Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 202301
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
401 PM MST Fri Jan 20 2017

.SYNOPSIS...The active weather pattern will continue across the
region, with a series of weather systems crossing the area through
the first half of next week.

&&

.SHORT TERM (Through 00z Tuesday...A progressive short wave trough
coincident with a 140+ kt NW-SE oriented pacific jet continues to
translate southeast into the lower Colorado River valley this
afternoon. H5 analysis depicts increasingly diffluent flow aloft
across the eastern Great Basin, and this can be noted in the
expanding cloud shield across portions of NV/UT/AZ in IR at this
time. Deep moisture advection is trending to increase across the
southwestern half of the state, with H7 speed convergence notable
across the southwest. Snow above 4500ft has increased in both
areal coverage and rates across the southwest over the last few
hours, with rain below.

A combination of continued moisture advection due to a sizable tap
off the eastern pacific, increased forcing aided by favorable left
exit region of the jet, increased orographics along the Staircase,
and continued diffluence aloft will bring very heavy precip to the
southern mountains/Dixie/Powell regions through the first half of
the night at least. Have issued a Winter Storm Warning for the
Southern Mountains where 12-20 inches of snow is likely to fall by
sunrise tomorrow, and a Winter Weather Advisory for the prone
upslope areas around Kanab and lower Staircase, primarily
elevations over 4500ft. Mainstem rivers in the Dixie/Zion areas
will likely see some rises due to an expected .50 inches of rain
during that time as well. Similar conditions will exist further
east across the Swell and into Castle County namely after
midnight. Have issued Winter Weather Advisories those areas as
well, outside of Dixie/Powell.

Further north the primary forcing mechanism for precip will be the
diffluent flow aloft tonight, with some lift aided by H7 speed
convergence within the increasingly moist airmass. In general
looking for light snow to fill in south to north this evening,
with light snow expected most all areas overnight into the morning
hours. Eastward translation of the trough into the desert
southwest tomorrow will drag the H5/H7 trough axis west to east
across the area likely bringing the best chance of moderate snow
and greatest rates of accumulation to the Wasatch Front/I-15
corridor tomorrow morning, with adequate NW flow and conditional
instability maintaining snow into or through the afternoon.
Although the Salt Lake Valley will likely suffer from some
shadowing effects due to modest south winds, and may flirt with
rain briefly at times overnight, accumulations are expected to
come rapidly Saturday morning with this axis passage. Have issued
Winter Weather Advisories for all northern valleys/Wasatch Back as
well.

The remaining mountain areas are expected to see modest snowfall
through tomorrow evening. Initially the southwesterly favored
areas will see the greatest totals, but this will switch to the
northwest with axis passage tomorrow allowing anticipated totals
to even out at that time as well. Have issued Winter Weather
Advisories for all of these other mountain areas as travel will be
impacted on all mountain routes over the next 24-30 hours.

Snowfall along the I-15 corridor and surrounding mtns will taper
fairly rapidly late tomorrow afternoon/evening with further
downstream translation of the trough and modest stabilization
aloft. All Advisories/Warning end tomorrow evening at the latest.

Any break in the action will be short lived as the next trough
races inland Sunday, likely spreading precip in west to east
Sunday afternoon. Integrated Water Vapor Transport is quite
impressive with this trough, as an atmos river looks to sneak
around the southern Sierra allowing penetration across the state
namely Sunday Night and Monday, this as the eastern pacific jet
backs SW-NE and positions overhead. Strong WAA will develop Sunday
with increasingly strong H7 winds aloft, and coupled with a
strengthening S-N mslp gradient do expect deep mixing/gusty south
winds to drive a net rise of snowlevels to around 6000ft Sunday
night into Monday. Passage of a cold Monday will allow for snow
back to most all valleys as CAA takes form, likely enhancing rates
Monday morning/afternoon. This deep moisture combined with strong
lift will once again aid widespread and potentially heavy precip
through Sunday night/Monday before waning in wake of frontal
passage. Expect the need for future highlights.

.LONG TERM (After 00Z Tuesday)...The midlevel trough axis will cross
Utah Monday night switching 700mb flow to northwest. Precipitation
will become more confined to the mountains and windward valleys
heading into Tuesday morning as colder air spreads into the area and
snow levels lower. Precipitation in all valleys but Dixie should be
all snow by Tuesday morning.

A cold and moist cyclonic northwesterly flow within a mean longwave
trough over the Western CONUS will keep unsettled weather over the
forecast area through at least Wednesday night. Embedded
disturbances/circulation centers within the mean trough will bring
periods of more widespread but generally light snow during this
timeframe and have trended PoPs upward as a result.

Models diverge by Thursday during the transition period when the
trough exits the area and high pressure builds into the West Coast.
With the GFS carving the trough farther back, it is slower than the
EC with the pattern shift and the arrival of the next weak but dry
disturbance. As a result, models are out of phase for Thursday and
Friday with significant differences in 700mb temperatures noted.

&&

.AVIATION...Winds at the SLC terminal should remain southerly
through at least 09Z, though there is a slight chance of variable
winds due to increased shower activity tonight. Snow showers will
increase across northern Utah starting at 00Z, but KSLC could be
shadowed through much of the night due to southerly winds, with
extended periods of VFR conditions likely at the terminal as snow
increases both to the north and south of the Salt Lake Valley.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until noon MST Saturday for UTZ005-
     011>016-020.

     Winter Weather Advisory until 8 PM MST Saturday for UTZ007>010-
     517.

     Winter Weather Advisory until 5 PM MST Saturday for UTZ001>004-
     006.

     Winter Storm Warning until 8 PM MST Saturday for UTZ518.

WY...None.
&&

$$

Merrill/Cheng/Schoening

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