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fxus65 kslc 200426 

Area forecast discussion
National Weather Service Salt Lake City Utah
926 PM MST Fri Jan 19 2018

Synopsis...a cold and moist storm system along the West Coast
will track into and then through the Great Basin this weekend.
This storm will impact much of Utah and southwest Wyoming tonight
through early Sunday.


water vapor satellite shows a trough shearing well to our west,
with the southern energy crossing central California. 400-200mb
mdcars wind observations place a 100-150kt cyclonic jet off the
West Coast through the northern rockies. GOES/slc 00z radiosonde observation/hrrr
indicate that precipitable water values vary from 0.20"-0.35"
mountains to 0.40-0.50" valleys. An atmospheric river stretches
from Hawaii through Southern California, with abnormally high
precipitable water values extending northeast into Utah and

The trough to the west is splitting earlier than previous days
model runs have shown, though the models have tracked in this
direction. That slowed down the cold front, which has already
outrun the cold air which tracks well west corresponding with the
trough. The cold front has in turn begun to stall and will pivot
north-south tomorrow. This will aid in increasing snow totals
across central and western Utah.

Going forecast on track, pulled pops back a bit across eastern
zones as everything continues to slow a touch more with time, and
tweaked quantitative precipitation forecast overnight with latest guidance. Winds have died down
after the initial push of the cold front. Have adjusted the T/relative humidity
trend overnight with latest guidance as well.

As happiness grows due to the arrival of an actual winter storm,
be very careful out there!


Previous discussion...
the eastern Pacific trough just off the West Coast late this
afternoon is in the process of shearing apart with the northern
piece racing east through the northern rockies. The southern
extent of the trough has captured a sizable piece of cold air
aloft and will likely maintain its structure as it moves east
across the Great Basin this weekend.

The surface cold front has moved south of slc, with precip
expanding along the trailing near 700mb baroclinic zone late this
afternoon. This precip expansion is in line with model guidance
which showed rapid frontogenesis along the boundary with an
ejecting mid-level shortwave from the main trough and the right
entrance region of jet over the top of the baroclinic zone. This
favorable synoptic setup will generate fairly strong lift along
and trailing the baroclinic as it sweep south into central Utah
later tonight. Precip-type will for the most part be snow due to
the strong upward motion and rapid cooling aloft. Accumulating
snow are expected, with these accumulations becoming greater as
the baroclinic works south tonight.

The aforementioned low-level baroclinic zone will stall over
central Utah, then pivot toward a more north-to-south orientation
ahead of the advancing mid-level trough later tonight/early
Saturday. A number of weak circulation centers coincident to this
thermal boundary along with the still favorably positioned upper
jet will maintain areas of moderate/locally heavy snow near the
boundary late tonight/early Saturday. The areal extent and
intensity of the snow will pick up and expand north and east as
the upper trough moves into the state Saturday evening. The
greatest storm accumulations in the mountains will likely exist
across the central mountains, though most areas near this thermal
ribbon will see significant accumulations through early Sunday.
For the valleys the greatest totals at this point look to be
focused on the west-central and southwest valleys, particularly
along the I-15 and western I-70 corridors.

The upper trough will pass through southern/central Utah Saturday
night/early Sunday. The bulk of the precip will end or become
greatly reduced in areal coverage as the favorable synoptic
conditions for lift deteriorate. Cold air instability will
continue to support loosely organized convection Sunday
morning/early afternoon, though little additional accumulation
will result.

Shortwave ridging should end precip late for Sunday afternoon
through early Monday. A weak shortwave will race east across the
northern rockies/northern Great Basin late Monday. This feature
will create some lift and light precip across the north out of
the sufficient low-level warm advection and dynamic support from
the passing shortwave.

The extended forecast period begins with a rather flat flow aloft
and not a building ridge, which should keep an unsettled pattern
in place for the extended. Models continue to show good agreement
in the early to mid week trough advertised to dig down the West
Coast. Timing looks to be out of sync still as well as the
strength of the trough.

The European model (ecmwf) is faster and stronger with the mid-late week system
compared to the GFS, which is about 12 hours later. This trough is
digging southward through the Great Basin but it remains on track to
move east through Nevada and into central Utah, in comparison with
previous tracks which have trended farther south. Ahead of this
trough, the airmass will warm with 700mb temps creeping towards -2c
across the western half of Utah. As the trough swings through
Thursday, southwest flow will accompany the trough and advecting
relatively warm air ahead of the front, which looks to move into
northern Utah Thursday evening.

Again, a moist northwest flow pattern with a decent looking front
associated with it points to another unsettled and active weather
weekend setting up.


rain/snow mix showers will likely continue through much of the
night in IFR/LIFR conditions at the slc terminal. Snow accumulation
on untreated surfaces is expected to be 1-3 inches through 12z
Saturday, with potential that snow continues past this time.


Slc watches/warnings/advisories...
Utah...Winter Storm Warning until 5 am MST Sunday for utz008>010-

Winter Weather Advisory until 9 am MST Saturday for utz002-005-

Winter Storm Warning until 11 PM MST Saturday for utz003-004-006.

Winter Storm Warning until 5 am MST Sunday for utz517-518.

Winter Weather Advisory from 3 am Saturday to 9 PM MST Sunday
for utz011-012.

Wyoming...Winter Storm Warning until 5 am MST Sunday for wyz021.




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