Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 151137

Area Forecast Discussion
National Weather Service Salt Lake City UT
437 AM MST Thu Feb 15 2018

.SYNOPSIS...The Pacific Northwest trough will move through Utah
today. This trough will bring colder temperatures and occasional
snow to northern and western Utah through the day today. Mild
weather is expected going into the weekend, with the next cold
weather system moving in on Sunday.


.SHORT TERM...The upper level trough diving south from the Pacific
Northwest continues to carve through the Great Basin this morning.
Afternoon and evening convection yesterday across northern Utah
may be a precursor to what will come this morning as the unstable
upper level trough swings overhead; 500mb temps have a compact
gradient with values reading -30C per the 12z KSLC RAOB. Added
isolated thunder to the morning package for this reason. The 700mb
baroclinic zone continues to push farther south through Utah this
morning. Water vapor imagery shows this tightening gradient
draped from southwest to northeast from Nevada into southern
Wyoming. With the tail end of the trough pushing through Utah this
morning, actually moving a bit faster than the models have been
advertising, precipitation is expected to come to an end by late
morning, with the mountains and higher elevations lingering on
through the afternoon hours. Valleys across northern Utah are
expected to remain mostly wet, although benches could see some
slush through the morning hours.

The airmass remains cold behind the trough with northwest flow
aloft Friday into Saturday. Forecast models have another shortwave
trough slide by to the north late Friday night into Saturday which
could provide a small potential for precip to the far northern
areas of Utah. A warm sector to the west begins to advect some
warmer air aloft Saturday as the ridge to the west flattens out
with another Pacific Northwest trough upstream.

.LONG TERM (After 12Z Sunday)...A significant pattern change is
expected in the medium range as a deep trough carves out across
the western U.S. As this transition occurs the focus shifts to a
winter storm expected to impact Utah late this weekend into early
next week.

On Sunday, the ECMWF carves the trough further to the west and is
deeper than the GFS. At this time, leaning toward the stronger ECMWF
solution as the ECMWF ensemble shows pretty remarkable agreement on
this solution, while the GFS exhibits more spread. Both bring a
significant cold front into northern Utah during the day on Sunday.
The ECMWF and GFS also are in agreement that the mid level
baroclinic zone is frontogenetic as the thermal gradient tightens as
it heads through northern Utah and into central Utah. A decent band
of precipitation would likely accompany this mid level baroclinic
zone as it moves south. The GFS indicates the potential for a layer
of unstable air above the the frontogenetic circulation which which
result in better response and a stronger frontal band.

Behind what could be a vigorous frontal band both models support an
impressive 500 mb cold pool with temps below -35C spreading over
northern Utah on Monday. The ECMWF brings -38C air at 500 mb over
northern UT on Monday which would be coldest air of the season. As
this cold air overspreads the region both models support deep moist
unstable northwest flow Monday into Monday night, which would result
in a continuation of orographic precipitation in the Wasatch
Mountains. As the front moves into southern Utah, the ECMWF is far
more optimistic with the precip threat there and leaned toward this

This storm ushers much colder air into the region with Monday`s
highs across northern Utah being about 20F cooler than Saturday`s.
After this system ejects out of the Great Basin the models indicate
that a splitting trough moves into the western U.S. by mid week,
which would dry us out, but continue the period of below normal
temperatures through most of the week.


.AVIATION...Light southeast winds this morning are expected to
switch back to the north between 1200-1400 UTC. However, there is a
20 percent chance that the switch back to north winds will be
delayed until after 1400 UTC. Ceilings will generally remain above
6000 feet AGL. However, snow showers are expected to re-develop
across northern Utah this morning and there is 40 percent chance of
intermittent ceilings below 6000 feet AGL before 1800 UTC.





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