Area Forecast Discussion
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FXUS65 KSLC 092301

Area Forecast Discussion
National Weather Service Salt Lake City UT
401 PM MST Fri Dec 9 2016

.SYNOPSIS...A persistent and quite moist westerly flow will allow
for another round of widespread precipitation across the north
Saturday through Saturday night. Southern Utah will remain south
of the storm track until Saturday night. A relative break from
active weather will occur early next week.


.SHORT TERM (Through 00z Tuesday)...Afternoon water vapor and H5
analysis depict a near zonal flow across the CONUS lying south of
an impressive long wave trough over Canada, anchored by a
deepening low dropping south from the Arctic. Deep moisture
continues to be tapped from the subtropics over the eastern
pacific with a significant plume penetrating into the western
CONUS near the NorCal coast and points interior. A strong and
near zonal jet remains aloft of this plume but has shifted north
over the northern Rockies states today.

Our forecast area is now well south of the focused jet energy, and
although deep moisture remains plentiful, lack of forcing has and
will continue to limit precip potential tonight across the north.
This said, the UT/ID border will continue to be most favorable for
continued precip due to closer proximity to the aforementioned
jet and associated short wave energy embedded in it. Have
continued to reduce PoPs across northern Utah and QPF/snow over
the next 12-18 hours this forecast, but maintained high PoPs along
the border and northern mtns for a high PoP low qpf period into
early Saturday.

A more favorable jet orientation and another round of weak WAA at
the mid/lower levels will aid to increase precip across the north
once again Saturday morning however, this driven by subtle pattern
amplification upstream as a progressive short wave tracks ESE out
of the PacNW. Good agreement in models regarding this. Snow levels
will remain high, near 7000ft across the north most areas, but
have retained a rain/snow mix in the trapped valleys such as the
Cache through the day.

As the aforementioned short wave gradually encroaches the area and
passes Saturday night, guidance remains in near lockstep of
tightening baroclinicity over northern Utah, and a shift to cold
advection with passage of a mid level cold front through the
middle of the night. Expecting areal coverage of precip to expand
south to roughly Cedar City at that time, with focused precip
within the tightest thermal gradient over northern/central Utah,
with lowering snow levels to valley levels early Sunday morning.

Do not foresee much snow at all in the northern valleys early
Sunday morning though the CAA will support it, as the deepest
moisture will be at that time shifting downstream ahead of the H7
-8C layer. Did however extend the Winter Storm Warning for the
northern mtns into Sunday morning as significant snow is expected.
See WSW for details. Additional headlines may be needed for the
Wasatch mtn valleys such as Park City, but will let later shifts
decide on this. By mid morning Sunday, precip largely will come to
an end as drier air advects in from the NW behind the front.

Weak WAA will then again set up across the far north Sunday Night
into Monday bringing a chance for light snowfall, but not
expecting anything of significance. Have generally chance/slight
chance PoPs for generally the Logan area mtns.

Global models continue to be at odds with each other and diverge
more as we head into the extended. The EC has been the most
consistent from run-to-run and actually handled the most recent
series of storms better than the the GFS building my confidence in
leaning heavily towards the EC solution. When comparing the last 4
runs of the GFS there has been a lot of variation in the handling
of the trough off the Pacific NW coast. Here is where the EC takes
the trough and stretches it westward allowing for more of ridge to
develop across the west coast Tuesday and eastward to over the
Great Basin by Thursday. This will keep the main moisture plume
confined to the northern portions of the CWA. Meanwhile, the GFS
brings this trough southeast across Oregon and strings it out as
its connection with the longwave trough over the central CONUS is
severed. The latest 18Z GFS run is heading towards the EC solution
with a much flattened wave crossing the northern Rockies.

Based on the aforementioned reasoning have lowered PoPs
significantly across southern Utah Tuesday through Thursday per the
EC guidance. By Friday, the EC brings the trough that it had
stretched westward on Tuesday and consolidates it over the eastern
Pacific and then brings it east southeast ward across Oregon into
the Great Basin at the same time a new long wave trough develops
over western Canada. The combination of these features could bring
widespread precipitation to the entire CWA Friday with colder temps


.AVIATION...At the SLC terminal...VFR conditions are expected to
prevail overnight. Northerly winds are expected to prevail through
about 02Z before shifting to the southeast. However, there is 30
percent chance they could remain in place until 03-04Z.
Lowering CIGs below BKN070 by about 14Z with a 30 percent chance of
MVFR conditions in rain possible after about 18Z.


UT...Winter Storm Warning until 9 AM MST Sunday for UTZ007>009.




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