Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 300259
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
800 PM MST SUN MAY 29 2016

.SYNOPSIS...
A slow-moving low pressure system across the desert southwest will
persist through Tuesday yielding cooler than normal temperatures and
mostly dry conditions. However, 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 continue through the
weekend.

&&

.DISCUSSION...
Weak upper low was spinning over far southeast California this evening,
clearly seen in the 00z plot data as well as on the water vapor
loops. Dry southwest flow is in place ahead of the low across
Arizona, with PWAT values at or below one quarter of an inch as seen
in the evening AZ raobs. Surface dewpoints over the central deserts
were low, mostly ranging from the mid teens to mid 20s this evening
and IR imagery showed generally clear skies area wide. The low will
shift a bit eastward and into far western AZ later tonight but will
have little if any effect on our weather with skies overnight
remaining generally clear. Winds will be mostly light with just some
occasional gustiness this evening, especially over the central
deserts, favoring the southwest to west. Current forecasts look to
be in good shape and no updates are needed at this time.

.PREVIOUS DISCUSSION...
Fairly status quo conditions this afternoon as a weak PV anomaly
continues to slowly progress eastward over the Los Angeles basin
pushing an unusually deep marine layer onshore into the higher
terrain of San Diego County. 12Z KNKX sounding depicted thicker 9
g/kg moisture through H8 and lesser 5 g/kg between H8 and H7.
Midtropospheric moisture drops off rather substantially as the
subtropical jet remains displaced well to the south over central
Mexico. Meanwhile over AZ, 12Z KTWC and KFGZ sounding data sampled a
very dry environment with only 3-4 g/kg of moisture around the H8
and H7 layers respectively. As a result, nearly clear skies have
prevailed over much of the forecast area though depressed heights
and thicknesses associated with the southern CA wave have kept
temperatures in a slightly below normal category.

As this aforementioned wave shifts slowly into western AZ Monday,
modest height falls and cool advection will become focused towards
central AZ while ridging builds towards southern CA. Overall
moisture profiles remain rather paltry Monday, however there are the
first inklings of some lower level Pacific moisture become absorbed
in the flow per model output.

The forecast becomes far more complicated Monday night and Tuesday
as numerous high resolution models indicate a shallow modified Gulf
surge early Tuesday morning, with moisture become entrained and
lifted orographically in the prevailing synoptic flow. With the
upper low shifted into SE AZ by this time, better divergent flow
aloft and adequate lapse rates should allow deeper convection over
higher terrain features. A deep boundary layer and inverted-v type
soundings would promote organized outflows descending into lower
elevation areas, and a majority of high resolution output indicates
such an event. While actually rainfall would be unlikely throughout
much of the forecast area, localized blowing dust in conjunction
with larger outflow boundaries may be more common, and have added
this mention late Tuesday afternoon/early evening.

The most significant weather story this week still looks to be the
steadily increasing heights/thicknesses associated with a building
ridge across the western CONUS. The latest midlevel height
percentiles from the NAEFS have trended somewhat higher and forecast
temperatures were adjusted accordingly, though daily highs could
still fall a few degrees short of records. The trend of higher
heights and thicknesses have continued in the 12Z iterations, and
several deterministic runs suggest expansive H5 heights near 594dm
and 1000-850mb thickness approaching 146dm. GEFS percentiles are
more bullish than the overall NAEFS suite denoting H7/H8
temperatures much closer to the 99th percentile and record
territory. Therefore, have further increased temperatures during the
end of the week closer to records just above 110F.

Latest operational GFS/ECMWF have come into better agreement for the
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.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL Weak and dry
upper low to the west of Phoenix will allow for some breezy west to
southwest winds this evening...with peak gusts to 20k or so at the
terminals thru 05z. Wind shift towards the east will likely be late,
especially at KPHX where there may be little if any return to the
east late tonight. Otherwise expect clear skies next 24 hours and
winds to again favor the south by late Monday morning with speeds
mostly aob 12kt.

Southeast California/Southwest Arizona Including KIPL and KBLH Weak
upper low over far srn CA will allow for some breezy southwest winds
this evening into the morning hours at both KIPL and KBLH with
speeds mostly aob 12kt. Winds to favor the southeast at KIPL after
12Z. Winds mostly from the south next 24 hours at KBLH with some
light variable winds during the morning on Monday, with speeds 5-
10kt.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
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 1o 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
range.

&&

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

&&

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

&&

$$

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DISCUSSION...CB
PREVIOUS DISCUSSION...MO/Hirsch
AVIATION...CB
FIRE WEATHER...Vasquez



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