Area Forecast Discussion
Issued by NWS Phoenix, AZ

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537
FXUS65 KPSR 251258 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
600 AM MST Mon Sep 25 2017

.UPDATE..Updated aviation and fire weather discussions.

&&

.SYNOPSIS...
Under sunny skies, temperatures will slowly warm over the next few
days though readings will remain slightly below average. An area of
low pressure may pull moisture back into eastern Arizona by the
middle and end of the week supporting some late season showers and
storms.

&&

.DISCUSSION...

Clear skies are overhead this morning as dry air on the backside
of an upper level trough, whose axis is currently positioned
squarely over Arizona, continues to filter moisture starved air
into our region. It is because of this trough that we have seen
such pleasant and anomalously cool weather lately and certainly
the coolest temperatures so far this season. Current temperatures
are running similar to what they were at this time yesterday which
should allow lows to bottom out in the lower 60s for Sky Harbor
with widespread mid to upper 50s for the surrounding areas.

Monday afternoon is forecast to be yet another exceptionally cool
day as the upper level trough remains overhead. Although H5
heights will remain nearly unchanged today, 850 mb temperature are
forecast to warm by several degrees as the airmass modifies under
the late September sun. This should bring temperature up to near
90 for almost all lower desert locations today which is still 7-10
degrees F below normal. Expect this trend to continue throughout
the week with upper 90s returning by the end of the work week.

Preceding this however, a portion of the aforementioned trough
will get cut off from the main flow and position itself somewhere
in the southwest US (there is still a fair bit of uncertainly
between models in the exact position). This will act to gradually
draw in moisture bringing a chance of thunderstorms to locations
mainly east of Phoenix starting Wednesday. The most bullish
solution (NAM) has approximately 1000 J/kg of CAPE in Phoenix
Wednesday afternoon with little to no CIN. This is in large
contrast to the GFS which is much drier (0.4 PW), has absolutely
no convective energy over Phoenix, and keeps activity over
elevated areas of eastern Arizona. For now, have a slight chance
of PoPs in Phoenix on Wednesday but that may certainly change if a
drier solution ends up presenting itself. Moisture looks to stick
around in the higher elevations of eastern Arizona through
Saturday with the upper level flow becoming more convoluted beyond
that. For example, the GFS depicts a glancing blow from an upper
level trough Sunday which would put us under a dry northwesterly
flow aloft while the ECMWF is quite the opposite, showing a more
moist southeast flow with ridging to our east. While there is
little agreement in the upper level flow pattern, slightly above
normal temperatures look to be a safe bet in our extended
forecast.

&&

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

Very dry conditions will affect the terminals for at least the next
24 hours, allowing for clear or mostly clear skies. Will go with FEW
to SCT cirrus decks but do not expect significant high cloudiness
during the period. Winds to stay rather light across south-central
Arizona, and should follow typical diurnal patterns with morning
east wind, becoming west by 19Z or so then returning to the east
after midnight.

A bit stronger winds out north today, with locally breezy conditions
developing during the morning, peaking late morning to early
afternoon and favoring the north at both KBLH and KIPL. Could see
occasional gusts over 20kt before winds taper off later this
afternoon. Wind to favor northwest tonight into Tue morning.
Otherwise no aviation concerns for at least 24 hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday: A large area of low pressure aloft will
dominate the weather pattern over the desert southwest for the rest
of the work week, keeping temperatures below seasonal normals, and
keeping drier air in place over southeast California and the
southwest Arizona deserts. Expect minimum RH values from the single
digits to low teens over the central deserts Wednesday, slowly
climbing mostly into the teens by the end of the work week.
Southerly flow ahead of the upper low will gradually advect moisture
into south central Arizona by Wednesday, bringing a chance for
afternoon and evening thunderstorms to most areas east of Phoenix.
There will initially be a slight chance for storms into the greater
Phoenix area Wednesday but afterward moisture will retreat slightly
to the east, focusing any chance for storms into the higher terrain
areas east of Phoenix Thursday into the weekend. Minimum RH values
will stay mostly in the mid to upper teens Friday into the weekend,
with values over 30 percent in the higher terrain. As the upper low
weakens this weekend, high temperature climb to around seasonal
normals with warmer deserts into the upper 90s. No strong winds are
expected during the five day period, just some minor and typical
afternoon gustiness favoring north to upslope.

&&

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

&&

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

$$

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DISCUSSION...Wilson
AVIATION...CB
FIRE WEATHER...CB



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