Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 110334
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
834 PM MST Mon Dec 10 2018

.SYNOPSIS...Two weather disturbances will impact the area through
Wednesday, The first will weaken as it crosses far northern Utah
and southwest Wyoming late tonight through early Tuesday. The second
stronger and colder storm system will bring significant snow to
the northern half of the area and breakdown the persistent valley
inversions over northern Utah.

&&


.DISCUSSION...
Unfortunately the weaker splitting solution is going to win out
later tonight and tomorrow. This means a much lower chance for
light snowfall, as well as waiting another day to bust the
inversion.

For tonight, kept idea of chance PoPs across the far north, closer
to the track of the northern piece. Also added chance PoPs to the
southern mountains downstream of the southern piece, and in the
vicinity of potential jet coupling.

If/where any light snow falls, there is the potential for locally
slippery travel as road temperatures fall into the 20s.

Added Haze back into the forecast for the Urban Corridor tonight with
no sign of breaking the inversion yet.

&&

.PREVIOUS DISCUSSION...
Two separate weather disturbances are scheduled to cross portions
of the forecast area through Wednesday. Strong high pressure
aloft will rebuild across the Great Basin on Thursday.

The first disturbance currently lined up over the west coast
states will split as it advances east toward Utah tonight. The
southern portion of the split will develop into a closed
circulation over southern California Tuesday, then drift southeast
into northwest Mexico with no real impact on southern Utah Tuesday
night through Wednesday.

The northern portion of the split will track due east through the
northern Rockies Tuesday. The bulk of the dynamic support for lift
along with the best cold air aloft will remain well north of Utah
as this feature tracks east overnight. The far southern portion
of this feature will generate a short period of low-level cold
advection and weak dynamic lift over far northern Utah late
tonight through early Tuesday. Anticipating little more than a
short period of light snow late tonight/early Tuesday with little
mixing to breakup the valley inversions in place across northern
Utah.

A second colder/stronger upper trough will take aim on the Pacific
Northwest Tuesday night, then continue southeast across the northern
Rockies and Great Basin Wednesday. Looking at solid baroclinic
structure with this second feature as it passes through
northern/central Utah and southwest Wyoming. The leading surface
cold front will reach the northern border late Tuesday night. This
front will continue at a fast clip through the state pushed along by
fairly strong low-level cold advection. Increasing instability along
with strong dynamic lift associated with the mid-level trough with
interact with the low-level forcing to generate deep convective lift
and periods of moderate to heavy snow behind the surface front
across northern/central Utah. Snow will pickup again along the
trough axis and within the mid-level cold pool, with orographic
enhancement of snowfall along the Wasatch range during the
afternoon. Dynamic subsidence along with steady warming aloft should
bring an end to most if not all snow showers Wednesday evening.

A secondary and equally important event will be the breakup of
valley inversions and the dramatic improvement in air quality over
the northern urban areas. This improvement will persist through
Thursday, then likely deteriorate once again after Thursday as
strong high pressure aloft returns to the region.

High pressure will quickly build back over the area for Friday,
bringing rapidly drying conditions with warming aloft. This will
result in mild conditions for the higher terrain, with some
inversion re-development for the valleys. However, the ridge axis
is progged to quickly shift east of the area by Friday night ahead
of the next approaching storm system.

EC/GFS coming into better agreement with this system, indicating
that it will quickly graze northern Utah Saturday morning.
Associated precipitation is not expected to be limited and confined
to the higher terrain of northern Utah, but the associated cold
front should provide enough mixing to keep inversions temporarily at
bay.

Behind the exiting system, high pressure is then expected to return
for the remainder of Saturday into Sunday, but global models
continue to indicate it will be bumped east in favor of another
Pacific storm system arriving for day seven. Latest GFS indicates
another grazer for northern Utah while the EC shows the system
diving into Arizona and New Mexico. Regardless, this progressive
pattern should keep inversions from getting too bad over the next
several days.

&&

.AVIATION...
MVFR conditions are expected through the night at the SLC
terminal in stratus, with a 30 percent chance of periods of IFR
ceilings overnight. Generally light and prevailing northwesterly
winds are anticipated during the overnight hours, with light
southeasterly winds developing around sunrise.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

PUBLIC...10/Conger/Traphagan
AVIATION...Traphagan

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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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