Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
748 PM MST Friday Dec 26 2014
Synopsis...high pressure aloft will slide southeast across Utah
later tonight through Saturday. The next cold Pacific storm system
will reach the area late in the weekend... and remain through the
first half of next week.
Discussion...water vapor loop shows the upper trough axis
shifting into The Rockies with an eastern Pacific Ridge. The next
storm system is over the Gulf of Alaska. Amdar 400-250mb wind
observations indicate a 100- 115kt northerly jet from western
Idaho through Nevada and Southern California. GOES/GPS/rap/00z
radiosonde observation indicate the precipitable water value ranges between 0.05"
mountains...to 0.20" most valleys. Blended total precipitable
water product indicates a Pacific tap into British Columbia where
values were greater than 150% of normal.
Until the aforementioned jetlet rounds the base of the trough...the
upper level trough will remain stuborn just to the east. This
continues to support areas of low and middle clouds across the state.
Where pockets of clearing take place...recent snowpack and
slackening winds may support patchy fog...especially near the Idaho
border...Wasatch back and I-15 corridor across central Utah.
For the few areas reporting precipitation...near Beaver and
Roosevelt...kept a mention of flurries. Otherwise no probability of precipitation for the
remainder of the night. Adjusted sky cover based on 11-3.9u and infrared
Pressure graident is still supporting a few gusts in the 30mph range
across southern and east central Utah...so updated grids to
represent those conditions.
Otherwise a cold night in store due to the drying dew points and
Previous discussion follows...
The upper low exiting to the east through the central/southern
rockies this afternoon should no longer impact the area after this
evening. A trailing vorticity maximum moving rapidly south through
eastern Nevada will generate clouds and a few light snow showers
near the Nevada/Utah border this evening. Gradual clearing...near
snow cover and cold air aloft will all serve to bring cold temperatures to
much of Utah overnight. Model guidance all suggest that plenty of
low-level moisture will linger across the western zones tonight.
This moisture banked into the higher terrain could maintain pockets
of low clouds and maybe some flurries during the night. Fog is not
likely to be a widespread issue late tonight/Saturday morning as
many areas will have sufficiently large dew point spreads and enough
wind to stir the boundary layer and inhibit fog development.
The shortwave ridge moving northwest to southeast across Utah
Saturday will be followed by the next cold Pacific storm arriving
late in the weekend. This next trough will reach the Pacific
northwest coast Saturday...then expand into a broad northeast to
southwest oriented trough across much of the region for late
Light precipitation should arrive ahead of the trough...driven by moist
warm advection overrunning cold low-level air mass Saturday night.
Dynamic lift from the trough and deep layer cold advection should
fuel an increase in snow for Sunday through Sunday night. Snow will
gradually taper off across the north Monday as the main baroclinic
zone orients west to east across central then southern Utah late
Have issued a Winter Storm Watch for the northern third of Utah late
Saturday night through early Monday. Assuming that the various model
guidance of /.45/ - ./60/ qpfs is correct with the expected low
density snow in this cold an air mass...the result should be heavy
snow across many areas of northern Utah. Snow totals will likely
bounce between heavy snow criteria and advisory criteria. The
tipping factor here will be the anticipated high impact due to
travel at the end of the Holiday weekend. Suspect that a mix of
warnings and advisories will be in place by the time this event
begins later in the weekend.
High amplitude ridge along the West Coast of Canada will allow a
storm system to dig due south along 120w before pinching off over
Nevada by Tuesday night. This will place the 700 mb baroclinic zone
between an Arctic air mass pushing in from Wyoming and a moderated
Pacific air mass from the southwest. Because the origin of this
storm is Continental it is lacking in deep moisture. Therefore
despite a prolonged time that this baroclinic zone will trek across
central and southern Utah Tuesday night through
Wednesday...precipitation amounts do not look to be that great.
There is still moderate uncertainty with this strength of this storm
over the southern half of the County Warning Area as there is still over 100 meters
of difference in the 500 mb heights between the GFS ensemble
members. The ec has trended a little towards the GFS with the core
of the closed low now near Reno at 12z Wednesday verses San
Francisco. Bottom line is have gone with above climatology probability of precipitation Tuesday
into Thursday but due to the uncertainty held off from putting
likely probability of precipitation in as of this time.
The main impact and highest confidence for this storm will be the
gusty easterly winds Monday night through early Tuesday across the
Aviation...light southerly winds are expected to prevail at kslc
overnight through middle morning Saturday. Occasional ceilings in the 7-8
kft range will persist through the evening hours before scattering
out for good by roughly 10z.
Utah...Winter Storm Watch from late Saturday night through Monday
morning for utz006>009.
Winter Storm Watch from late Saturday night through Monday
morning for utz001>003.
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http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)