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Area forecast discussion 
National Weather Service Salt Lake City Utah
320 PM MST Sat Dec 20 2014

Synopsis...a very warm and moist stream of Pacific air extends
from the Pacific northwest into northern Utah. This will continue
into Monday. A cold front will move through the area from north
to south on Monday bringing a cooler but drier airmass.


Discussion...there was not much of a break between the remnant
instability showers from last nights cold front and trough and the
beginning of the atmospheric river event that started late this
morning. A well defined atmospheric river stretches from near the
dateline through Oregon and then southeast into the Great Basin.
As the event unfolds the airmass will become more
moist...warmer...and the winds will increase in strength.
Precipitable water values rise to around .70 inches...700 mb temperatures
over the north rise to 1.5 c and the upper jet comes right over
top of the area.

12z GFS/NAM both generate storm total quantitative precipitation forecast around 2.5-2.9 inches in
the mountains with nearly an inch in northern valleys with the
12z ec a bit less. Models have been quite consistent for a number
of days so although it is a bit unusual for Utah to see so much
precipitation from a northwest flow warm air advection event like
this...forecast confidence is above average.

As the trough at the west end of the atmospheric river finally
crests the Flat Ridge over the west and plunges into the central
U.S. A backdoor cold front pushes from north to south through Utah
on Monday. This frontal passage Marks the end of the event as a
much cooler and drier airmass spreads south through the area.

From Sunday night through Monday evening the winds aloft are very
strong. Ridgetop gusts at the 11k elevation could reach 100 miles per hour
from Sunday night through Monday. The combination of the very wet
dense snow and the high winds is going to create a high avalanche
hazard in the backcountry. Coordinating with uafc on upgrading
their avalanche watch to a warning this afternoon.

Pretty quiet Monday night through early Wednesday. The models keep
going back and forth on the depth and strength of the Christmas
storm. The key seems to be how they handle the splitting of a
trough near 150w on Tuesday. The ec keeps the southern portion of
the split near or west of 150w while the GFS puts the southern
portion east of 150w. The reason this is important is because of
the location of the ridge that forms just downstream of this
closed low on Wednesday. With the ridge further east in the
GFS...the Christmas storm tends to be shunted further east while
the ec ridge position allows the storm to dig more into the Great
Basin. Todays 12z GFS/ec were pretty close with the ec slightly
colder/deeper. Quick look at the 18z GFS shows it wandering back
to the weaker solution that we saw from the 00z and 06z GFS runs.
Stay tuned.

Beyond Thursday models diverge considerably on details and
forecast confidence is low.


Aviation...ceilings at the slc terminal are expected to remain at
or below 7000 feet above ground level through the afternoon and evening. VFR
conditions and isolated light rain showers possible at times
through the period. There will be a 50 percent chance of MVFR
conditions due to lowering ceilings in heavier rain showers after
00z. Generally light southerly surface winds are
expected...although erratic winds are possible in some heavier


Slc watches/warnings/advisories...
Utah...Winter Storm Warning from midnight tonight to 6 PM MST Monday
for utz007>010-517.

Wyoming...High Wind Warning from 5 am Sunday to 2 am MST Monday for wyz021.




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http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)

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