Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 201631

Area Forecast Discussion
National Weather Service Salt Lake City UT
1031 AM MDT Tue Mar 20 2018

.SYNOPSIS...A developing storm system along the west coast will
bring rain and high elevation snow to much of the state for the
latter half of the week. This storm will move east through the
area this weekend, leading to cooler temperatures and lowering
snow levels through Sunday.


.DISCUSSION...Morning water vapor and H5 analysis depicts low
amplitude ridging building into the area from the west, this
downstream of vertically stacked closed low off the central Cali
coast. Orientation of this low and minimal movement over the last
24 hrs coincident with mid level ridging over Baja has allowed for
a solid tap to sub-trop moisture and an intensifying atmos river
beginning to impinge on coastal Cali. Remnant upper level moisture
in lee of the Sierra continues to stream downstream with variable
mid level cloud cover noted across much of the CWA at this time.

The primary focus of the shift moving forward will key in on the
Thu-Fri timeframe when the aforementioned atmos river will round
the south end of the Sierra allowing for penetration into the
eastern Great Basin. Purely looking at integrated vapor
transport (IVT) and PWAT anomalies during the day Thursday, this
upcoming event is very notable in regards to the breadth of deep
layer moisture that will advect into the area. This said, BUFR
data indicates the potential for some upstream shadowing at the
lowest levels (notable dry layer below H5 at KSGU) which could
limit much of the heaviest precip potential to the orographically
favored areas within strong SW-S flow. Starting to key in this
potential more and more as areal extent of significant QPF has
been decreasing over the last few runs, even though NAEFS
anomalies continue to be on the climb and looking even more

More orographic or not, still looking at the potential for quite a
few impacts from this event. Expecting notable river rises across
the Virgin River basin at the least, though lack of low/mid
elevation snowpack will keep much of the hydro concerns in check,
dense/heavy snow and blowing snow for elevations in excess of
9kft (slightly higher at times), and strong winds across the
western valleys. Will check out the full suite of 12z guidance and
provide more detail in the afternoon package.

The only update made this morning was to increase cloud cover
slightly for today. No other updates made or planned attm.

.PREVIOUS DISCUSSION...Snow levels statewide will be rather high
as near 700mb temps remain at or above freezing through Thursday
evening. Some modest cooling aloft later Thursday night will
translate into lowering snow levels, though even then the valleys
should see precip-type remain as rain overnight through Friday.
The areal coverage of precip should trend downward Friday
afternoon as the passing dynamic feature moves east of the area.

The main storm system is expected to be along the Pacific
Northwest coast Friday night before moving inland to impact the
forecast area over the weekend. However, there remains
disagreement among the models with respect to how the scenario
evolves. The latest GFS brings the trough across as a more or less
cohesive unit Saturday into Sunday, pushing the associated cold
front through northern Utah during the day Saturday and through
the rest of the area by Sunday morning.

The situation remains a bit more complicated in the 00Z EC, which
splits off a wave that grazes northern Utah during the day Saturday,
bringing an initial push of cooler air into northern Utah. That wave
is followed by the main system on Sunday, which phases with another
weaker Pacific wave as it does so. It pushes the cold front into
central/southern Utah by Sunday night but the whole feature stalls
by Monday, with the center shifting to Arizona on Tuesday as it
continues to merge with the other wave and closes off.

What these solutions have in common is that they indicate unsettled
weather, particularly over northern Utah and southwest Wyoming,
through the entirety of the long term period. As such, have kept at
least isolated POPs in through day seven. I am most confident about
precip over northern Utah with the initial arrival of the front, so
POPs are highest there, but have generally broadbrushed POPs for the
remainder of the period, lacking confidence in the precise timing of
best precip.

With regard to temperatures, there is relatively high confidence in
the cooldown from the front as the main system moves through, with
the EC and GFS both dropping 700mb temps into the -10C to -12C range
over northern and central Utah, so have maintained a drop to maxes
up to 10F below seasonal normals by Sunday with a slow warming trend


.AVIATION...Southerly winds are expected to continue through the day
at the SLC terminal with only a 30 percent chance winds will become
either light and variable or light northerly between 20-01Z.





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