Flash Flood in Arizona, December 24, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
517 AM MST Sat Dec 24 2016

.UPDATE...Updated Aviation and Fire Weather Discussions...

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.SYNOPSIS...
The next weather system will move through the region today leading
to another round of rain, isolated thunderstorms, mountain snow,
windy conditions, and much cooler temperatures. Conditions will dry
by Sunday with temperatures lingering cooler than average through
Monday. Slightly above normal temperatures will return Tuesday and
continue through much of the week.

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.DISCUSSION...
Approaching winter storm has a classic appearance on satellite, with
a divergent signature evident along with backbuilding colder tops
stretching into the Eastern Pacific. Models remain locked in on a
solution, which takes the cold front and associated precipitation
steadily through the lower deserts today and into the higher terrain
this evening. Only minor refinements were made to the forecast this
morning.

Very strong vorticity and jet-forced ascent is currently producing
widespread moderate to heavy rain across southern California. Suite
of hi-res WRFs continues to indicate this activity will become better
organized as it reaches the Phoenix area early this afternoon. Latest
model soundings suggest wind gusts to 30 to 40 mph will accompany the
line of storms. Although traditional convective parameters are
generally unimpressive, SPC objective lightning probabilities
indicate a slight chance of thunderstorms and this has been
maintained in the forecast. Although the hasty movement of the front
will generally limit rain totals, reflectivity depiction in several
of the WRFs also suggests bright banding/higher reflectivity will
produce an hour of heavy rainfall. Most likely rainfall totals
include around a tenth of an inch near Yuma and a third to a half of
an inch in the Phoenix area.

Main forecast concern in terms of sensible weather will be the
impact of the weather on travel. Snow levels will plummet from
around 6000 feet during the afternoon to 3500 feet this evening as
the trough acquires a negative tilt and colder air rushes in.
Thicknesses near 540 dam this evening will actually be lower across
central Arizona than in parts of Montana. Although the intensity of
the precipitation will drop off dramatically behind the front, deep-
layer southwesterly flow will remain favorable for orographic lift
across much of Gila County. QPF combined with the cold air surging
east will support accumulations of 3 to 5 inches above 5000 feet
before moisture is scoured from the -15C dendritic layer Sunday
morning. At lower elevations, precipitation will change over to snow
later in the night down to 3500 feet and a dusting to an inch of snow
is possible in the foothills around Globe area through Sunday
morning. The Winter Weather Advisory has been extended through 8 am
and modified to include elevations down to 5000 feet.

Snow showers will linger across the higher terrain Sunday morning. Across
the lower deserts, a drier more subsident pattern will develop as the
aforementioned trough becomes more negatively tilted and lifts into
the northern Plains. Despite the decrease in clouds, temperatures
will be roughly 10 to 15 degrees below average. The forecast high
temperature in Phoenix of 55 degrees would be the coolest day since
February 2nd.

Latest operational GFS and ECMWF have come into somewhat better
agreement for next week. Both models depict another Pacific trough
cutting off, but remaining well west of the Baja Peninsula through
much of the weak. Further eastward a weak ridge will build across the
Desert Southwest, resulting in height rises and a return to slightly
above normal temperatures through much of the week. The aforementioned
cutoff will likely migrate eastward late in the week. However, there
is still considerable uncertainty with regard to how much moisture
will be transported into the region. Latest model consensus yields
PoPs between 10 and 20 percent Friday/Saturday, though this system
bears watching.

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.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

CIGS will gradually lower through the morning hours, eventually
lowering to MVFR for much of this afternoon as a cold front moves
through the area. Southeast winds will increase in the late morning
and may make for some strong crosswinds on approaches. When the front
arrives, it will be a fairly sudden onset. Expect very strong winds,
poor visibility in moderate rain, and low ceilings when it moves
through (IFR conditions). Confidence in the timing of the front is
medium to high with high aviation impacts lasting for an hour or two.
Gusty west winds will continue through the afternoon behind the front
along with ceilings low enough to slow arrivals, but quickly
improving conditions will occur starting early this evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Cold front will affect the area this morning and early afternoon with
increasing aviation impacts. Winds will pickup ahead of the front
followed by much stronger westerly winds behind it. With the frontal
passage, expect low ceilings and possible IFR visibility in showers.
Closer to the western slopes of the coastal mountains, strong
downsloping may actually yield higher ceilings but will mean very
strong mountain wave activity which must be avoided. Gusty winds will
continue into the early evening hours before gradually diminishing
into tonight.

Aviation discussion not updated for amended TAFs.

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.FIRE WEATHER...
Monday through Friday:
Modest drying conditions and warming temperatures are expected into
the middle of next week as highs reach to slightly above normal
starting Tuesday. Minimum humidities decrease through Tuesday, but
overall remain somewhat high through the week, mostly between 25 and
35 percent for the deserts. A potential weather system for late in
the week may bring a return of precipitation and increasing
humidities. Winds will overall be light, but some breeziness down
the Lower Colorado River is likely on most days.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Winter Weather Advisory from 5 PM this afternoon to 8 AM MST
     Sunday for AZZ024.

CA...Wind Advisory from 10 AM this morning to 7 PM PST this evening
     for CAZ033.

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DISCUSSION...Hirsch
AVIATION...Kuhlman
FIRE WEATHER...Kuhlman