FXUS65 KSLC 171633

Area Forecast Discussion
National Weather Service Salt Lake City UT
933 AM MST Sun Dec 17 2017

.SYNOPSIS...The weak storm system exiting Utah will be replaced by
high pressure aloft beginning later today. A substantially colder
storm system will arrive in Utah late Wednesday, and impact the
state through Thursday.


Water Vapor Satellite shows a shear axis bisecting Utah with a
closed low over northwest Mexico. 400-200mb MDCARS wind
observations place a 100-145kt anticyclonic jet from off the
British Columbia coast south down the Northern Rockies, Great
Basin, and central California. GOES/SLC 12Z RAOB/HRRR indicate
that precipitable water values vary from 0.05"-0.15" mountains to
0.20"-0.30" valleys.

Gusty canyon/downslope winds affecting outlying Washington County
due to a 9mb northerly pressure gradient, 20kt northeast flow at
700mb with weak cold advection. Expect these winds to remain just
below advisory criteria, gradually diminishing during the next 30

Going forecast in good shape. Updated sky based on GOES 16
Nighttime Microphysics imagery. Cooled daytime highs a couple
degrees across the north, especially in the Cache Valley owing to
snowpack. Increased minimum RH in areas expected to remain in
stratus deck the longest.


The remnant of the weak upper trough over Utah will drift east of
the area by late this afternoon. Building heights in the wake of
this exiting trough will bring generally dry conditions to the
area through Tuesday night. The next Pacific storm system diving
southeast from the Gulf of Alaska will reach the Great Basin early
Wednesday, with the first impacts for Utah late Wednesday.

For this morning the weakening deformation axis associated with
the upper low along the Arizona/Mexico border will maintain a few
areas of light snow across southwest and central Utah early this
morning. North of the deformation an extensive area of low clouds
exists in the cold/moist air mass trailing the upper trough.
Dissipation of these low clouds may be a bit slow as the flow
aloft remains fairly light and the drier air to the north will
hold off spreading across northern/central Utah until later this
morning through the afternoon.

The increasingly mild west-northwest flow aloft early in the week
will bring in plenty of moisture to the northern half of the
forecast area. Sustained deep lift does not appear to exist in
this pattern, though weak low-level warm advection underneath
slight mid-level cooling could generate some light precip over
the higher terrain of the far north Monday.

The cold core upper trough diving southeast out of the Gulf of
Alaska will enter the Pacific Northwest Tuesday night, then
settle into the western Great Basin on Wednesday. Somewhat
concerned that this next trough will also show a pronounced split
as it moves east across the region. Strong amplification of the
eastern Pacific ridge suggests that this trough may take a more
south-southeast trajectory into the basin, with the southern
portion of the trough likely splitting off and heading into
Arizona. At this point confidence is low with the current global
model solutions, so will adjust timing a bit to favor a slower
split solution.

This midweek trough, unlike the one that moved through yesterday,
will still show good baroclinic structure even it splits as it
presses through northern and western Utah late Wednesday. Precip
Wednesday will develop behind the surface front and will be
confined to far northwest Utah late in the afternoon.

The extended global models continue to be in agreement with the
trough feature digging south through the Great Basin and swinging
through Utah during the latter part of the week. The EC remains
the faster solution while both the GFS and the EC depict cold
700mb temperatures -15C or so. The baroclinic zone is better
attuned for a frontogenetic front pushing through Utah with
favorable dynamics to produce widespread snowfall through the day
Thursday. This trough is being advertised (right now) as not
shearing apart until it is well south and east of Utah, so
confidence is growing that this will be a much better precip
producing storm. However with the recent low confidence in the
model guidance in regards to drastically weakening storms, not
putting all my confidence in this system just yet.

Behind this trough, shortwave ridging builds in under northwest flow
aloft. Another shortwave trough may be in the cards for next
weekend, however long range models aren't in great agreement with
this feature. Continued unsettled northwest flow aloft with minimal
to almost no warm advection aloft should help keep valleys well
mixed and shunt out any inversion building for the short term.


The SLC terminal will see VFR conditions throughout the day with
some scattered clouds. Northwest winds are expected throughout the





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