868
FXUS65 KPSR 070537
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1037 PM MST Sun Oct 6 2024

.UPDATE...Updated 06z Aviation Discussion...

&&

.SYNOPSIS...
Temperatures will remain above the seasonal normal through the week,
however exhibiting a slow cooling trend during the latter half of
the week. Nevertheless, afternoon highs near record levels are likely
through the middle of the week before a somewhat cooler airmass
arrives. A weak disturbance late in the week could yield some light
showers over mountainous areas of eastern Arizona.

&&

.DISCUSSION...
A North American longwave pattern featuring sprawling subtropical
ridging over the southwest 2/3rd of the Conus with negative height
anomalies across SE Canada and the NE Pacific will remain nearly
stationary through much of the week due to downstream blocking over
the North Atlantic. This pattern will only begin to dislodge late in
the week as the Atlantic block begins to retrograde poleward and
stronger Pacific energy pushes onshore acting to dampen the
anomalous ridging over the Southwest. However, until this occurs,
strong subsidence with only gradually decaying H5 heights will
ensure a continuation of much above normal and record breaking
temperatures across the forecast area.

Over the next 72 hours, strong high pressure with H5 heights 585-
590dm will be maintained over the region, albeit slowly weakening
with time. Temperature guidance spread is very narrow with excellent
confidence of more record breaking temperatures, though confidence
is also good that readings each day will be a degree or two below a
persistence forecast. By the middle of the week, models continue to
advertise the southern extent of a positively tilted trough axis
leaking into the SW Conus further eroding midtropospheric heights
and pushing HeatRisk back into a moderate category and below
excessive warning thresholds. While jet structure and vorticity
advection suggests increased ascent over the region, any imported
moisture will be relegated to 600mb and above likely yielding only
areas of virga.

While there is good ensemble agreement depicting an East Pacific
shortwave moving into the western Conus at the end of the week,
greater uncertainty exists with respect to the magnitude and depth
of this feature. It`s influence should keep midlevel heights from
rebuilding substantially, although all guidance does suggest
temperatures persisting in an above normal range through the
weekend. The evolution of this shortwave into the Rockies appears to
be dictated by the resolution of hemispheric blocking with many
ensemble members attempting to form some temporary Rex block feature
over the western or central Conus. Not surprisingly given this
blocking pattern, there is a wide range of model solutions with a
growing minority of members suggesting a weak cutoff low forming the
block base near the CWA, but a larger subset indicating a more
sheared and northward displaced feature. This outcome would favor a
warmer regime with temperature anomalies still around 10F above
normal into the beginning of next week.

&&

.AVIATION...Updated at 0535Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No major aviation weather concerns under FEW mid and high-level
clouds are expected through the TAF period. Easterly winds will
prevail into Monday afternoon with sustained speeds generally
under 10 kts although there will likely be a period mid to late
Monday morning of some occasional gusts into the mid to upper
teens before subsiding during the afternoon hours. There is
uncertainty of whether or not wind directions late Monday
afternoon/early evening switch out of the west with very light and
variable winds more likely.


Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather concerns are anticipated through the TAF
period. Extended periods of light and variable winds are expected.
Clear skies will give way to some passing high clouds heading into
Monday afternoon.

&&

.FIRE WEATHER...
Very dry and extreme heat conditions will continue for many areas
through Tuesday with lower desert highs ranging between 104-112.
MinRH values in the afternoons in the 5-10% range will be common
through at least Tuesday, while MaxRH readings are expected to be
between 20-30% with locally higher values in Yuma and Imperial
Counties. The driest conditions are expected today through Monday
afternoon, with RH values under 25% in most areas for the next 36-48
hrs. An increase in winds above the surface during the overnight
hours, this morning and again tonight/Monday morning may lead to
very localized elevated fire weather conditions in South-Central AZ.
Easterly winds during the overnight hours on and near ridgetops will
be capable of reaching 25-35 mph at times, mostly in the mountains
east of Phoenix.

There will be a very slight increase in moisture beginning midweek
that may keep MinRH values from falling into the single digits,
but most areas will still see 10-20% readings in the afternoon
through the end of this week. There will also be potential for
some regional rain showers with the moisture increase, but at this
time the chances are very low (5% or less) across the lower
deserts and CWR is near zero. Winds, beyond Monday, will follow
familiar diurnal trends, with occasional afternoon gusts near
15-25 mph.

&&

.CLIMATE...

Record highs through Friday:

Date        Phoenix         Yuma         El Centro
----        -------         ----         ---------
Oct 6     105 in 1917    108 in 1980    106 in 1964
Oct 7     104 in 1991    108 in 1987    105 in 1991
Oct 8     104 in 1987    107 in 1996    106 in 1996
Oct 9     103 in 1996    106 in 1996    104 in 1996
Oct 10    105 in 1991    107 in 1991    105 in 1996
Oct 11    102 in 1991    107 in 1950    106 in 1995

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ530>536-538-
     539-553>555-559.

     Excessive Heat Warning until 8 PM MST Tuesday for AZZ537-540>544-
     546-548>551.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ562-566-567-
     569-570.

&&

$$

DISCUSSION...18
AVIATION...Lojero
FIRE WEATHER...Benedict
CLIMATE...18