Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 181123

Area Forecast Discussion
National Weather Service Salt Lake City UT
423 AM MST Mon Dec 18 2017

.SYNOPSIS...High pressure aloft over the Great Basin will bring
dry and mild conditions to Utah early this week. A cold Pacific
storm system will generate warm and windy conditions early
Wednesday, followed by much colder temperatures and snow late
Wednesday through Thursday.


.SHORT TERM (Until 00z Friday)...The low amplitude upper ridge
extending from the eastern Pacific through the Great Basin will
bring dry conditions with seasonal temps across most of the area
through Tuesday. Temps will show a warming trend across the higher
elevations and southern valleys in response to warming aloft
through Tuesday. The northern and eastern valleys may struggle a
bit as the stable air mass in place, some cloud cover and low sun
angle will limit mixing both days. Urban haze will also increase
through Tuesday, though the increase will not be a severe as
during the recent poor air quality event of last week.

The cold core upper trough currently moving across the Gulf of
Alaska will reach the Pacific Northwest Tuesday night, then
continue southeast into the Great Basin by late Wednesday. The
latest global model guidance maintain a progressive and reasonably
consolidated trough entering northern/western Utah Wednesday
night. Suspect that the short wavelength between this feature and
the next upstream system cresting the eastern Pacific ridge could
be the reason for the progressive nature in the current model

The leading cold front looks to be strongly frontogenic as it
sweeps across northern and central Utah Wednesday afternoon/night.
Strong low-level forcing along with a rapidly destabilizing
airmass trailing the front will likely lead to a couple of hours
of fairly intense convective snows along and behind the frontal
boundary. Snow showers will likely persist in the cold, moist, and
unstable air mass in the core associated with the upper trough
late Wednesday night and into at least Thursday morning. Snow in
general will be loosely organized, with most of the activity tied
to the higher terrain and possibly along the I-15 corridor
through central/southwest Utah.

At this time snow amounts and impacts will likely justify winter
weather highlights in subsequent forecasts. Will mostly likely see
winter weather highlights to address this event.

By Thursday afternoon one last area of snow will likely be in the
wrap around associated with the now developed circulation in the
base of the trough near the four corners region. The heaviest
snows will likely be east the forecast area, though widespread
light/moderate snow could exist over the far east/southeast zones
through late Thursday afternoon.

.LONG TERM (After 00Z Friday)...The main picture is that a long wave
trough will set up over the western CONUS this weekend. Prior to
this trough taking shape will be a short wave ridge moving across
Utah early Friday before the arctic front moves southward across the
region. The EC and Canadian are much farther west with the jet axis
over Oregon into Nevada vs the GFS which has the jet axis from Idaho
into Wyoming by 00Z Saturday. Looking at the EC ensemble, there is
high uncertainty between its members so hard to chose which model
is on target. Have gone with the SuperBlend solution for now but see
merit in the EC and Canadian solutions of the trough digging farther
west due to the likelihood of super-geostrophic flow over the top
of the ridge in the Northwest Territory of Canada to allow digging
farther to the west.

Have increased PoPs some across northern Utah Friday as the arctic
front slips southward and also extended PoPs farther south Friday
night and Saturday to accompany the passage of the Arctic front.
Believe PoPs are under done, but will wait for better agreement of
models to get timing and potential down.

The 700mb temp difference between the GFS and EC is only 20C by 12Z
Mon. Even if the GFS was to be right, it would be a cold day because
of the arctic air that infiltrates the region Saturday into Sunday.


.AVIATION...Southeast winds will occur at the SLC terminal this
morning before shifting to the NW about 19Z. There is a 40% chance
that cigs of 6K ft AGL will become scatter at times between 15 and





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