062
FXUS65 KPSR 250533
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1033 PM MST Tue Sep 24 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
An unseasonably strong high pressure system building over the region
will strengthen through the weekend bringing a return of excessive
heat to much of the region. In fact, temperatures resembling the
middle of June will be more common through early next week. Dry air
will also remain entrenched over the area well into next week
resulting in mostly clear skies and a lack of rainfall.

&&

.DISCUSSION...
An unseasonably strong high pressure ridge will build and continue
to strengthen into the SW Conus and Rockies over the next several
days as the entire flow pattern across the country becomes more
blocked. An essentially stagnant synoptic pattern will take hold
across the country later this week with zonal jet energy retreating
into southern Canada yielding a near persistence forecast across the
CWA into at least the weekend. As a result of the evolution, H5
heights currently analyzed around 586dm this morning will rise
closer to a 588-591dm range during the latter half of the week with
excellent agreement among ensemble members that H7-H8 temperatures
feeding the boundary layer will reach the 99th or greater percentile
of climatology for this time of year. Correspondingly, guidance
spread remains extremely narrow yielding excellent forecast
confidence that surface readings will warm into a 6F-12F above
normal range. Areas of Major HeatRisk will become fairly widespread
Wednesday, initially focused more over south-central Arizona due to
the ridge center setting up over New Mexico, however then expanding
westward as the breadth of high pressure expands.

With numerical guidance continuing to push temperatures higher with
each model run over the last several iterations due to blended
models finally catching up to the majority of operational and
ensemble members, the hottest days and most expansive major HeatRisk
appears to be on tap Friday and Saturday. Forecast confidence is
excellent that multiple days of temperatures nearing and exceeding
record territory will be common through the latter half of the week
(see Climate section). In fact, its becoming a near certainty that
Phoenix Sky Harbor will experience its latest 110F day on record
(since 1896) as the previous latest 110F was September 19, 2010.
Given the increased magnitude and expansion of major HeatRisk, have
expanded the warning across much of the CWA with the Friday-Saturday
time frame highlighted as the most impactful.

Despite the mandated, blended NBM suggesting cooler temperatures
arriving early next week, the majority of ensemble members suggest
this blocking pattern persisting with anomalously high H5 heights
remaining stationed over the SW Conus into the middle of next week.
Thus, there is now higher confidence this early autumn heat episode
will continue through the weekend and perhaps well into next week
with some expectation that the ongoing warning may very well be
extended. With a large subset of models indicating H5 heights being
maintained above 590dm into the middle of next week, many in the
region may be dealing with record or near record temperatures well
into the middle of next week.

&&

.AVIATION...Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather concerns are expected through the TAF period. A
prevailing easterly wind component has begun to take hold across
the terminals and will persist through at least tomorrow morning.
A surge of SE`rly winds is expected between 16-19Z, with gusts
into the mid teens, before directions settle more out of the SSE
heading into the afternoon. Confidence in exact wind directions by
late tomorrow afternoon is once again low, and the evolution may
be somewhat similar to what was observed over the past few days
(VRB to TEMPO SW/W for several hours). A FEW-SCT deck of clouds
aoa 12 kft will begin moving overhead tomorrow afternoon and
persist through the evening, otherwise, skies will be clear.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are expected through the next 24
hours under clear skies. Winds will generally favor a westerly
component, with extended periods of light and variable winds
throughout the period.

&&

.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
Sep 30    107 in 2010    109 in 2012    108 in 2020

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Thursday to 8 PM MST Saturday
     for AZZ530-533-534.

     Excessive Heat Warning from 10 AM Wednesday to 8 PM MST Sunday
     for AZZ537>544-546-548>551-553>555-559.

     Excessive Heat Warning from 10 AM Friday to 8 PM MST Saturday
     for AZZ531-532-535-536.

CA...Excessive Heat Warning from 10 AM Thursday to 8 PM PDT Saturday
     for CAZ569.

     Excessive Heat Warning from 10 AM Friday to 8 PM PDT Saturday
     for CAZ566-567-570.

&&

$$

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