Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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991
FXUS65 KSLC 212302
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
402 PM MST Sat Jan 21 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 Wednesday)...Low amplitude ridging is
gradually shifting across the eastern Great Basin this afternoon
and have noticed subtle height rises locally in latest H5
analysis. This said, presence of the cold core of the departing
trough, ample moisture and light to moderate NW flow at the
low/mid levels continues to support orographic snowfall across the
Wasatch mountains south to Skyline and prone east bench areas
along the Wasatch Front. A continuation of building heights over
the next few hours will allow remaining snowfall to gradually
taper, with the valleys likely to see little if any significant
snowfall after 5pm. Advisories have been left in place across the
Wasatch Front and northern/central mtns as such.

Aforementioned mid level ridging will bring a brief reprieve from
active weather for the latter half of night and through much of
tomorrow morning, outside of the extreme southwest where precip
will again begin to fill in after sunrise within strengthening WAA
in advance of the next trough. The strong eastern pacific jet
that has been largely south of the area for the last two storm
events will buckle north in advance of eastward translation of
this trough, spreading overhead during the day tomorrow.

Strong SW flow both aloft and at the surface will strengthen WAA
across the area allowing precip to increase both areally and in
intensity tomorrow afternoon across much of the area. A prolonged
period of WAA within this strong flow is expected through Sunday
night, with widespread precip through that time. Anticipated snow
levels along the Wasatch Front south of Ogden at times will rise
to or just above 5000ft, with valleys across southern Utah closer
to 5500ft. Snow levels north of Ogden and in the Cache are
expected to remain at valley floors in a somewhat less mixed
environment. Anomalously high values of water vapor transport
coupled with deep layer ascent will drive heavy at times precip
overnight, especially areas favored in southwest flow. Gusty
southerly winds will likely shadow much of the SLC valley/metro
area however until cold frontal passage Monday morning. This front
will again lower snow levels to all valley levels whilst
maintaining heavy precip into the afternoon hours. Have issued
Winter Storm Watches for all mountains, the northern Front/Cache,
Wasatch Back/southwest WY, and Castle Country anticipating heavy
snow throughout much of the event. Anticipate additional headlines
will be needed for other valleys during/after frontal passage
Monday by later shifts.

Globals continue to diverge on Frontal timing Monday morning, with
the GFS remaining faster than the ECMWF. Have maintained rain/snow
wording Monday morning for the SLC Metro area, but will likely
see a rapid change to all snow shortly after sunrise due to
increased PI rates alone as winds begin to veer. Currently
accumulating snow looks to begin towards the tail end of the
commute in the SLC valley.

H7 flow in excess of 50 kts coupled with strengthening S-N mslp
gradient and surface pressure falls will drive strong and gusty
southerly winds across the western valleys. Will likely still
need wind advisories for some valley areas across the west
tomorrow afternoon/night.

Do expect precip to taper shortly after frontal passage Monday
afternoon in the north as the bulk of the deep layer
moisture/forcing will occur along and ahead of this. Will continue
to monitor as globals begin to diverge on trough evolution
overnight into Tuesday, specifically details/timing of potential
wrap around precip in the north whilst cyclogenesis occurs
downstream. For now have maintained chance PoPs through Tues.

.LONG TERM (AFTER 00Z Wednesday)...A cold deep upper trof will be
in place across pretty much the entire western conus at the start
of the long term period. An unstable and somewhat moist
northwesterly flow is forecast to be over the CWA to the west of
the axis of this trof.

The main center of this trof will be moving into the plains states
Tue night with a trailing center forecast to drop across our CWA.
Med range guidance is somewhat out of phase with this wave with
the GFS now faster than the EC. It shows instability snow showers
continuing along the Wasatch Front and adjacent mtns Tue night
into Wed while the EC fills these back in early Wed.

Both do indicate we will have a threat of snow showers lasting
thru Wed in mainly northwest flow favored areas and the cold
airmass in place implies the snow could stick to roads and
continue to impact travel.

Kept pops in the chance category for Tue night/Wed for the nrn
CWA at this time but these may need to be boosted once we get
closer to this time frame.

These snow showers taper off Wed night into Thu in the GFS while
the EC has one more trailing wave that could keep them going Thu.
Have kept a slight chance mention in the mtns for these in case
the EC is right.

The upstream ridge will be moving into the eastern Great Basin on
Fri with the EC a bit faster. Either way, looks like we are in for
a period of ridge conditions bringing dry weather over the next
weekend and into the following week. Also expect increasingly
strong valley inversions, especially with new snow on the ground.

&&

.AVIATION...At the SLC terminal...Ceilings continue to
improve to 5000 feet or above through the evening. Expecting
southerly winds to increase after 12z ahead of a cold front passage.
Snow showers likely to begin around 18z, dropping to MVFR conditions.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 8 PM MST this evening for
     UTZ007>010.

     Winter Storm Watch from Sunday morning through late Monday night
     for UTZ007>010-517.

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

     Winter Storm Watch from Sunday morning through Monday afternoon
     for UTZ001-002-006.

     Winter Storm Watch from late tonight through late Monday night
     for UTZ518.

     Winter Storm Watch from Sunday morning through Monday morning
     for UTZ012.

WY...Winter Storm Watch from Sunday morning through Monday afternoon
     for WYZ021.

&&

$$

Merrill/Wilensky/Dewey

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