Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
520 am MST Sat Dec 7 2013
Synopsis...the cold winter storm system centered over the
western Great Basin will track east across Utah this afternoon
through tonight. Widespread snow through Sunday will be followed
by very cold temperatures heading into the first half of next
Short term (through 00z Tuesday...the cold core upper trough
moving into the northern/western Great Basin from the Pacific
northwest will continue east into and then across Utah this
afternoon through tonight. Light snow has already formed ahead of
the trough in the moist warm advection across southern and
eastern Utah. This snow will expand and intensify fairly quickly
this morning as the moist warm advection strengthens and the exit
region of the upper jet works into southwest Utah.
The entire region should be in accumulating snows by late morning
or early afternoon as the upper trough settles into northeast
Nevada. The heaviest snowfall likely to develop across the region
around middle to late afternoon as good low-level cold advection
moves into northwest Utah. Destabilization of this moist airmass
with the arrival of the middle-level trough will bring a period of
heavy snow to the mountains and many northern/western valley
locations through late tonight.
Snow will begin to decrease in both areal coverage and intensity
late tonight/early Sunday as the axis of the upper trough tracks
off to the east of the forecast area. Showery precipitation will linger
in the cold air instability Sunday afternoon. A trialing shortwave
moving south-southeast behind the exiting main trough will likely
bring short period of snow to the north Sunday evening. A stable
northerly flow across the region will keep temperatures bitterly cold
heading into the first half of the week.
Long term (after 00z tuesday)...a dry northwesterly flow will
develop Monday night and Tuesday...with only far northern Utah
retaining enough moisture to potentially allow for mountain showers
on Tuesday. Conditions will stabilize even more on Wednesday...as
the ridge shifts from the West Coast to over the Great Basin. The
ridge axis will likely slide over the County Warning Area by midday Thursday.
This rising of heights will also be associated good warm advection
aloft Tuesday through Thursday...which should raise mountain
temperatures significantly from the bitter cold of the past week.
However...any valley warming is likely to be modest in most
locations...with inversions expected to set up. While the forecast
still indicates a slow warming trend...have tried to keep fairly
cold temperatures for both maxes and mins in many basins.
Details beyond Thursday are still low confidence...as models
continue to have issues on how to to handle the next trough forecast
to move through the western Continental U.S.. while details differ...both the
European model (ecmwf) and GFS have trended toward digging this trough further south
into Baja California...which would result in less precipitation and less mixing
for the County Warning Area. Have kept a slight chance of probability of precipitation in the northern half
of the County Warning Area for Thursday night...as models still hint at some low
level moisture moving into the area with weak forcing. Other than
that however...have mostly kept probability of precipitation near or a little below
climatology...as models indicate a mean trough moving through the region
but little dynamic forcing over Utah. As such...have also tried to
keep valley temperatures inverted through the end of the forecast.
Aviation...ceilings will lower at the slc terminal through the
morning hours...likely dropping below 7000ft between 13z and 16z.
Winds will increase from the south through the morning. There is a
30 percent chance that gusts reach 30 miles per hour or higher...with the most
likely timeframe between 14z and 18z. Snow will develop at the
terminal later today...with IFR conditions most likely starting
between 20z and 23z...but intermittent IFR snow showers could occur
as early as 15z.
Utah...Winter Storm Warning until 10 am MST Sunday for utz011>013-
Winter Storm Warning until 6 PM MST Sunday for utz002>004-
Winter Storm Warning until noon MST Sunday for utz001-005-
Wyoming...Winter Storm Warning until 6 PM MST Sunday for wyz021.
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
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)