Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 181232
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
531 AM MST Thu Jan 18 2018

.SYNOPSIS...An upper level trough currently over the eastern
Pacific will move into and across the Great Basin Friday through
early Sunday. Precipitation will develop across northern Utah late
Friday, then spread across most of the region for Friday night
through Saturday night. High pressure will return to the area late
Sunday.

&&

.DISCUSSION...High clouds continue to stream across Utah this
morning as the ridge centered over the area shifts east today.
Southwest flow aloft will increase throughout the day and should
become fairly strong tonight as 700mb winds increase to 40-50kt.
With an upstream trough approaching the area, the MSLP gradient will
also gradually tighten throughout the day and night. A fairly well-
mixed and warm airmass should result temperatures that may reach or
exceed records this afternoon and overnight for multiple sites. The
one spoiler would be the high clouds this afternoon.

The trough is progged to push a cold front into far northwest Utah
tomorrow morning, then into SLC around midafternoon, southwest and
central Utah during the late evening to overnight hours and finally
through southeast Utah on Saturday. The reason for the slow movement
is due to the models indicating the trough will carve south and
potentially pinch off into a closed low over southern Utah. Given
this evolution, the front is not particularly strong as it sags into
northwest Utah initially, with weak convergence into the 700mb
baroclinic zone. However, as the trough carves south, it becomes
more frontogenetic and precipitation should increase in intensity
along/behind the boundary as it crosses the Great Salt Lake. A cold,
moist, and unsettled airmass will remain in place behind the cold
front into Sunday.

With a fairly well-mixed and warm airmass ahead of the cold front
and with the colder air aloft lagging behind the surface boundary,
precipitation in most valleys is expected to begin as rain. For
example, when the surface boundary reaches SLC around 21z, 700mb
temperatures are progged to be around -5C on the GFS. The -10C
isotherm does not reach northwest Utah until around 00z on both the
GFS and EC. Precipitation intensities could help lower snow levels,
but based on BUFR sounding wet-bulb zero heights from both the NAM
and GFS, it does not appear that snow will be likely along most
northern and west-central Utah valleys until tomorrow evening (or
late afternoon closer to the Idaho border). Farther south, southwest
Utah is not expected to see snow until after midnight. Eventually,
the airmass will be cold enough for all valleys, including Dixie and
the Lake Powell area, to see snow. However, for far southern and
southeastern Utah, this will occur towards the tail end of the storm
on Saturday so any accumulations there should remain minimal. That
being said, accumulating snow is expected for much of northern,
central, and western Utah. Significant accumulations are expected
across most mountains as well as west-central and southwest Utah
given favorable flow, the strength of the cold front as it crosses
those areas, and the location of the entrance region of the upper
jet. Portions of the central and southern Wasatch Front and
southwest Wyoming could potentially see significant accumulations as
well. No winter weather highlights yet, but it looks like some will
eventually be needed; just need to refine the timing and amount of
snow.

.LONG TERM (After 12Z Sunday)... The GFS has the 500mb cold core
axis over the western half of the CWA and a moist NNW 700mb flow
continuing along the I-15 corridor from about SLC to Cedar City
between 12Z and 15Z before height rises begin and stabilization
ensues by afternoon. The EC is a little faster in drying it out so
decided to hold back from going likely PoPs in aforementioned area
at this time figuring that chance category PoPs is a good start.

On the other side of the spine of mountains in central Utah
downsloping winds gusting into the 30s is expected Sunday morning
but the gradient relaxes by afternoon so winds should drop off as
well.

A short wave ridge temporarily builds in Sunday evening, but by late
Sunday night 700mb warm advection develops already across the
northern quarter of Utah. While the EC keeps the PoPs mainly along
the Idaho border during the day Monday the GFS is more bullish with
this system with moist NW flow in cold advection penetrating across
the northern third of the state (as far south as Soldier Summit).
Since the EC ensemble mean from 12Z was farther south with this
shortwave (albeit a broad trough) than the operational EC model,
have leaned toward the GFS solution with support from the 12Z EC
ensemble and boosted PoPs across the north. Some PoPs will linger
into Monday night but then a ridge rebuilds across the region and
have removed PoPs for Tuesday altogether.

Another good looking trough is poised to make a dive southeast into
the Great Basin late in the work week but the global models are not
in agreement in timing. The EC has cold advection beginning by 00Z
Thursday across NW Utah while the GFS is 12 hours slower. Both
models show this system to be quite cold with -36C to -38C 500mb
temps and -16C to -18C 700mb temps across the northern half of the
CWA by Thursday night or Friday morning. Winter is here finally!

Have undercut MOS guidance over the northern and western valleys the
first couple days in this extended period to account for snow cover.

&&

.AVIATION...The main concern for the SLC terminal today is whether
south winds will prevail or will a shallow northerly surface wind
develop for a few hours late this morning into early this afternoon.
The chance for northerly winds is about 20 percent. VFR conditions
will prevail through the TAF period.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Cheng/Struthwolf

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