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Area forecast discussion 
National Weather Service Phoenix Arizona
909 am MST Thursday Dec 25 2014

Synopsis...
a somewhat dry low pressure system will slide through the area today
before slowing and lingering over The Four Corners through the
weekend. This system will bring cooler and breezy conditions along
with a chance of some light showers...and possibly a light dusting of
snow...over the higher terrain of south-central Arizona this
afternoon and evening into Friday. A dry and continued cool weather
pattern will persist from through early next week.

&&

Discussion...
middle morning water vapor imagery reveals low pressure over Utah with
a shortwave trough extending southward through portions of
Arizona...southeast Nevada...and southeast California. A surface
cold/cool front is positioned roughly from Flagstaff to Lake Havasu
as of 16z with some light showers noted on radar across northern
Arizona. Across the lower Arizona deserts...conditions remain dry but
clouds are increasing just a bit ahead of the front.

In the short term...the front will move through the area relatively
quickly...a bit faster than models had previously indicated...and
could very well be exiting the state by early evening. Latest hi-res
guidance including the hrrr and local wrfs suggest a few showers
developing early this afternoon in the higher terrain locations
North/East of Phoenix...with more coverage across portions of
Southeast Arizona. Across the Phoenix area...any showers that
develop would be widely scattered at best and would be very short
lived. The inherited forecast of 10 probability of precipitation across the metropolitan still
seems valid with 30-40 probability of precipitation certainly warranted east of Phoenix. I
trimmed things back just a bit after 06z as the front should be well
south/east of the area by then and drier air should be moving in
from the north.

In terms of snowfall...it will be a race between how quickly the
colder air moves into place vs how quickly the precipitation exits
the area. Snow levels are expected to fall below 5000 feet this
afternoon although given the rapid nature of the front...precipitation
should be coming to an end right around the same time. Could be a
brief mix of rain/snow with little to no accumulation above 5000ft
this afternoon/early evening but that's about it. No major
winter-type concerns expected with this system.

Finally...there should be some breeziness coincident with the
frontal passage later this afternoon...generally in the 10-20mph
range with occasional higher gusts on the ridges North/East of
Phoenix. Overall the forecast is in good shape. Look for highs to
reach the lower 60s this afternoon...about 2-3 degrees below normal
for late December. Should be a chilly night especially in the
protected areas of La Paz County and the low lying areas near the
Colorado River where the winds calm down after sunset. Even colder
temperatures are expected Friday/Friday night.

&&

Previous discussion...
Friday morning into early next week...
upper troughing will linger over The Four Corners as the jet stream
dips further to our south Friday. Clear skies and lighter winds will
take full advantage of the cooler airmass behind the front and result
in some of our coolest mornings so far this late fall/winter for
Friday am and the coming weekend. Daytime temperatures Friday could
be anywhere from 8 to 10 degrees below seasonal normals with
widespread 50s across the lower deserts. Friday night has the
potential to bring freezing temperatures to northern Pinal
County...including the far southeast and western fringes of the
Phoenix metropolitan...as well as rural areas of southwest Arizona. High
temperatures for the weekend could struggle to break into the 60s
across many of the Lower Desert locales with overnight lows dipping
into the 30s...and briefly the upper 20s...in spots. In anticipation
of the upcoming cold mornings...a Special Weather Statement (spspsr)
has been issued for La Paz Colorado. For Friday morning and a freeze
warning has been issued for Saturday morning across La Paz...northwest
Maricopa...and northwest Pinal counties.

Taking a broad brush approach from the widely varying model forecast for
the extended...mostly dry and cool northwest flow with embedded
shortwaves will hold over the region for early next week...keeping
temperatures at or below seasonal normals while periodically bringing
clouds into the region. 00z operational solutions from the GFS/ECMWF/Gem
all advertise a large upper low developing into the SW states with
considerable moisture fields and quantitative precipitation forecast values by Tuesday.
Meanwhile...only a small percentage of ensemble members support
their operational counterparts. For now...maintained a cool forecast
for both highs and lows and blended into climatology pop influence in the
extended for Tuesday and beyond.

&&

Aviation...
south-central Arizona including kphx...kiwa...and ksdl...
a cold front will pass through the Phoenix terminals around 18z
causing a rather abrupt wind shift. While a brief higher gust may be
possible...speeds should largely remain below 15kt. Coincident with
frontal passage...a rather narrow band of ceilings in a 5k-8k above ground level will affect the
Aerodrome space with some accompanying virga. There is moderate
confidence ceilings will only prevail for a couple hours and remain
mostly above 6k feet...but some partial mountain obscuration may be an
impact. Rapid clearing by sunset will yield no aviation concerns
overnight.



Southeast California and southwest Arizona...including kipl and
kblh...
a cold front will pass through southeast California this morning
causing a period of gusty winds. The initial Post frontal pressure
packing has caused gusts over 35kt at kipl...though these speeds
should decrease rapidly while the direction turn more to the north/northwest
through the morning. Otherwise...persistent higher gusts near 25 knots
will prevail into the late afternoon. Some residual moisture behind
the front may cause a brief period of ceilings in a 5k-7k above ground level...but
these ceilings should be short lived. Wind speeds will decrease by
sunset with no aviation impacts overnight.

Aviation discussion not updated for amended tafs.

&&

Fire weather...
Saturday through Wednesday...
tranquil dry and abnormally cool weather will prevail over the
weekend into early next week with no fire weather concerns. Only a
brief period of somewhat gusty winds down the Colorado River valley
will disrupt otherwise nearly calm conditions. Afternoon humidity
values will only fall into a 20 to 30 percent range with good to
excellent overnight recovery. The potential exists for another wind
producing and wetter weather system affecting the district by the
middle of next week.

&&

Psr watches/warnings/advisories...

Arizona...freeze warning from midnight Friday night to 8 am MST Saturday
azz021-022-028.

California...none.

&&

$$

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Discussion...leins/Nolte
aviation...MO
fire weather...MO

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