Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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499
FXUS65 KSLC 172322
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
422 PM MST Sat Feb 17 2018

.SYNOPSIS...A significant and quite cold winter storm will reach
the Great Basin Sunday night, then move slowly south and east
across the region through Tuesday. The coldest air of the season
will accompany this storm, and remain over the area through much
of the upcoming week.

&&

.SHORT TERM (Until 00z Wednesday)...The two vorticity maxima in
close proximity off the BritCol coast this afternoon will separate
quickly this evening. The southern most vorticity maxima will
weaken and shear quickly off to the east along the Canadian
border. The second trailing maxima will continue to intensify as
it moves into the Pacific Northwest early Sunday, then expands
south and east across the interior west early in the upcoming
week.

The leading low-level baroclinic zone will stretch out west-to-
east along the Idaho border later tonight, then exhibit strong
frontogenesis as it moves slowly south across northern Utah
through early afternoon. The initial convective precip along and
north of the boundary early Sunday morning could start off on the
light side as the mid-level cold air and dynamic support from the
advancing trough will lag back to north of Utah. The precip-type
will also be a mix of rain and snow due to the still fairly mild
temps aloft and generally weak lift.

The rapid intensification of the low-level baroclinic zone with
the advance of the mid-level trough will result in fairly rapid
expansion and intensification of precip across northern Utah
during the morning and early afternoon hours. Snow levels will
drop quickly with snowfall rates exceeding 1 inch/hour at many
locations. The baroclinic zone will remain active as it moves
south into central/southern Utah overnight. Low-level forcing into
the boundary along with a deep conditionally unstable air mass
and solid dynamic support for lift will lead to a continuation of
moderate to heavy snow into Monday morning.

Various model guidance remain insistent that a significant push
of drier low-level air will spread across northern Utah Monday.
Model QPF fields show little or no precip from early Monday
morning onward. While the dry low-level cold advection exists, on
top of this is a rather moist confluent mid-level flow which
should be able to generate sufficient for precip. The drier air
may serve to inhibit precip in the valleys, but the higher
elevation could maintain at least some light snow Monday. Across
southern/central Utah the old low-level baroclinic zone will
stall along the I-15 corridor, and serve as a focus to maintain
light/moderate snow triggered by vorticity lobes rotating through
the base of the trough still to the west of the area.

Looking at one last round of snow as the main trough axis finally
moves east through the state Tuesday. Behind this trough axis
increasingly dry/stable and very cold air will end precip and
lead to some of the coldest temperatures of the season for most of
the state.

.LONG TERM (after 00Z Wednesday)...The timing and strength of
shortwave troughs moving through the overall longwave trough over
the western CONUS between global models from Wednesday through
Saturday is poor to say the least. As far as confidence, we have
high confidence that temperatures will be below normal during this
extended period and southern Utah should have dry conditions both
Wednesday and Thursday. Low confidence really ramps up after this
point, especially across northern Utah and southwest Wyoming where
shortwave troughs are out of phase between the EC and GFS as well as
within the ensemble members of the GFS itself. Have leaned towards
climo for PoPs across the northern half of the CWA from Friday
through Saturday until there is a clear signal on the timing and
trough between models.

&&

.AVIATION...Gusty southerly winds will prevail at the SLC terminal
through 18-20Z Sunday before shifting to the NW behind the cold
front. CIGS will lower rapidly to IFR levels within an hour two
the cold front passage with any period of rain short lived and
going over to snow with IFR visibilities by 22Z. Accumulating snow
will occur on untreated surfaces by 22-24Z and continue through
the night.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 9 AM to 9 PM MST Sunday for UTZ013-020-021.

     Winter Storm Warning from 9 AM Sunday to 4 PM MST Monday for
     UTZ006-008>010.

     Winter Storm Warning from 5 AM Sunday to 5 AM MST Monday for
     UTZ007.

     Winter Weather Advisory from 5 AM Sunday to 5 AM MST Monday for
     UTZ005.

     Winter Storm Warning from noon Sunday to noon MST Monday for
     UTZ003-004.

     Winter Storm Warning from 5 AM to 11 PM MST Sunday for UTZ001-
     002.

     Winter Storm Warning from 5 PM Sunday to 9 PM MST Monday for
     UTZ014>016-517-518.

     Wind Advisory from 9 AM to 8 PM MST Sunday for UTZ015-016.

WY...Winter Storm Warning from 9 AM Sunday to noon MST Monday for
     WYZ021.

&&

$$

SHORT TERM...CONGER
LONG TERM/AVIATION...STRUTHWOLF

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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