Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 091700
Area Forecast Discussion
National Weather Service Salt Lake City UT
1000 AM MST Fri Dec 9 2016
.SYNOPSIS...A persistent and quite moist westerly flow aloft will
generate significant precipitation across much of northern Utah
and southwest Wyoming into early next week. Southern Utah will
remain south of the storm track until late in the weekend.
.DISCUSSION...Strong low/mid level convergence continues to exists
across the eastern pacific this morning with near zonal flow extending
east into the interior west, this maintaining a focused moisture
plume penetrating into the intermountain region attm. Have seen
the strong anticyclonically oriented upper jet begin to shift
north however and short term progs indicate the core will settle
over Montana late today through tonight. Northern Utah will remain
on the southern periphery of this jet into tomorrow morning, with
progressive short wave energy most likely impacting the UT/ID
border and points north over the next 18 hours or so, this after
one last embedded wave translates east across the area north of
I-80 come midday.
Though very deep moisture remains in place across the north,
forcing is limited attm limiting PI rates in the mountains and
areal extent of activity across the valleys of the north. Updated
PoPs to follow these trends, and also to add shower wording in the
northern valleys vs. more persistent stratiform precip. P-type
remains a challenge north of the Salt Lake metro area, with the
full gamut of mixed precip at times in the northern Wasatch Front
and areas like the Cache still experiencing near isothermal snow
showers at times. Still expecting a transition to mixed precip in
the Cache this afternoon as WAA aloft will continue to drive warming
profiles and a net rising of snow levels.
Temps quite the challenge in some of the protected basins such as
Great Salt Lake desert, Uinta Basin, and Castle Country as well. Cold
air remains more entrenched in these areas with strong low level
inversions present. Opted to lower Max temps fairly significantly
those areas as mixing looks hard to come by today.
Focus of shift will be to delve into the forecast details through
Sunday morning. The best period for widespread and heavier precip
remains on tap Saturday afternoon into Sunday.
Previous Discussion below...
Rising snow levels will also confine accumulating snows to the
higher elevations of the northern mountains. Have scaled back snow
accumulations a bit the next couple days as dynamic/thermal lift
not particularly strong and the warmer temps aloft not the best
for good ice crystal growth.
By Sunday a somewhat stronger embedded shortwave will cross the
northern Great Basin. This feature will bring significantly colder
air aloft back into northern Utah to go along with the improved
dynamic lift and the rather moist air mass still in place. Looking
at the heaviest snows of this long duration event to develop
across the far north late Saturday night, then spread south during
the day Sunday. Lowering snow levels will also bring the threat
for valley snow back to the higher northern valleys during the day
Southern Utah will finally get a decent shot at rain/snow as the
colder air works south through western Utah Sunday. Not looking at
any substantial precip outside of the central mountains as the
mid-level cold air and dynamic support for lift remain well to the
north of the low-level cold air working south through the area.
A slight increase in heights across the region Sunday night/Monday
will push precip back to the north, with the best chance for a
continuation of snow confined to the far northern portion of Utah
The active pattern looks to continue through the extended period,
though the global models continue to disagree on some of the
details. After a brief break, latest EC indicates warm advection
precip developing again during the day Tuesday while the 00Z GFS
showed this starting a little earlier. EC then moves this
precipitation primarily north of the area Wednesday into Thursday
with high pressure developing, while the GFS shifts this area of
precipitation into central and southern Utah during this time.
With this divergence, have taken a broad brush approach to POPs
with limited confidence in the location of precip over Utah and
southwest Wyoming during the Wednesday/Thursday timeframe. By day
seven, all guidance indicates a broad Pacific storm system
entering the forecast area, with the potential to produce
widespread precipitation. Models have been consistent in showing
this for a few runs, so have kept POPs on the high side for the
end of the forecast period.
.AVIATION...At the SLC terminal...VFR conditions are expected to
prevail through the TAF period. There is a 30 percent chance that
CIGS will not lower to BKN060 or lower between 18 and 21Z. Southerly
winds are expected to prevail through this TAF period but could
occasionally become light and variable, primarily through 21Z
UT...Winter Weather Advisory until noon MST today for UTZ010.
Winter Storm Warning until 4 PM MST Saturday for UTZ007>009.
Winter Storm Warning until 3 PM MST this afternoon for UTZ006.
WY...Winter Weather Advisory until 3 PM MST this afternoon for WYZ021.
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