701
FXUS65 KPSR 222343
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
443 PM MST Sun Sep 22 2024

.UPDATE...Updated 00z Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will steadily warm the next few days reaching into an
above normal category as strong high pressure builds over the
region. Dry air will also remain entrenched over the area yielding
little to no chance for precipitation through the entirety of the
week.

&&

.DISCUSSION...
Lingering positively tilted troughing across the SW Conus will give
way to high amplitude ridging over the next 48 hours as the entire
western hemisphere flow pattern begins to exhibit a blocking regime.
Anomalous H5 high pressure over the East Pacific will translate into
the western Conus with an expansive area of heights in excess of
588dm extending from Arizona into the northern Rockies by Tuesday.
While a strong northern stream zonal jet will eventually erode
heights along the Canadian border during the latter half of the
week, blocked flow across the southern Conus will allow anomalous H5
heights to be maintained across the forecast area. Ensemble
memberships offers little deviation from this pattern evolution with
the only notable uncertainty tied to the magnitude of heights aloft
with some indicators suggesting H5 levels reaching 590dm. Not
surprisingly, forecast temperature guidance during the middle and
end of the week has increased such that previous 75th-90th
percentile output is now closer to the mean while also incorporating
a much narrower spread. Thus, confidence is very good that high
temperatures 5F-10F above normal will be common with some areas
easily flirting or exceeding record levels (see Climate section).
This forecast results in an increasingly expansive area of major
HeatRisk, albeit more a result of the seasonal climatology and
method of calculation rather than a shear magnitude compared to what
was experienced over the past 4 months.

By the weekend, model spread increases considerably with some
members attempting to dampen or dislodge the ridge in response to a
cutoff low along the southern California coast migrating inland.
However, a majority of members maintain ridging over the SW Conus,
and even suggest some increase in H5 heights above 590dm due to the
continuation of Conus blocking. While the influence of this
potential cutoff low would bring slight cooling, there is growing
evidence against this outcome with somewhat better confidence
towards persistence of much above normal temperatures. The overall
uncertainty is depicted in widening guidance spread with the
mandated official NBM output highlighting a median forecast, but
likely not the eventual outcome. Regardless of the pattern
evolution later this week and weekend, forecast confidence is
excellent that dry conditions will continue to prevail.

&&

.AVIATION...Updated at 2345Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;
No aviation weather concerns will exist through the TAF period
under mostly clear skies and distant daytime cumulus to the
north. Winds have finally shifted W this afternoon after a period
of variability, and an E`rly shift is expected earlier than usual
across the terminals this evening (between 04-06Z). Tomorrow,
winds will again struggle to shift W during the late
afternoon/early evening and may instead exhibit variability before
reestablishing out of the E. Speeds should remain aob 7 kt
sustained through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are anticipated through the next 24
hours under clear skies. Winds will favor W-NW this evening and
overnight, and directions will begin to favor NE by tomorrow
morning. Speeds should remain light, mostly aob 6 kt. Extended
periods of variability or calm winds will be possible,
particularly at KIPL.


&&

.FIRE WEATHER...
High pressure will gradually build over the region early this week
resulting in warmer temperatures and dry conditions. Above normal
temperatures with lower desert highs topping 100 degrees are
expected starting Monday. MinRHs will fall from 10-15% over the
lower deserts today to 8-12% starting Monday with higher terrain
areas seeing up to 20%. Expect a more traditional diurnal wind
pattern across the region over the next several days with little
to no breeziness. 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...None.
CA...None.
&&

$$

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