Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 162255

Area Forecast Discussion
National Weather Service Salt Lake City UT
355 PM MST Tue Jan 16 2018

.SYNOPSIS...High pressure will remain over the Great Basin into
tomorrow. A weak system will move through the high late today
through tonight bringing some clouds. The high then gives way to
a warm southwest flow on Thursday. A strong cold front will cross
Utah Friday through Friday night with a cold upper trough
remaining over the area Saturday.


Water Vapor Satellite shows an amplified ridge over the Interior
West. 400-200mb MDCARS wind observations place a 120-140kt
cyclonic jet off the Pacific Northwest coast. GOES/SLC 12Z
RAOB/HRRR indicate that precipitable water values vary from
0.05"-0.15" mountains to 0.25-0.35" western valleys.

Ridge axis briefly gives way to weakening shearing wave, and this
area is only on the very tail of height falls centered over
Saskatchewan. Given dry lower levels, and owing to very weak
forcing, we`re looking at a narrow shield of mid and high clouds.
However the forecast did have PoPs and there is slight low level
cold advection, so since I could not completely rule out a few
measurements, I kept slight chance PoPs in the mountains tonight.

Height rises second half of tonight and tomorrow morning with
ridging re-building into Thursday. Stable conditions with light
winds tomorrow could restrict high temperatures, though by
Thursday southerly flow increases allowing warming temperatures
with better mixing in the thermal ridge. Temperatures may be
underdone, but did go above most guidance Thursday night ahead of
the cold front.

Looks like the next storm system splits more than previous days
guidance indicated, with best height falls Friday to our west and
southwest and the upper level PV field crossing right over
Arizona. So the frontal timing slows down Friday as a result. We
had data reception problems much of the day so only saw portions
of the 12Z GFS and European. But from what we saw, things split
more and were slower. As a result of not having a strong
confidence to make wholesale changes Friday for PoPs, just pulled
them further north for now. Details likely will lock in within 24

This leads additional opportunity for strong southerly flow
within the warm sector, especially southwest and southern valleys
where the 700mb jet is nearing 50kts with decent pressure falls
and tightening pressure gradient.

It should precipitate across the entire state, and some areas will
even do well along the frontal band.

The surface cold front will continue its slow trek across the
CWA, exiting the CWA Saturday afternoon. A relatively robust
frontal precip band and cold temps aloft will likely produce
accumulating snow over all but the lowest elevations of the CWA
near St George, with substantial road impacts in many areas.
Total snow accumulations are still uncertain, but several inches
are possible along the heavily populated Wasatch Front region by
late Saturday.

The GFS and EC ensembles are in agreement that a second shortwave
trough will enter the region late Sunday night. Recent EC runs have
moved toward the less amplified GFS forecast, although whether this
trend will continue is unknown. With preexisting cold ground
surfaces and cold temperatures aloft, snow associated with this
trough will likely bring another round of road weather impacts
into Tuesday, although amounts and precise timing of whatever snow
may fall are very uncertain. Current guidance indicates that the
main impacts from this system will be in the northern half of the
CWA, however.

Beyond Tuesday, Jan 23, the GFS and EC ensembles and deterministic
models diverge substantially on the evolution of the upper level
pattern. However, unlike much of the last couple months, both
systems indicate that a relatively progressive pattern will continue
for the western US through the 2 week period, with regular trough
passages likely for the Intermountain West.


Little operational concerns at KSLC this afternoon as VFR
conditions will prevail with gradually increasing mid/high-lvl
clouds this afternoon/evening. Northwest winds will prevail until
around 06-08Z before switching to southerly. There is a 30
percent chance that winds switch earlier around 04-05Z or simply
become light and variable for the start of the overnight period.




SHORT TERM...Rogowski
LONG TERM...Jeglum

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