Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 301153

Area Forecast Discussion
National Weather Service Phoenix AZ
453 AM MST MON MAY 30 2016

Added Excessive Heat Watch to W/W/A section.


.UPDATE...Updated aviation discussion.


A slow-moving low pressure system across the desert southwest will
persist through Tuesday yielding cooler than normal temperatures and
mostly dry conditions. Thereafter high pressure builds over the
regions for a sharp warming trend. By the weekend, many lower desert
locations will reach the 110 degree mark.


Today through Wednesday...
A weak upper low is centered over the Mohave Desert early this
morning. To the north is an upper trough centered with an axis over
the northern Rockies. In between the low and the trough is a
deformation zone. Upstream of our low is a ridge just offshore.
Satellite imagery shows indications of outflows and gravity waves
pushing westward to the Continental Divide due to convection over the
Plains (especially northern Texas). As the low slowly drifts east-
southeastward, it will try to tap into moisture from the Plains as
well as subtropical moisture from Baja and Sonora. Being that it is
weak, the moist advection will be meager. However, both hi-res and
larger scale models depict convection developing over northern AZ
today with more coverage Tuesday. With the low center dipping south
of the Mexico border by Tuesday afternoon, northeasterly steering
flow will help move storms from the Rim and Yavapai County into
south-central AZ. This would primarily affect southern Gila and
northern Maricopa Counties. The nature of the convection will be
high based and shallow given the very modest moisture availability.
Thus, no real precip opportunity but rather there will be gusty wind
potential and thus blowing dust potential. Our local WRF-NMM is the
most aggressive with the coverage and even has La Paz County and
Pima County getting into the mix. Other models (including NCEP WRFs
and UofA WRFs) are not as optimistic. Held on to the patchy blowing
dust in the grids for late Tuesday afternoon and evening but pared
back the area. By Wednesday afternoon, the low will be centered over
the Sonora/Chihuahua border. With lingering moisture and
northeasterly steering flow, added mention of slight chance t-storms
for our higher terrain zone northeast and east of metro Phoenix for
Wed. aft/eve.

As for temperatures, they will be a little bit below normal today and
near normal Tuesday (slightly above near and west of the Lower
Colorado River Valley). There will be additional warming Wednesday as
the low exits and the ridge advances inland. That will be the first
day of this week for 100 degree mark over the Phoenix area.

Thursday through Sunday...
The short story is that a strong ridge keeps building inland with
500mb heights in the 592-594dm range Friday and Saturday. NAEFS
depicts 850mb temps reaching and exceeding the 90th percentile both
of those days (especially Saturday). This pattern will result in well
above normal temperatures (even for early June) with most lower
deserts reaching the 110 degree mark Friday and Saturday. One minor caveat
is a backdoor front pushing against the Continental Divide late in
the week which could conceivably keep temps over south-central AZ
slightly below forecast but probably would only be felt over the
higher terrain - if at all. The ridge starts shifting eastward Sunday
in response to an advancing low pressure system leading to slight
cooling (more noticeably for southeast CA).


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

A weak low pressure system will move into Arizona today, though no
direct aviation weather impacts are anticipated through Tuesday
morning. Winds will generally remain less than 10 kt and follow
typical upslope/downslope patterns across the Phoenix area.

Southeast California/Southwest Arizona Including KIPL and KBLH...

Lighter than normal winds are expected through Tuesday. At KIPL,
directions will vary while at KBLH, winds will generally retain a
southerly or easterly component.

Aviation discussion not updated for amended TAFs.


Wednesday through Sunday...
A strong high pressure system is forecast to build into the region
providing some of the warmest afternoon temperatures of the year by
the weekend. Afternoon temperatures will be 5 to 8 degrees above
normal. Typical afternoon and evening southwest winds 10 to 15 mph
are expected each day. Good recovery is expected at night. Minimum
relative humidities are generally expected in the 8 to 12 percent


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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