Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 202337 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
435 PM MST Tue Oct 20 2020

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Temperatures will remain close to 10 degrees above normal the next
several days with a steady cooling trend arriving over the weekend.
Readings may actually fall below average for several days early next
week. An approaching weather disturbance early next week may be
capable of squeezing out some shower activity, but it is expected to
be focused mostly over the higher terrain areas north and east of
Phoenix.

&&

.DISCUSSION...
The SW Conus remains stuck in persistent weak NW flow pattern
between broad troughing across the Upper Midwest and an East Pacific
blocking ridge. H5 heights around 585dm will only slowly be eroded
through the next 72 hours as the entire longwave pattern begins a
readjustment phase and flow aloft becomes somewhat more cyclonic
ahead of energy digging into the Great Basin. As has been the case
the past several days, multi-model guidance spread is extremely
narrow yielding excellent forecast confidence and conditions
resembling almost direct persistence. Even towards the end of the
week as flow aloft strengthens, heights change very little and
guidance suggests cooling somewhat more gradual than previous
forecasts indicate.

Very little has changed with respect to forecast spread among the
suite of ensemble guidance early next week during a
realignment/retrogression of longwave features over the western
hemisphere. However, a trend towards a colder/wetter soln does seem
to be developing. First of all, a set of shortwaves is forecast to
break over East Pacific ridging, carving out deeper troughing
through the Great Basin and is expected to drive the first
substantially deep front of the cold season into the forecast area.

A second feature that will likely affect the forecast for our region
is a cut-of low that has remained quasi-stationary over the eastern
Pacific for the last few days. This low is forecast to begin moving
very slowly eastward towards the southern CA coast over the next
several days. This is a rather moisture-rich system (current PWAT
anomalies aoa 4 SD above the mean), it actually originated from the
remains of tropical storm Norbert. A significant number of the GEFS
and (especially) the EURO ensemble members attempt to phase the
energy from this EPAC system with energy from the afomentioned
northern wavetrain, producing a rather deep and cold closed-off low
over the SW US. However, the majority of GEFS and EURO emsemble
members still show a more progressive system at this time, with it
remaining further to the north, resulting in a noticeably
warmer/drier soln.

Given the still-wide ensemble spread and no preferred outcome, but
still honoring the trend towards a colder/wetter soln, going with a
forecast very close to the current NBM soln, which continues to pull
some better defined energy/cold air aloft towards the southwest, but
still keeps the entire wave train more progressive, resulting in
only a brief period of modest POPs focused over higher terrain areas
north and east of Phoenix. However with many NAEFS members
indicating some semblance of a cutoff cold core feature and the
recent tendency for blocking patterns, cannot discount the potential
for one of the much slower, potentially "wetter" scenario through
the first half of next week. Regardless of the precise outcome, much
cooler temperatures (likely falling into a slightly below normal
range) will filter into the region, whether it be more abrupt with
the rapid frontal passage or gradual with a slower system.

&&

.AVIATION...Updated at 2335Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT, and
Southeast California/Southwest Arizona including KIPL and KBLH:

Dry northwest flow aloft will continue across the region through
Wednesday evening for generally clear skies. A weak disturbance
moving through the flow may bring a bit of high cirrus to the
central deserts later tonight and into the day Wednesday with FEW-
SCT decks aoa 20k possible. Winds will be on the light side across
all of the deserts. In the Phoenix area expect typical diurnal
tendencies with speeds typically 6kt or less. Winds to shift to the
east between 04z and 06z, and back to the west again after 21-22z
Wednesday afternoon. Winds over the western deserts will be largely
light/variable next 24 hours, although they will favor south to
southeast during the afternoon Wednesday especially at KBLH. No
aviation concerns at any of the terminals for at least the next 24
hours.

&&

.FIRE WEATHER...
Friday through Tuesday:
Dry weather will prevail into the weekend with temperatures slightly
above the seasonal normal, then cooling closer to average early next
week. A fast moving weather disturbance and modest moisture increase
may support shower activity in eastern districts early next week,
however wetting rains will likely be limited in extent. Minimum
afternoon humidity levels will generally fall to 15-30% while
overnight recovery steadily improves from a 30-50% range to 40-70%.
Wind speeds will gradually increase over the weekend ahead of an
approaching weather disturbance, potentially become quite gusty on
Monday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Percha/MO
AVIATION...CB
FIRE WEATHER...Percha/MO



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