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Area forecast discussion 
National Weather Service Phoenix Arizona
955 PM MST Thursday Dec 25 2014

Synopsis...
expect clear skies and cold temperatures tonight and through the
weekend. Another low pressure system is expected to move through the
area Tuesday through Thursday.

&&

Discussion...
skies have cleared across most of the forecast area this evening. An
update to the forecast was made earlier in the shift to drop probability of precipitation
from the Phoenix metropolitan area. Ceilings linger along and south of
Interstate 8 with all of Southeast Arizona covered by cloudiness.
Southeast Arizona is also where the showers are...along with some
isolated thunderstorms. This is due to the frontal zone as well as a
vorticity maximum and associated jet streak within the upper trough.
Cloudiness over our forecast area is shallow and not producing any
radar echoes/precip. Will make some additional updates to drop the
probability of precipitation altogether. Anticipate colder temperatures Friday morning than this
morning though lingering west and northwest winds may help curb the
radiational cooling a little bit. But expect many desert locations
to be in the 30s tonight. The next night will be colder yet with a
little bit of additional cold air filtering in and even lighter
winds. Thus there is a freeze warning in effect for portions of the
forecast area. Some areas...including the Phoenix metropolitan will be more
marginal for freeze warning conditions and thus a Special Weather
Statement is in effect there.



&&

Previous discussion issued 145 PM MST/1245 PM PST...
cold front continues to rapidly move through the area this afternoon
and is positioned roughly from Winslow to Yuma as of 2040z. Showers
have increased across the region since about noon as the front has
moved into somewhat deeper moisture across the lower deserts. Across
the Phoenix area rain showers have been mostly in the form of virga
but more intense echoes are starting to fill in across portions of
Gila County and far NE Maricopa County. The cloud cover has managed
to put the brakes on our warming for the day with most spots stuck
in the low/middle 60s.

Forecast remains largely on track as the front will continue its
rapid push through Arizona this afternoon and evening. Moisture is
mainly confined to the 700-500mb layer with a fairly dry airmass in
place in the lower levels. With surface dewpoints in the middle 20s
it's hard to envision much precipitation making its way to the ground
across the deserts. Will continue to mention 10-20 probability of precipitation for parts of
the East Valley and northwest Pinal County but the remaining desert
locations are likely done with precipitation chances today. Meanwhile to
the North/East of Phoenix...it is still looking to be a race between
falling snow levels and the back edge of the precipitation. Snow
levels should fall to around 5000ft this evening but this is right
around when the precipitation will be coming to an end. Will
continue with the idea of a dusting of snow in the highest locales
east of Phoenix but nothing of significant impact. As for
winds...the gustiness will taper off quickly behind the front this
evening. The combo of relatively light winds and clearing skies
should allow temperatures to drop quite a bit overnight. Cannot rule
out some pockets of freezing temperatures in the protected areas of
La Paz County as well as portions of Joshua Tree National Park
/although their winds could stay elevated overnight/. Will issue a
Special Weather Statement for these areas. I don't think the areal
coverage and temporal extent of the freezing temperatures is enough
to warrant a warning at this point.

Looking ahead to Friday however...cool northwesterly flow aloft will
continue to usher well below normal temperatures into the region.
Daytime highs from bias corrected and raw guidance suggest highs
won't break out of the middle 50s. Consequently low temperatures will
fall well below normal and numerous locations across the deserts
could see freezing temperatures. Will stick with the idea of the
coldest temperatures in La Paz...northwest Maricopa...and northwest Pinal counties
although the normally colder southeast valley locations could also
see freezing temperatures early Saturday morning. Freeze warning will
remain intact as issued earlier today...only question at this point
is if the Phoenix metropolitan should be included within the warning based
on the southeast valley's freeze potential.

Saturday and Sunday will see continued northwesterly flow but
heights and bl temperatures will rise slightly each day. While highs will
still top out in the middle/upper 50s each day...clouds will be on the
increase ahead of a fast moving shortwave which should help mitigate
the chance of another round of early morning freezing temperatures.

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...
southeast California and southwest Arizona...including kipl and
kblh...

A cold front extends across New Mexico and far Southeast Arizona as
of 04z. Meanwhile the upper low was centered near southwest Utah. A
batch of broken-overcast clouds with bases 8-10 kft mean sea level was across
southeast and far southwest Arizona with and a portion of Imperial
County in California. Shower activity was near and southeast of a
line from cft-tus. Only taf site in the list above affected by
ceilings is kipl otherwise the ceilings are confined mainly along
and south of Interstate 8. The clouds and showers will slowly shift
southeastward this evening with kipl clearing by 06z. Lingering west
and northwest winds will slowly become light and variable late
tonight while winds in the lower levels above the surface remain
moderately strong. Northwest and west winds resume Friday afternoon.

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...aj
previous discussion...leins/Nolte
aviation...aj
fire weather...MO

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