Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
1126 am MST sun Dec 21 2014
Synopsis...a very mild and moist northwest flow will continue
across northern and central Utah today into tonight. A cold front
will move south across the area late tonight and Monday bringing
a cooler but drier airmass into the region through Wednesday.
Discussion...broad ridge axis situated off of the West Coast
placing Utah under strong northwest flow. Over the top of the
ridge a significant inland penetrating atmospheric river is
bringing a substantial amount of moisture into northern Utah and
southwest Wyoming today. The 1200 UTC kslc sounding had .58"
precipitable water...which is 2 Standard deviations above normal
for this time of year. In addition to the substantial moisture
plume...northwest flow aloft is very strong with 80-90 knots of
flow expected at 500 hpa over northern Utah and southwest Wyoming
this afternoon. The combination of anomalously deep moisture and
strong flow points to an unusual event underway with forecast
integrated water vapor transport values having return intervals of
once every 20+ years. The strong and very moist flow is occurring
in a modest warm advection environment which yields significant
isentropic upglide through the evening hours. With this in mind
expect widespread precipitation to continue across northern Utah and SW
Wyoming through this evening with an increase in intensity for most
Also...with very strong wind aloft anticipate that high
winds..with gusts to 60 miles per hour...will develop across SW Wyoming today.
Across northern Utah Ridgetop winds may be quite significant with the
highest exposed peaks in the Wasatch potentially receiving 90+ miles per hour
wind gusts this afternoon and evening. Ongoing winter storm
warnings for Utah/S northern and central mountains look to be in
good shape as does the High Wind Warning for southwest Wyoming. Believe
that most significant snow amounts will be in the Wasatch Range
north of I-80 where storm totals of 24+ inches of snow are
possible...in the most favored areas...by the time the storm winds
down on Monday. Further south...the 10-20" range still looks good
for the Wasatch mountains south of I-80...although locally higher
amounts will be possible above 8500 feet.
Snow levels have been a bit stubborn and have been slow to rise
this morning. However...still anticipate that they will rise this
afternoon. Have trended the snow levels down a bit and now
anticipate that the snow level will rise to 7000-7500 above ground level this
A shortwave dropping into the backside of a trough in the northern
plains will nudge a backdoor cold front into northern Utah and SW Wyoming
later tonight. This will turn the flow to the north and begin
ushering drier air into the region. As the front pushes south the
precipitation will diminish across northern Utah on Monday while it
continues in central Utah through much of the day.
The upper level ridge axis off of the West Coast sharpens
substantially and moves over the interior west for Tuesday and Wednesday
resulting in a period of dry weather over the eastern Great Basin
ahead another approaching cold front which will impact the region
during the latter portion of the week.
With this update made some minor adjustments to the snow
levels...limited how much the snow level rises this afternoon...which
will keep a mix of rain and snow in southwest Wyoming through the day.
Also...made some minor adjustments to the maximum temperatures
along the Wasatch front and east central valleys based on current
trends. Finally...increased winds along ridgetops in northern Utah to
match expected gusts.
Aviation...VFR conditions are expected to prevail at the slc
terminal during the daytime hours with ceilings below 6000 feet above ground level in
rain. Some periods of high-end MVFR conditions will occur in heavier
rain through the afternoon. Southeast winds are expected to shift to
the northwest with the passage of a cold front between 00z and 02z.
Utah...Winter Storm Warning until 6 PM MST Monday for utz007>010-517.
Wyoming...High Wind Warning until 2 am MST Monday for wyz021.
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