Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171254

Area Forecast Discussion
National Weather Service Phoenix AZ
554 AM MST Sun Dec 17 2017

A compact weather disturbance will bring chances for light rain and
mountain snow to the eastern half of Arizona today into early
Monday. Temperatures will cool substantially with this system
bringing the coolest temperatures of this winter so far. Beyond
Tuesday, dry conditions with some warming will settle over the area
during the middle of the week before another potentially colder
weather system swings into the region by the end of the week.


Early this morning, an impressive upper low continued to develop and
spin across the northern Gulf of CA; this low will bring scattered
showers along with a few thunderstorms to southern Arizona today
with showers decreasing from the west tonight into Monday. IR
imagery at 2 am showed quite a bit of lightning activity associated
with the low as thunderstorms formed in the difluent area to the
northeast of the low center. So far most of the CAPE (surface
based/most unstable/MLCAPE has been focused south of Arizona and
that is where most of the storm activity has been as of 2 am, but
there are isolated storms increasing over far southern AZ, moving
north and they will likely be entering portions of southern Maricopa
and northern Pinal Counties by sunrise.

Guidance has become consistent in calling for the upper low center
to track just south of the Arizona border today into tonight, with
by far the strongest dynamics/UVV fields/difluence & PVA/ focused
across southeast Arizona in areas mainly from Phoenix east and
south. Models are still all over the map regarding moisture and POPs
for today, but the preponderance of POP guidance still suggests a
good chance of showers over portions of south central Arizona
including southeast Phoenix. POPs are into the likely category for
southern Gila County where orographics can help to play a role. We
still expect this event to be a high POP but low QPF scenario, and
current SREF Plumes guidance suggest meager QPF in the Phoenix area
with most locales to see less than one tenth of one inch. There is
just enough CAPE, with MLCAPE values approaching 250 j/kg, to
warrant isolated storms mainly this morning and rain totals could
approach one quarter of an inch near any of the stronger storms that
form. The clouds/showers as well as thickness cooling will result in
a much cooler day today with highs in the lower deserts falling into
the low to mid 60s, and despite significant clearing Monday temps
will likely change little, maybe rising one or two degrees.
Overnight tonight there will be a gradual clearing trend with
decreasing showers as the low slowly wobbles off to the east, and
south central AZ comes under a much more subsident regime behind the
main low. There will be lingering wrap around moisture but weak UVV
over the higher terrain east of Phoenix even into the day Monday so
we will keep continued slight chances going thru the afternoon hours

For Tuesday into Wednesday, a flat upper ridge will build into the
area leading to a gradual, modest warming trend under mostly sunny
skies. High temps over the warmer lower deserts should reach into
the low 70s by Wednesday.

For the latter portion of the extended period, Thursday into the
weekend, we are looking at cooler but likely continued dry
conditions area-wide. Operational runs from the GFS and ECMWF have
actually become very similar and both initially drop another closed
upper low through the four corners area Thursday then quickly and
progressively shift the low off to the east leaving behind a dry but
cool northwest flow aloft for Friday. Little change is advertised
Saturday as a high amplitude ridge remains in place near the west
coast while a monster upper low becomes situated over most of the
CONUS, centered over the central upper midwest. A much cooler
airmass will be ushered into the area Thursday dropping highs back
into the low to mid 60s over the lower deserts with little change
expected Friday. Slight warming may then occur Saturday.

It should be noted that despite the good agreement between
operational GFS and ECMWF for the extended, there continues to be
massive flip-flopping around between the models run to run and day
to day, with significant disagreement between most of the GEFS
ensemble members. There is still the potential for a large closed
low to take shape somewhere over the desert southwest, or even near
or over Arizona, by later in the week allowing for even cooler temps
as well a potential for precipitation. Given the uncertainty, we do
have single digit POPs in the forecast each day with values
approaching 10 percent over the higher terrain east of Phoenix.
Hopefully, over the next couple of days our guidance can lock in on
the real solution for the second half of the week, whatever that may


South-Central Arizona including KPHX, KIWA, and KSDL:
Scattered showers, isolated thunder, variable shower-driven winds
and lowered cigs will continue to be the primary aviation weather
concern through midday. South-southeast winds will generally
prevail, however more variable and sometimes gusty winds will
result from passing showers. CIGs will drop 4-5kft in the heaviest
showers, but generally range 7-10kft in between shower bands.
Shower activity transitions east of Phoenix during the afternoon,
with the remote possibility of showers wrapping around from the
northeast across the Metro. Maintained VCSH through the early
evening. After shower activity wraps, mid-level clouds will linger
and winds will be on the lighter side, with slight easterly
drifts to headings if not periods of variable headings.

Southeast California/Southwest Arizona including KIPL and KBLH:
Gusty north winds to continue today as an upper low moves through
Arizona, wrapping northerly flow on the back side of the system.
Gusts around 20kts will be possible for both KBLH and KIPL with
skies clearing into the afternoon. Mid-level cloud layers will
generally stay above 10kft for the day. Airfield conditions will
stay dry as the focus for precipitation will stay east across

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Winds will be light and variable with subtle trends toward
drainage slopes and diurnal patterns. Dry northwest flow will
linger for Tuesday and Wednesday before another colder and more
potent storm system sweeps in from the northwest. Breezy west-
northwest winds will develop overnight Wednesday and into
Thursday, impacting area ridgetops first then mixing into the
lower desert elevations during the daytime. Model forecasts have
backed off any precipitation chances with this system, so
conditions will stay mostly dry over the Districts. Daytime
humidities will be range higher than the last few weeks, 15 to 20
percent for the lower elevations and 30 percent or greater for the
higher terrain. After temperatures rebound in the early week to
slightly above normal readings, they will drop to near normal
readings (60s for the deserts and 40s/50s for the foothills/higher
terrain) by Thursday into the coming weekend.


Spotters should follow standard reporting procedures.




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