Scientific Forecaster Discussion
fxus65 kpsr 261208 aaa
Area forecast discussion...updated
National Weather Service Phoenix Arizona
510 am MST Tue Sep 26 2017
Update...updated aviation and fire weather discussions.
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 mainly in the higher elevations. Warmer temperatures will
materialize by the end of the work week and persist through early
next week &&
A broad upper level trough is currently situated over the West
Coast with ridging just downstream centered over the eastern half
of the United States. This is bringing below average temperatures
to many locations on the West Coast while just the opposite is
being observed in the eastern US. Only a few cirrus clouds can be
seen on satellite this morning thanks to the extremely dry air
overhead. The latest 00z sounding from Tucson shows pwats of only
0.28 inches, just above yesterday's record minimum of 0.22! With
the lack of water vapor, temperatures have again cooled quickly
with morning lows forecast to be near to slightly above what was
observed Monday morning.
Heading into this afternoon, another below average day is expected
as 850 mb temperature warm just slightly over yesterday.
Subsequently, many locations will only see a degree, maybe two
of warming from Monday. Other than a few more high clouds, a very
similar day is expected.
By midweek, moisture gradually seeps in from the east as energy
shears off from the aforementioned trough and develops into an
upper level low near the California/AZ/NV border. Some hi-res models
show this more moist air nearing Maricopa County by late
Wednesday, but is positioned farther east than what models were
indicating 24 hours ago. Nonetheless, Gila County looks to have a
shot at some showers and storms come Wednesday afternoon although
activity in Phoenix is looking rather slim (<10%), especially
given the southwest steering flow that will quickly push off any
storms to the north and east. Locations west of the Phoenix Metro
have virtually no chance of precipitation.
Isolated afternoon storms in eastern Gila County look to persist
through at least the end of the week until drier air moves in over
the weekend. All the while, temperatures will gradually warm and
approach the century mark in the lower deserts come Saturday as
500 mb heights gradually recover from the exiting upper level low.
By Sunday, models show a passing trough to our north that may
bringing subtle variations to our temperatures although nothing
that deviates far from climatology.
south-central Arizona including kphx, kiwa, and ksdl:
Expect rather light winds next 24 hours following typical diurnal
tendencies - morning east wind becoming west after 20z, returning to
the east well after midnight. Variable amounts of high clouds,
mainly few-scattered with some mainly thin broken decks at times, all above
20k feet. No aviation concerns at any terminal for at least 24 hours.
Southeast California/southwest Arizona including kipl and kblh:
Expect relatively light winds next 24 hours, mostly less than 10kt,
favoring the north at kblh and the west/northwest at kipl. Few-scattered
high cloud decks at times. No aviation concerns for at least 24
Aviation discussion not updated for amended tafs.
Thursday through monday: a large area of low pressure will dominate
the weather pattern through Thursday, keeping high temperatures
below seasonal normals, and drawing a bit of moisture northward and
into the higher terrain of eastern Arizona, allowing for a slight
chance of afternoon and evening thunderstorms over higher terrain
areas east of Phoenix. The low will weaken Friday into the weekend
allowing high temperatures to climb above seasonal normals with
hotter lower deserts reaching around 100 by Saturday. Humidity
levels will stay relatively low through the period, mostly below 15
percent over the deserts and below 30 percent in the higher terrain.
Storm chances will diminish Friday into Saturday, with just isolated
storms possible over higher terrain areas east of globe; by Sunday
drier air will move in mostly eliminating any threat for convection.
Winds each day will stay relatively light, favoring north to west
during the afternoon hours, with stronger winds possibly Sunday,
becoming breezy from the southwest during the afternoon on that day.
Spotter information statement...
spotters should follow Standard reporting procedures.