545
FXUS65 KPSR 172129
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
229 PM MST Tue Sep 17 2024

.SYNOPSIS...
A series of upper-lvl troughs progressing through the Desert
Southwest will result in temperatures around 5 to 8 degrees below
normal for the remainder of the workweek. Highs will range from
the upper 80s to mid 90s and lows will be in the 60s and 70s
across the lower deserts through at least Saturday. A gradual
rebound in temperatures with lower desert highs returning to the
triple digits is anticipated late this weekend into early next
week. Dry weather will persist for the next several days.

&&

.DISCUSSION...
Latest mid-lvl WV imagery and 500 mb streamline analysis depicts a
shortwave trough progressing through the Intermountain West. The
trough axis is located directly over our forecast area, resulting in
a transition from southwesterly flow aloft to dry westerly flow.
This dry air will continue to filter into southcentral AZ this
afternoon, keeping any rain chances well north and east of the
region. The passing trough has also resulted in lowering of 500 mb
hghts to below 582 dam. Coincidentally, temperatures will be much
cooler this afternoon with most lower desert communities seeing
highs in the low to mid 90s or around 5 degrees below normal. Of
importance, the record streak of 113 days of 100 degree or hotter
high temperatures in Phoenix should finally come to an end. Another
dry and mild night is anticipated with temperatures settling into
the mid 60s to low 70s across the region.

In the wake of the current upper level trough, our region will
remain positioned within a considerably drier and cooler air mass
as a secondary shortwave trough begins to deepen over the West
Coast during the Wednesday-Friday timeframe. Mean 500 mb hght
anomalies will remain around 1-2 sd below normal through late-week
resulting in seasonably mild temperatures. Highs will continue to
be around 5 to 8 degrees below normal, mainly in the low to mid
90s across the lower deserts through Friday. No rain chances are
anticipated with the next low pressure system as moisture
continues to be scant with PWAT values remaining well below 1.0".
The main weather impact with the next trough will be locally windy
conditions both Wednesday and Thursday afternoon, particularly
across western Imperial County. Otherwise, just general afternoon
breeziness can be expected.

This weekend into early next week, overall troughing will begin to
weaken with more of a zonal pattern taking shape. Global ensembles
show increasing 500 mb hghts over the Desert Southwest, with mean
values up to 584-587 dam on Sunday. As hghts begin to rise,
temperatures are expected to trend back up to near normal, possibly
reaching above normal by early next week. Probabilistic NBM output
shows an 88% chance of the high temperature at Phoenix Sky Harbor
Airport reaching or exceeding 100 deg F on Sunday and a 98% chance
by Monday (25% or lower chances each day before Sunday).

It is important to note that cluster analysis reveals some key
uncertainties Sunday onward. This is indicative by the larger
spreads in the 25th-75th percentile NBM temperature forecast.
This is in part due to large differences in the amplitude of the
ridging and its eastward extent early next week. About 10% of the
ensemble members comprise a cluster that shows a negative height
anomaly dipping down into the Intermountain West, which would
likely continue to moderate temperatures in a near normal range.
In any case, after this morning, rain chances remain essentially
zero across the CWA through the next 7 days, and our below normal
temperatures are likely to come to an end over the weekend.

&&

.AVIATION...Updated at 1804Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will tend to follow typical diurnal patterns, with speeds
mostly aob 10 kts, under mostly clear skies. Periods of
variability are expected at all terminals in the morning and up
until the more defined W/SW wind by 20-22Z.

Southeast California/Southwest Arizona including KIPL and KBLH:
Clear skies and light winds will present no aviation concerns
through the period. Expect westerly winds to prevail at KIPL,
while KBLH will favor NW in the mornings before shifting SW. Both
terminals will see periods of light variability.

&&

.FIRE WEATHER...
Chances for wetting rainfall are expected to remain around zero
through at least this weekend, as a drier and cooler airmass
encompasses the region. Temperatures 5-10F below normal can be
expected through the end of the week. Winds will return to
generally light and diurnal patterns today, with typical upslope
gusts to 15-25 mph during the afternoon each day. Afternoon minRHs
in the teens will be common in lower elevations and closer to a
20-30% range across higher terrain. Overnight recovery will range
widely from fair to good at 30-70%. As temperatures warm closer to
normal this weekend, humidity levels will decrease further with
single digit readings more likely in lower desert communities.

&&

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

$$

DISCUSSION...Salerno/Whittock
AVIATION...Benedict
FIRE WEATHER...Whittock