Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 290330

Area Forecast Discussion
National Weather Service Phoenix AZ
830 PM MST SAT MAY 28 2016

A broad area of low pressure will persist over the western United
States through the beginning of next week yielding somewhat cooler
than normal temperatures and dry conditions. Strengthening high
pressure much of next week will bring a warming trend with high
temperatures in the lower deserts finally pushing above 100 degrees
by the middle of the week. A stretch of well above normal
temperatures will become widespread later in the week.


As with the previous weak upper level low that passed through the
region fairly harmlessly earlier this week, another similar low is
now entering southern California and forecast to move through
Arizona late Sunday into Monday. Nearly all system moisture again
will stay to our north and the only thing the weak low will do for
our CWA is to halt the warming trend for a couple days. Highs today
continued their upward trend, but that trend will flatline through
Monday as the low quietly passes through the region. The forecast
through the rest of the evening and overnight looks good overall,
just some minor dew point tweaks needed to account for the slightly
drier current conditions.


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

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.


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

Weak area of low pressure centered over far southern California will
lead to relatively light diurnally driven winds along with clear
skies next 24 hours at the terminals. No aviation impacts through
Sunday evening.



Tuesday through Saturday...A ridge of high pressure begins moving
into the Desert Southwest on Tuesday for the beginning of a warming
trend. Expect most lower elevations to reach 100 by Wednesday.
Additional warming will make for well above normal temperatures
Friday into Saturday. Minimum humidities will be in the 10-15% range
on the lower elevations (dipping to single digits east of the Lower
Colorado River Valley by Friday). Overnight recovery will be fair to
good. No strong wind events are expected. Anticipate Wednesday to
have the strongest afternoon breezes and Thursday the lightest.
Otherwise, familiar warm season wind patterns can be expected.


Spotter activation is not expected.




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