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923
FXUS65 KPSR 230933
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
233 AM MST Mon Sep 23 2024

.SYNOPSIS...
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Strong high pressure will build into the region over the next
couple of days resulting in a quick warm up and temperatures
reaching well above normal by Wednesday. High temperatures topping
105 degrees are expected over a good portion of the lower deserts
by Wednesday and likely lasting through the rest of the week. 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 clear to mostly clear skies
persisting all week.
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&&

.DISCUSSION...
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Record high temperatures later this week are looking even more
likely, particularly in the Phoenix area. The broad troughing
over the region that has brought several days of cooler
temperatures will begin to collapse today, while an unseasonably
strong high pressure ridge shifts onshore across the Pacific
Northwest and northern California. The ridge is then forecast to
spread over all of the Interior West starting Tuesday with the
ridge center tracking across the Great Basin. For our region,
heights aloft will quickly rise into Tuesday allowing the heat to
quickly build under continued dry atmospheric conditions. Forecast
H5 heights for our area are expected to rise from the current
583-585dm to 587-589dm on Tuesday before even climbing a bit
higher later this week.

Highs today are likely to top 105 degrees over portions of the
western deserts to between 100-104 degrees in the Phoenix area. As
the ridge settles over our region starting Tuesday, even more of
the lower deserts will see highs topping 105 degrees. Moderate
HeatRisk will also expand over the majority of the area starting
Tuesday. Over the past few days, NBM forecast temperatures have
continued to trend up for this heat event, particularly for the
latter half of the week. Starting Wednesday, NBM guidance now
shows the majority of the lower deserts reaching between 105-110
degrees with expansive areas of Major HeatRisk across south-
central Arizona centered over Phoenix. As a result, we have issued
an Excessive Heat Watch for the Phoenix metro area from Wednesday
through Saturday. Daily record highs for Phoenix starting
Wednesday and likely through the upcoming weekend are likely to be
threatened with NBM probabilities of tying records between 60-80%
each day. Records in Yuma and El Centro are roughly three degrees
higher than Phoenix during the period, so the probabilities of
those two cities tying records are closer to 20%.

This early fall heat episode will last for several days, likely at
least into the weekend as the weather pattern becomes blocked
with cut-off lows developing on either side of the ridge. The
ridge center could meander a bit through the period, but ensemble
guidance shows a high correlation of H5 heights staying around
588-590dm through at least Saturday, possibly longer. Some
members are still trying to have the upper level low move closer
to or even into southern California starting this weekend, but
overall trends have been more toward keeping the ridge the
dominant feature over our region into next week. At this point, it
is difficult to say how long this heat episode will last, but
there is at least some hope as the NBM highs for now start
trending away from the 110 degree mark by around Sunday. Either
way, we should keep temperatures well above normal into next week
with little to no change in moisture levels.
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&&

.AVIATION...Updated at 0540Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;
No aviation weather concerns will exist through the TAF period
under mostly clear skies. A prevailing easterly wind component is
expected through much of the period. The primary forecast
uncertainty is whether a westerly component will develop late
tomorrow afternoon into the early evening. Confidence is low to
moderate that W winds will take hold at KPHX for several hours.
Speeds should remain aob 6 kt sustained through the period, though
there could be a stronger surge of easterly winds tomorrow
morning.

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 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...
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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.
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&&

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

CA...None.
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&&

$$

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