Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 172145

Area Forecast Discussion
National Weather Service Salt Lake City UT
345 PM MDT Sat Mar 17 2018

.SYNOPSIS...A slow-moving storm system will bring cool and
unsettled weather for remainder of the weekend. High pressure
aloft will return for the first half of next week and bring a
warming trend to the region, before another storm system
approaches for the latter half of the week.


.SHORT TERM (Through 00Z Wednesday)...

An active weather day continues across Utah and southwest
Wyoming. Infrared satellite imagery shows cyclonic circulation
across the Great Basin with a circulation center over Nevada.
Farther west, cooling cloud tops in the lee of the central Sierra
indicate another developing trough axis shifting east towards our
area. Radars show convective activity over mainly north Utah at
this time, with a couple lightning strikes reported over the last
few hours. Cams suggest that steady snow continues in the central
Wasatch with road snow lingering in Little Cottonwood Canyon,
while roads are now clear elsewhere with precip appearing more
intermittent. Gusty winds continue over southern and central Utah,
though diminished somewhat from earlier today.

Models indicate that the baroclinic zone which slowed and became
more diffuse this afternoon over northern Utah will tighten and
reinvigorate by late this evening as the parent trough axis
mentioned earlier approaches. High-res models are lighting up with
precip along this boundary late today and tonight, with good
convergence at 700MB along the baroclinic zone. The average
placement of the feature looks to be SW to NE from around Delta to
Evanston. Precip should intensify overnight as this band shifts
southeastward and a 700MB closed low develops over southeast Utah.
Current thinking is that the high precip rates in southern Utah
combined with colder road surfaces overnight will lead to snow-
covered roads, including higher spots on I-15 from Beaver to
Cedar City and the I-70 corridor higher elevations including the
San Rafael Swell. Have expanded the winter weather advisory to
include these locations as well as the Uinta Basin. Did not elect
to include central Utah valleys or Castle Country as models
suggest downsloping in those areas will limit precip amounts.

While the trough axis slides out of southeastern Utah tomorrow,
the remainder of the forecast area will remain under a cold and
unstable air mass. Model soundings suggest a drying air mass
except near the surface, so expecting lesser coverage of showers
tomorrow. The NAM is the most pessimistic, with no showers
suggested generally north of Utah County, while other models do
indicate continuing showers. Also, despite 700MB temps dropping to
-12 to 13C, local lake effect guidance suggests only minimal
chances of lake enhancement given the unfavorable saturation

A ridge will build over the Great Basin Monday and Tuesday,
bringing an end to the precip except for an outside shot of
showers over the Uintas. Temperatures will be slow to rebound
Monday, then eventually landing close to normal for this time of
year Tuesday. Enhanced moisture rounding the periphery of the
ridge will preserve some mid and high cloud cover over northern
Utah Tuesday and an outside chance of precip at the higher

.LONG TERM (After 00z Wednesday)...Primary focus for the long term
period keys in on the next (and potentially significant) series of
troughs that will impact the area beginning during the latter half
of the week. To begin the period the first trough to note will
reside off the central Cali coast near 135W as a closed upper low.
Orientation of this low will allow for a notable tap into sub-trop
moisture across the southeastern Pacific, with trajectories
advecting the plume into the SoCal region inducing a heavy precip
event there.

This trough will begin to be influenced by a high amplitude northern
branch trough that will translate southeast along the Britcol coast,
kicking it northeast and opening the trough up an increasingly
negatively tilted wave that will track across the eastern Great
Basin sometime in the Thu/Fri timeframe. What is most notable
regarding this trough and associated plume of sub-trop moisture are
the IVT anomalies and depth of penetration into the intermountain
region. 00z NAEFS progs indicate a return interval of similar IVT
values ranging from 10 to 30 years across central/southern Utah
(respectively) Thu, with a standardized anomaly ranging from +4 to
+6 for those areas. What this means is that although specific trough
evolution and resultant timing has been off by some 12 hours at
times in operational globals, the globals all point to a similar
evolution and outcome. This increases confidence for a potentially
significant precipitation event that would yield heavy valley
rainfall and significant upper elevation snow late week, and the
potential for some hydro related issues across southern Utah.

Have continued to up PoPs days 5/6, though have blended a bit on
either side as specific timing regarding onset and timing of the
most significant precip is less certain. Did opt to retain a slower
trend of reducing snow levels Friday as events similar to these
primarily occur within the warm sector.

Upstream evolution of the northern branch trough remains a lower
confidence forecast, but all indications point towards unsettled and
cooler conditions either beginning in the first or second half of
next weekend.


.AVIATION...Light southeast winds at the SLC terminal will
likely persist through the evening hours, and may become light and
variable at times as precip redevelops nearby. Small chance for less
than half an inch of slush accumulation on untreated surfaces
overnight. Ceilings will likely bounce up and down to MVFR through
05z before improving to VFR.


UT...Winter Weather Advisory until noon MDT Sunday for UTZ009-010-517-

     Winter Weather Advisory until midnight MDT tonight for UTZ008.

     Winter Weather Advisory from 9 PM this evening to noon MDT
     Sunday for UTZ011.

     Winter Weather Advisory from 3 AM to noon MDT Sunday for UTZ013.

     Winter Weather Advisory from midnight tonight to 10 AM MDT
     Sunday for UTZ016.

WY...Winter Weather Advisory until noon MDT Sunday for WYZ021.



Van Cleave/Merrill/Dewey

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