Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 291305 CCA
AFDPSR

Area Forecast Discussion...CORRECTED
National Weather Service Phoenix AZ
605 AM MST SUN MAY 29 2016

.UPDATE...
Updated Aviation and Fire Weather sections.

&&

.SYNOPSIS...
A slow-moving low pressure system across the desert southwest will
persist through Tuesday yielding cooler than normal temperatures and
dry conditions. Strengthening high pressure thereafter will bring a
warming trend with high temperatures in the lower deserts eclipsing
the 100 degree mark by the middle of the week. Well above normal
temperatures will then continue through the weekend.

&&

.DISCUSSION...
Latest water vapor imagery shows a mid-level vort max off the
southern CA coast while streamline analysis indicates that the
cyclonic circulation extends as low as 925 mb. However, the storm
system is moisture-starved and there`s little to show for it in
terms of clouds or precipitation. Latest models continue to show the
low pressure system sliding slowly eastward across AZ over the next
few days. Main impact will be to neutralize and briefly halt the
warming trend. Below normal temperatures will persist through Monday
under sunny skies.

Global models continue to indicate that there is some potential for
convection in the wake of the low Tuesday and Wednesday. Although
Q-vectors point to widespread subsidence, hi-res CAMs suggest
residual moisture should be sufficient for an isolated shower or
thunderstorm, even across the lower deserts.

Big weather story this week still looks to be the steadily increasing
heights/thicknesses associated with a building ridge across the
western CONUS. Latest mid-level height percentiles from the NAEFS
have trended somewhat higher and forecast temperatures were adjusted
accordingly, though daily highs will still likely fall a few degrees short
of records.

Latest operational GFS/ECMWF have come into better agreement for next
weekend, both depicting a trough in the eastern Pacific fracturing
and heading towards the West coast. Timing differences are, not
surprisingly, still apparent this far out. Higher weight was
assigned to the ECMWF-based solutions, which have exhibited somewhat
lower spread than the GFS ensembles. With regard to this system, the
ECMWF evolution is slower and consequently results in warmer
conditions across the desert southwest through Sunday.

&&

.PREVIOUS DISCUSSION...
A flattened ridge was prevalent over the SW Conus early this
afternoon, however a more defined vorticity center and sharper
cyclonic circulation was descending down the central and southern
California coast. This wave has acted to increase
heights/thicknesses downstream over Arizona today; and as a result,
afternoon temperatures were trending some 1F-5F warmer than Friday.
With clear skies and a seasonally dry airmass, evening and overnight
temperatures should trace fairly similar to last night and model
output strongly agrees eventual lows will fall near or just above a
persistence forecast.

Operational models remain in good agreement through the remainder of
the holiday weekend, indicating that the aforementioned trough will
briefly close off Sunday while moving through southern CA. This
system is almost entirely moisture starved and its main impact will
be on temperatures, which will be a degree or two cooler on Sunday.
The trough will weaken slightly Monday while continuing to slide
eastward into AZ allowing H5 heights to fall 1-3dm. As a result, an
additional degree or so of cooling is likely across AZ, however
warmer temperatures will rebound across SE CA as a ridge in the
eastern Pacific builds towards the west coast.

Residual moisture in the wake of the compact low pressure system
should be sufficient for isolated showers and thunderstorms Tuesday
and Wednesday afternoon. However, both global and high resolution
guidance suggests that the activity will be almost exclusively
relegated to the higher terrain north and east of Phoenix. There is
still a remote chance some dying shower leaking into the local
forecast area, however chances still do not justify a higher pop
mention.

Latest guidance continues to indicate that the ridge will build
across the western CONUS, yielding steady height rises and a
continued warming trend during the mid to late week period.
Temperatures were increased for Thursday and Friday and generally
represent the 70th percentile of all available guidance, which is
well short of records. Looking ahead to next weekend, operational
GFS and ECMWF diverge markedly, with the GFS depicting an eastern
Pacific trough fracturing and moving into the intermountain west.
This solution is not supported by other ensemble members and was
dismissed, with the expectation that the dry conditions and above
normal temperatures will persist.

&&

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

Weak upper low centered near SBA this morning will keep southerly
winds going in the lower troposphere. As surface inversions erode
this morning, south and southeast surface winds will prevail for
much of the day (a bit lighter than yesterday). Distinct westerly
component may not take hold over the Phoenix area until after 00Z.
Skies will be clear except for some forest fire smoke over portions
of south-central AZ. Thin smoke will affect portions of Phoenix
metro this morning (mainly East Valley) and there could be an impact
to slant visibility (but surface vsby will remain above 10SM).

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday...
A weak upper low slowly exits Arizona Tuesday and Wednesday and is
replaced by a ridge of high pressure building in from the west. Thus
a warming trend can be expected. Highs will be near normal on
Tuesday then climb to 8-10 degrees above normal by Friday and
Saturday. Minimum humidities will be in the 10-15% range on the lower
deserts before dropping to single digits at most locations by late in
the week. No strong winds are forecast through Saturday - just
familiar warm season patterns with occasional breeziness in the
afternoons and evenings.


&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.

&&

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

$$

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DISCUSSION...Hirsch
AVIATION...AJ
FIRE WEATHER...AJ


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