Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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357
FXUS65 KSLC 182258
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
358 PM MST Wed Jan 18 2017

.SYNOPSIS...High pressure aloft will remain over the Great Basin
for the remainder of the day today. This high pressure will give
way to a series of weather disturbances beginning tomorrow and
lasting into next week.

&&

.SHORT TERM (Through 00z Sunday)...The mid level ridge axis that
has been over the area as of late has shifted slightly downstream
today, this in advance of the approaching trough beginning to
impact northern/central Cali attm. Increasing southwesterly flow
aloft continues to nose into western UT, and with a strengthening
S-N mslp have noted well mixed environments across the
southwestern valleys and portions of west central Utah. Temps
have rebounded as such and many areas in those locales are in the
40-49F range . Stratus across the Uinta basin has thinned some,
but remains largely in place. Little change in stratus has
occurred over the northwest deserts. Modest inversions in place
those areas continue to hold temps in the low to mid 20s or lower,
and will be significant moving forward.

Aforementioned trough will continue to amplify tonight due to
increased influence of a 160-180 kt jet upstream over the eastern
pacific, this splitting the trough along the CA coast while
driving a more negative tilt/upper diffluent wave passage across
the eastern Great Basin tomorrow. A modestly deep moisture tap on
the eastern periphery does look to prime the mid levels, with lift
focused within the upper diffluent sector tomorrow morning into
the midday hours across namely western and southwestern Utah.
Onset of precip across the west will occur late tonight as this
diffluence starts to become established. Although forcing will be
generally weak, with light precip over a widespread area, keying
in on higher impacts/greater totals over the northwestern deserts
and west central Utah where inversions are more pronounced and
dynamic lift should be focused, especially with trough axis
passage during the afternoon/evening hours. Do anticipate seeder
feeder process to aid increased accumulations where stratus is
present and likely to remain into tomorrow. Have issued Winter
Weather Advisories for these valley areas, as very cold sub
surface/cold surface temps exist and any snowfall will accumulate
rapidly.

Have also issued Advisories for the mountains of central and
southern Utah. Upslope combined with the diffluence aloft and
closer proximity to the PVA looks to aid modest snow totals across
the southern mountains especially, with up to or just over a foot
for prone areas such as the Pine Valleys/Cedar Breaks areas by
Friday morning. Further north forcing is primarily from weak
convergence and the diffluent flow aloft, but still expected 4 to
8 inches across the central mountains impacting travel over
Skyline, US 6 near Soldiers, and the passes near Cove Fort on
I-15. Coordinated with UDOT regarding this.

Elsewhere, generally light snow or a mix of rain and snow (valleys
along the central/southern Wasatch Front) as valley inversions
gradually attempt to mix beneath a marginal vertical profile for
periods of mixed rain...especially in the afternoon. Limited
impact if any anticipated attm. The progressive nature of the
pattern will allow rapid downstream propagation of this trough
Friday morning allowing a brief break, though clearing looks
limited. Have ended all Advisories by Friday morning.

The next trough will race in coincident with the encroaching jet,
with mean placement from central CA into the desert southwest. The
deepest moisture and forcing does look to remain across southern
Utah and points south, but upper diffluence and passage of a
trough axis early Saturday morning should aid to yield widespread
light precip (mostly light snow) across the area. Will be focused
over southern Utah however where more moderate PI rates are likely.
Have upped PoPs across the south for the first half of the event,
and maintained high chance/likely PoPs along the I-15 corridor and
adjacent mtns coincident and in wake of trough passage Sunday, as
light northwest flow will aid orographic influence.


.LONG TERM (After 00Z Sunday)...The forecast area is expected to be
under weak ridging Saturday night into Sunday morning. However,
lingering moisture will keep at least some precipitation going,
primarily over the higher terrain.

Precipitation is progged to become more widespread again with the
arrival of the next wave Sunday night/Monday. Initial wave is not
hugely impressive but is accompanied by a good amount of moisture
and a strong jet. EC/GFS the associated cold front will cross the
area late Monday morning/early Monday afternoon with some precip
lingering into Tuesday with cold pool instability as the trough
tracks slowly east. EC shows a greater focus of precip for southern
Utah during this time as it takes the center of the wave along a
more southerly track. Have not tried to get too detailed with POPs
at this time given these subtle differences.

A drier northerly flow is then expected to develop beginning
Wednesday evening/Thursday behind the exiting wave. Latest ECMWF
keeps some lingering moisture and shortwave energy over the eastern
half of Utah through Thursday while the GFS is quicker to push this
out of the area. Have kept some isolated POPs in through day seven
to account for this. All guidance shows strong ridging and thus a
return of inversion conditions after day seven.

&&

.AVIATION...VFR conditions are expected to prevail at the SLC
terminal through the evening with increasing clouds. There is a 30
percent chance of MVFR conditions developing for a time this
evening. Generally light northwest winds are expected to shift to
the southeast between 03Z and 05Z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 2 AM Thursday to 5 AM MST Friday
     for UTZ010-015-517-518.

     Winter Weather Advisory from 2 AM to 11 PM MST Thursday for
     UTZ005.

WY...None.
&&

$$

Merrill/Traphagan/Schoening

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