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Area forecast discussion 
National Weather Service Salt Lake City Utah
426 PM MST sun Dec 21 2014

Synopsis...a very moist and strong northwest flow will continue
across northern and central Utah tonight into Monday. A cold
front will move south across the area late tonight and Monday
bringing a cooler but drier airmass into the region through


Short term (until 00z thursday)...a broad ridge axis remains
situated off of the West Coast keeping Utah and southwest Wyoming
beneath a region of strong northwest flow. This northwest flow is
associated with an inland penetrating atmospheric river which is
bringing an anomalously moist airmass over Utah. The precipitable
water value of .58" on this morning/S kslc sounding was two
Standard deviations above normal. For this time of year...the
forecast 700 hpa winds for this evening over northern Utah are 3 to 4
Standard deviations above normal with a return interval of
occurrence of about once every ten years. So...certainly a
significant event underway. This strong moist northwest flow is
associated with significant isentropic upglide and speed
convergence across northern Utah which has led to widespread
precipitation last night through today. Anticipate this will
continue through tonight before precipitation begins to decrease on
Monday. Some very significant snow water equivalent
excess 2... coming in this afternoon and believe that some
locations will receive an additional 1" of swe tonight.

Snow levels this afternoon have risen to at least 7500 feet and
they will remain high through the evening. However...a shortwave
dropping into the backside of a trough in the northern plains will
nudge a backdoor cold front into northern Utah and SW Wyoming later tonight.
As the the cold air associated with this wave moves into Utah
snow levels will drop and anticipate that we will at least see
some slushy roads in the Wasatch Mountain valleys and over Parleys
Summit for the Monday morning commute. As the front moves through
northern Utah the flow will switch to the north and begin ushering drier
air into the region. Anticipate that the Wasatch mountains will
continue to receive snow through Monday morning before
diminishing. While the precipitation decreases across northern Utah by midday
on Monday...the remnants of the moisture plume will shift south
continuing precipitation in central through the day. The winter storm
warnings currently in place for the northern and central mountains
will continue through 600 PM Monday evening.

In addition to the heavy precipitation...there is a significant
wind event underway. Exposed ridgetops have experienced
increasingly strong gusts through the afternoon with a couple of
sites gusting over 90 miles per hour. The strong winds will continue tonight
with the potential for 100 miles per hour gusts in the most favored
locations above 11000 feet. Winds will remain strong along
ridgetops in northern Utah on Monday although they will back down
a fair amount from this afternoon's values. Strong winds will also
continue across southwest Wyoming this evening before diminishing
overnight. There will also be the potential for a bit of a
downslope wind event in Utah/S Castle country Monday afternoon and
evening as strong northwest flow continues and an inversion
develops above crest level. The window for such an event is fairly
short as the flow aloft becomes northerly later Monday night.

Monday night drier overspreads the region as the upper level
ridge currently off of the West Coast sharpens and moves over the
western U.S the ridge then shifts east across the Great Basin
resulting in a period of dry weather over the eastern Great Basin
for Tuesday and Wednesday ahead another storm system.

Long term (after 00z thursday) models continue to indicate
a storm system impacting for forecast area Wednesday night through
Christmas but continue to have a hard time with the details. The
storm is much more of a direct hit in the latest European model (ecmwf)
solution...with a quick moving cold front and a decent amount of
moisture. 700 mb temperatures as low as -14c would quickly bring snow
levels down to valley floors. Track of the system would confine
precipitation to primarily northern and central Utah. Latest GFS brings a
similar amount of cold air with the front...but keeps the main
system north of the area...resulting in a moisture starved front for
northern Utah. Think at this time it looks like at least the
mountains of northern Utah will see something on Christmas day...but
lack of run-to-run consistency means lower than usual confidence on
anything else relating to the system. Going pop grids seemed to
cover the potential scenarios well so made few changes.

Ec/GFS show a second trailing wave behind the main system for
Thursday night/Friday but again...this is much stronger in the European model (ecmwf)
solution. This does keep at least a small threat of northern
mountain precipitation through Friday evening before high pressure moves
back in. GFS moves another Pacific northwest trough into the
forecast area on Sunday. the European model (ecmwf) this system holds
off until Monday/Tuesday. Raised probability of precipitation a bit around day 7 to account
for this potential instability.


Aviation...VFR conditions are expected through the early evening
with ceilings tending to stay at or below 6000 feet above ground level with rain showers
in the vicinity of the terminal. There is a 30 percent chance of a
brief period of MVFR conditions if heavier showers develop. The best
chance for this to occur is before 03z. Southeast winds are expected
to shift to the northwest between 00z and 02z with the passage of a
cold front. There is a 30 percent chance that the front will move
through later than expected.


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

Wyoming...High Wind Warning until 2 am MST Monday for wyz021.



Short term...Graham
long term...traphagan

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