Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 250341

Area Forecast Discussion
National Weather Service Salt Lake City UT
941 PM MDT Fri Mar 24 2017

.SYNOPSIS...The upper level trough along the west coast this
evening will move east across the Great Basin on Saturday. High
pressure aloft will return briefly on Sunday, followed by another
storm system for early in the upcoming week.


.SHORT TERM (Through 12z Monday)...The upper level trough just off
the west coast this evening will continue east across the Great
Basin Saturday through Saturday evening. This trough has tapped
into a substantial moisture plume, with this moisture already in
place across western Utah this evening.

Shortwaves ejecting northeast out ahead of the main trough will
bring an increasing chance for precip across western/northern Utah
through Saturday morning. The first shortwave that moved into
northwest Utah this afternoon generated more clouds than precip as
there was little in the way of thermal advection to support deep

The second shortwave just now reaching the central Sierras looks
far more promising in terms of generating widespread precip late
tonight through early Saturday afternoon. The near 700mb
baroclinic zone moving across Nevada this evening will reach far
northwest Utah shortly after midnight, then extend from northwest
Wyoming through west-central Utah early Saturday. Precip will
initially looks to be light and lagging back to the west of the
baroclinic zone over far northwest Utah. Precip will then intensify
and turn more convective Saturday morning as the mid-level
shortwave ripples northeast along the low-level baroclinic
boundary and trailing cold advection becomes stronger and deeper
behind the passing shortwave. Also thinking that convection could
for a short period generate heavy precip as the entrance region
of the upper jet sets up over northern Utah.

The convective precip should remain well organized along the
low-level thermal boundary over central/southwest Utah as the
low-level cold advection remains substantial and the main upper
trough begins to work into western Utah with the associated mid-
level cold pool to further destabilize the air mass.

Instability showers will likely persist in the cold air aloft
associated with the upper trough across northern Utah Saturday
afternoon. Convective precip should gradually wind down Saturday
evening as the upper trough exits to the east and drier more
stable air is ushered by the next shortwave ridge moving across
the Great Basin late Saturday night/Sunday. Increasing clouds
ahead of the next Pacific trough will spread across the region
Sunday night, with the potential for a few showers working into
western/northern Utah by late Sunday night.

.PREVIOUS LONG TERM (After 12Z MONDAY)...The long term period
begins with a trough centered upstream of the area over the
Pacific coast. Since yesterday`s 12Z cycle, guidance has come into
much better agreement regarding the evolution of this feature...
essentially keeping it as an open wave while it moves over Utah
late Mon/early Tues before finally closing it off as it digs south
over the four corners. Given this projected track expect
widespread precip to impact much of the state Mon-Tues... with
relatively high forecast. precip totals over Northern/Central Utah
Monday afternoon-evening in association with an active (although
not particularly tight) mid-level baroclinic zone and adequate
moisture (~90 percentile PWATS are forecasted by the GFS
ensembles). As this trough and associated front pass through the
state, average snow levels will fall from around 7k feet Mon
afternoon to around 5k feet Tuesday morning. Despite most guidance
keeping snow levels just above N.Utah valley floors for the
duration of precip can`t completely rule out some snow falling on
the northern valley floors Monday night/Tues morning. However,
this period of snow would likely be brief with limited impacts (as
the bulk of the precip should occur when snow levels are over 6k
feet). There is potential for some higher elevation impacts with
this system, however, fairly low snow ratios may limit
accumulations even in locations where most of the precipitation
falls as snow.

As the low progresses eastward into New Mexico during the day on
Tuesday, precipitation should taper pretty quickly across the state
from west to east as heights rise and mid level temps warm. This
warming trend and tranquil weather should continue into Thursday,
before another colder Pacific trough is progged to dig/amplify over
the state of Utah. This scenario would result in a fairly potent
frontal system impacting the state and also a rather cold
postfrontal airmass settling over the state late Thurs/early Friday.
For example the GFS forecasts H5 temps around -29C and H7 temps
around -11C in Northern Utah early Friday, which if verified would
be the coldest temps we have seen in over 3 weeks. Such a system
would likely result in snow levels falling to valley floors and at
least a day or two of rather cold (by late March standards)
temperatures. However, being 6 days out (despite relatively good
agreement between guidance at the moment) did not make too many
forecast. edits besides lowering snow levels/decreasing temps
slightly and raising PoPs Thursday into Friday


.AVIATION...Ceilings will continue to lower overnight at the SLC
terminal ahead of the next upper level feature. Ceilings will
likely drop below 6k feet around 09-11Z although there is a 20
percent chance ceilings could lower earlier. Winds will shift to
the west ahead of the advancing cold front late tonight, with the
front passing through the terminal between 12-14Z. Rain will
remain heaviest behind the front, with MVFR cigs/vis possible in
this heavier precip.





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