Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 152249

Area Forecast Discussion
National Weather Service Salt Lake City UT
349 PM MST Thu Feb 15 2018

.SYNOPSIS...A cold upper trough will move out of Utah tonight. A
weak trough will clip far northern Utah Friday night. A warmer
westerly flow will be over the area on Saturday. The next cold
upper trough will start to move into Utah on Sunday and will
remain over the area into Tuesday.


.SHORT TERM (Thru 00Z Monday)...The axis of the cold upper
trough is just moving out of the CWA at this time. A cold airmass
covers the CWA with enough low level instability to keep snow
showers going over and upstream of the mtns.

Expect the snow showers to end fairly rapidly once the sun sets
and the gusty nw winds should drop off thru the evening.

With clearing skies under weak short wave ridging in the nw flow,
temps should get chilly overnight. A low amplitude trof in the
flow clips the north tomorrow aftn/eve with clouds across the
north and a small chance of snow showers far north. Looks dry for
the rest of the CWA on Fri with near seasonal temps.

The flow backs and warms on Sat ahead of the next upper trof. This
should send temps up to about 10-15 deg above normal and local
south winds should start up in the wrn valleys.

The models have come into good agreement with this next trof that
looks to start sagging into the Great Basin on Sun. Gusty south
winds will develop ahead of the associated cold front and temps
should be mild once again. The front slowly crosses nrn UT on Sun
reaching SLC late afternoon. Precip is forecast to be widespread
along and north of this front and the airmass behind the front is
quite cold, with precip changing to snow soon after fropa. This
front and the associated cold upper trof will usher in a period of
cold snowy weather lasting thru the first part of the week, as
detailed well below.

.LONG TERM (After 00Z Monday)...Guidance is in general agreement
that the 700 baroclinic zone will push through the northern half
of the state Sunday evening reaching the vicinity of Cedar City
near or just after midnight. This baroclinic zone is frontogenic
throughout most it not all of its trek across the area, and is
also coincident with good jet support (much of the region will lie
in the left exit region of a cyclonic upper level jet). These
factors should result in a fairly healthy band of frontal
precipitation impacting most of the area. Post frontal cold
advection will result in snow levels quickly dropping to the
valley floors Sunday evening in northern Utah and by Monday
morning in southern Utah (except Dixie). Despite increasing
agreement among guidance regarding frontal passage timing there
are still some differences regarding the amplitude (and
consequently speed) of the upstream mid-level trough. The EC is
more amplified/slower with the trough with results in better
frontal dynamics and more snowfall for southern Utah. The
Canadian/GFS are a little lower amplitude and faster with the
trough which would result in less snowfall for southern and
central Utah. Both ensemble camps support their deterministic
models so went with a blend for PoPs in southern/central Utah for
this forecast package.

Unlike many of our quick hitting systems so far this winter,
guidance (even the more progressive GFS and Canadian solutions)
currently indicates that this trough will take its time moving
through the area. This will result in an extended period of cool
moist northwest flow over the area on Monday creating a good
orographic precipitation setup in favored locations across the
area. Additional (albeit lesser) valley snowfall accumulations
are also possible during the day on Monday across most of the
forecast area. The trough axis/mid-level cold pool will move east
of the area Monday night or Tuesday morning (depending which
solution verifies) which will help suppress any lingering showers.
All guidance is currently depicting a rather cold (potentially the
coldest of the entire winter) airmass in place behind the trough
on Tuesday with H7 temps near -20C over the north. If this airmass
verifies highs would struggle to reach freezing through much of
the state with single- digit highs possible in the mountains

The next feature of interest is a compact mid-level low which
dives southward down the west coast Tuesday into Wednesday.
Guidance is currently not in agreement on the handling of this
feature which more or less becomes cut-off by midweek resulting in
split flow over the southwest US. The GFS solution places most of
the state under southwest flow, resulting in a gradual warming
trend with increasing mid-level moisture while the EC keeps the
area (at least on Wednesday) under the influence of a weak
northern stream disturbance slowing down warming. Temperatures in
the mid-week time frame are a challenging at this point given the
multitude of current unknowns, namely how much snow cover will be
generated by the Monday storm and which flow regime will prevail
in the vicinity of the west coast low. The blended solution
employed in this forecast package resulted in a gradual warmup
Wednesday into Thursday, with temperatures still running several
degrees below normal throughout the extended period.


.AVIATION...VFR conditions are expected at the SLC terminal
through the evening. Cigs around 6000 feet AGL should scatter
out between 00Z and 02Z as showers in the vicinity of the terminal
diminish. There is a 20 percent chance these cigs will hang on
longer than expected. Northwest winds are expected to shift to the
southeast between 03Z and 05Z.




SHORT TERM...Wilensky

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