701
FXUS65 KPSR 241700
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1000 AM MST Tue Sep 24 2024

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
Record high temperatures are expected for at least the Phoenix
area during the latter half of the week and possibly into next
week. This is all due to an unseasonably strong high pressure
system that is currently building over the region and could even
strengthen further this weekend. Early to mid June like high
temperatures are forecast for much of the area starting Wednesday,
lasting through at least the weekend. An Excessive Heat Warning
has been issued for the Phoenix area for Wednesday through Sunday.
Dry air will also remain entrenched over the area well into next
week with mostly clear skies persisting.

&&

.DISCUSSION...
Even though we are now into the fall season, temperatures and
humidities through the rest of the month are expected to be more
like what we would expect during the month of June. An
unseasonably strong high pressure ridge is already pushing into
our region from the northwest, setting up what will become a
strong blocking pattern by late week as a pair of cut-off lows
develop on either side of the ridge. The high center is currently
over Oregon, but it is forecast to quickly shift southeastward
into New Mexico over the next day or so. H5 heights over our
region have already risen to around 585dm, but will rise further
to 588-590dm by Thursday and likely even higher over the weekend
as guidance continues to push heights and temperatures higher with
each model run over the last couple of days. Guidance now shows
H5 heights reaching into the 90th percentile of climatology
through the weekend, while also showing record or near record
700mb and 850mb temperatures for the period. The blocking high is
now also favored to persist into at least early next week,
probably longer. We now have high confidence this fall heat
episode will last through the weekend and medium confidence
through early next week.

Starting today, high temperatures are expected to top 105 degrees
over the majority of the lower deserts before rising another few
degrees over the south-central Arizona deserts starting Wednesday.
Since we are now into early fall and normal highs have dropped
nearly 10 degrees from their summer peak, these temperatures will
more easily reach Major HeatRisk. Areas of Major HeatRisk begin to
develop today before becoming fairly widespread on Wednesday,
focused more over south-central Arizona due to the high center
setting up over the eastern portion of the region. The Excessive
Heat Watch for the Phoenix area has now been upgraded to a Warning
and extended out through Sunday. Forecast highs starting
Wednesday will mainly top out between 105-110 degrees across the
lower deserts with the hottest days likely falling from Friday-
Sunday. NBM forecast highs for Phoenix Sky Harbor are now
eclipsing daily records each day from Wednesday through Monday
with the peak highs of 111 degrees on Friday and Saturday. It is a
near certainty that Phoenix Sky Harbor will see its latest 110
degree day ever as the previous latest 110 degree day was
September 19 in 2010. Temperature records for Yuma and El Centro
are around three degrees warmer than Phoenix, but now the NBM is
showing anywhere from a 20-60% probability of tying records for
those locations. Needless to say, do not underestimate the
upcoming heat and keep practicing proper heat safety. I wish I
could definitively say long range models are showing this
unseasonably strong ridge weakening or getting displaced at some
point next week, but so far there are no strong signs of either
one happening. In fact we may be dealing with record or near
record temperatures through the rest of next week, time will tell.

&&

.AVIATION...Updated at 1700Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation weather concerns are expected through the TAF period.
Winds will overall favor light, diurnal tendencies with extended
periods of variability. There is uncertainty in regards to whether
or not winds this afternoon will make a definitive westerly
switch. As of now, expect light and variable winds to prevail
while favoring a light westerly component at times this afternoon.
Clear skies will persist.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are expected through the next 24
hours. Winds will generally favor the west at KIPL and NNE at KBLH
with extended periods of light and variable winds throughout the
period. Clear skies will continue.

&&

.FIRE WEATHER...
Strong high pressure will settle over the region today and remain
in place at least through the weekend. This will result in well
above normal temperatures and continued dry conditions. Lower
desert highs temperatures are likely to reach 9-13 degrees above
normal by Wednesday and stay there through the weekend. MinRHs
will continue to dip down to around 10% over the lower deserts to
15-20% over higher terrain areas through the rest of the week,
while winds remain light generally following diurnal wind
patterns.

&&

.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 Warning from 10 AM Wednesday to 8 PM MST Sunday
     for AZZ537-540>544-546-548>551.

CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Smith/RW
FIRE WEATHER...Kuhlman
CLIMATE...18