-- Discussion containing changed information from previous version are highlighted. --
447
FXUS65 KPSR 232344
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
444 PM MST Mon Sep 23 2024

.UPDATE...
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Updated 00z Aviation Discussion.
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&&

.SYNOPSIS...
Strong high pressure will build into the region over the next couple
of days resulting in rapid warming and temperatures reaching well
above normal by Wednesday. An Excessive Heat Watch has been issued
for the Phoenix area for Wednesday through Saturday. Dry air will
also remain entrenched over the area this week with mostly clear
skies persisting.

&&

.DISCUSSION...
Residual positively tilted troughing will finally exit the region
over the next 24 hour replaced by anomalous high pressure ridging
folding equatorward from the Pacific NW as the entire Conus pattern
transitions towards a higher amplitude blocking regime. By the
middle of the week, pronounced ridging approaching 2 normalized
standard deviations above normal will extend from Arizona through
the northern plains yielding an expansive area of much above normal
temperatures. With H5 heights over the forecast area hovering very
near 588dm, high temperatures 5F-10F above normal should be common
across the CWA during the middle and end of the week. Numerical
guidance seems to have consolidated into a narrow range yielding an
increasingly large area of major HeatRisk across south-central
Arizona and high probabilities of multiple record high days in
Phoenix. In fact, it would not be out of the question for KPHX to
reach 110F during the latter half of the week which would easily be
the latest recorded in the period of record (since 1896 the latest
110F day in Phoenix is Sep 19 2010). While much above normal
temperatures will also occur in SW Arizona and SE California,
readings should not be quite as excessive and remain a few degrees
short of records.

This early autumn heat episode will likely be prolonged through the
weekend as the weather pattern remains blocked with a deep cutoff
low meandering around the middle Mississippi River valley forming
the base of an eastern Conus Rex block. Although the SW Conus ridge
center could oscillate a bit into the weekend, ensemble guidance
shows a high correlation of H5 heights remaining around 588dm with
some guidance indicating a potential strengthening above 590dm. Not
surprisingly, trends in NBM forecast temperatures have continued to
increase during the weekend pointing towards a more persistence type
forecast instead of any substantial cooling. There remains a
minority of ensemble members suggesting a deeper low pressure system
entering central/southern California early next week, however the
preponderance of modeling shows a general maintenance of anomalous
ridging over the Southwest. As has been the case the past several
days, it would be fully expected that the mandated official NBM
forecast will warm in future iterations such that any notable
cooling would not enter the region until the middle and end of next
week. Regardless, there is still essentially no evidence of any
measure of moisture return such that rainfall chances the remainder
of September are almost nil.

&&

.AVIATION...
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Updated at 2340Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation weather concerns are expected during the TAF period
under mostly clear skies. A briefer than usual SW/W wind shift is
anticipated late this afternoon into this evening, but a
prevailing easterly component should reestablish by 04-05Z. If
winds do not fully establish out of the SW/W over the next few
hours, they may instead continue to exhibit southerly variability
and somewhat erratic behavior. Speeds throughout the period
should remain light (mainly aob 6 kt sustained), but occasional
gusts into the teens were observed this afternoon and may persist
over the next few hours. Another weak, briefer than usual W`rly
shift is anticipated once again tomorrow around the same time.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns expected over the next 24 hours.
Northwesterly winds will become favored within the next few hours
(between 02-03Z), followed by a light shift out of the northeast
tomorrow morning a few hours after sunrise. Periods of
variability will be common at both terminals, particularly during
the daytime tomorrow at KIPL. Clear skies will prevail through
Tuesday afternoon.
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&&

.FIRE WEATHER...
High pressure will build into the region over the next couple of
days resulting in warmer temperatures and continued dry
conditions. Above normal temperatures with lower desert highs
topping 100 degrees are expected today with many locations topping
105 degrees by Wednesday, or near 10 degrees above normal. MinRHs
will fall to around 10% over the lower deserts to 15-20% over
higher terrain areas. Winds are expected to remain light through
at least mid week, generally following diurnal wind patterns.
Much of the same is expected through the rest of the week as the
high pressure system keeps above normal temperatures and dry
conditions over the region.

&&

.CLIMATE...

Record highs this week:

Date        Phoenix         Yuma         El Centro
----        -------         ----         ---------
Sep 25    108 in 1989    112 in 1963    110 in 2015
Sep 26    108 in 1989    111 in 2010    110 in 2010
Sep 27    107 in 2009    111 in 1963    111 in 2009
Sep 28    108 in 1992    108 in 1994    109 in 2009
Sep 29    107 in 2003    110 in 1980    110 in 1969

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Wednesday morning through Saturday
     evening for AZZ537-540>544-546-548>551.

CA...None.
&&

$$

DISCUSSION...18/Kuhlman
AVIATION...Whittock
FIRE WEATHER...Kuhlman
CLIMATE...18