern and conceptual reasoning.

The aforementioned upper level low becomes even further
disassociated with the forecast area moving into northern Chihuahua
Wednesday while more pronounced height rises build over the forecast
area. While isold/sct showers and storms may be possible over the
White Mountains and SE Gila County, increasing subsidence should
become the more dominant feature, and more than likely deeper
convection will be relegated to New Mexico. More notably, Wednesday
will herald the beginning of a significant heat wave with a strong
ridge axis already shifting onshore in southern California. Model
guidance is in excellent agreement that 100F or greater highs will
be common in just about all lower elevation communities Wednesday

Thursday through Sunday...
The strong ridge keeps building inland through the end of the week
with 500mb heights in the 592-594dm range Friday and Saturday
(possibly even persisting into Sunday). NAEFS output now depicts
850mb temps exceeding the 90th percentile both of days (especially
Saturday) while the GEFS subset suggests record level heat. This
pattern has been well forecast with excellent model consistency and
most lower desert communities will reach or exceed the 110 degree
mark Friday and Saturday. Model trends have been continually
warming forecast highs during the end of the week, and historically
this is not unusual where record breaking temperatures become
inevitable (see CLIMATE section below for records list). With all
these trends considered, have continued the process of edging
forecast values towards the upper tercile of numerical guidance.

A couple minor caveats to consider that may disrupt more extreme
heat: 1) a backdoor front pushing against the Continental Divide
late in the week could conceivably moderate temperatures over central
AZ slightly below forecast, but likely would remain trapped over
higher terrain of New Mexico, and 2) somewhat more possible could be
thicker cirrus streaming northeast from the subtropical Pacific
ahead of a weak circulation center approaching the southern CA
coast. Regardless, the ridge starts shifting eastward Sunday in
response to this advancing low pressure system resulting in some
gradual cooling (more so in SE CA versus AZ), however the immediacy
of these minor height falls may be delayed given the larger scale
ridge strength. Therefore, the Excessive Heat Watch spans the entire
weekend and historically will correlate to around a top 10 or 15
hottest day of the year.


South-Central Arizona Including KPHX, KIWA, and KSDL
A weak Pacific low pressure system will move slowly from western AZ
Monday morning to southeast AZ Tuesday afternoon. Although this
system will be relatively dry initially, sct high cld base/mtn tstms
are expected along the Mogollon Rim and White Mtns Tue aftn and
evening. Through 06z Tue, mostly clear skies. Light west wind under
8 knots.  From 06z Tue through 23z Tue, sct clds aoa 18 thsd agl.
Light and vrbl winds under 6 knots, becoming west under 10 knots
after 19z Tue. Distant Mtn tstms north and east of PHX are expected.

Southeast California/Southwest Arizona Including KIPL and KBLH
A weak Pacific low pressure system will move slowly from western AZ
Monday morning to southeast AZ Tuesday afternoon. No affects from
this system are expected. Through 23z Tue, mostly clear skies.
Light and variable wind under 8 knots.

Aviation discussion not updated for amended TAFs.



Record high temperatures later this week:

Date        Phoenix           Yuma
----       ---------         ------
June 2     110 in 2014       115 in 1957
June 3     112 in 2006       113 in 2006
June 4     113 in 1990       114 in 1990
June 5     112 in 1990       115 in 1957
June 6     110 in 2013       114 in 1928


Thursday through Monday...
An unseasonably strong high pressure system will build over the
region this period, bringing some of the warmest afternoon
temperatures of the season. Many desert locations will slightly
exceed the 110 degree mark. Minimum relative humidities will range
from 7 to 10 percent. Typical warm season upslope afternoon winds
are expected in the 10 to 20 mph range. Good overnight recovery is
also forecast.


Spotter activation is not expected.


AZ...Excessive Heat Watch from Friday morning through Sunday evening
     for AZZ020>023-025>028.

CA...Excessive Heat Watch from Friday morning through Sunday evening
     for CAZ031>033.



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