Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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637
FXUS65 KSLC 202217
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
417 PM MDT Wed Sep 20 2017

.SYNOPSIS...A cold upper trough will move into Utah starting
tonight. This colder storm system will linger over the area
through the end of the week and into the upcoming weekend.

&&

.SHORT TERM (Through 12Z Saturday)...Mostly sunny to partly cloudy
skies have developed over Utah and southwest Wyoming this
afternoon, as the area is awaiting a storm system moving in from
the northwest. This storm is already producing increased cloud
cover over far northern Utah, which will push slowly southward
through the night. Combined with continuing southwesterly winds in
many locations, nighttime temperatures should remain relatively
warm.

The upper level trough swings over the Great Basin tomorrow, with
the cold front slowly pushing across northern and western Utah
through the day. Ahead of the front, southwesterly winds will ramp
up considerably, with 700mb winds generally forecast to be 35-50
knots by 21Z Thursday. Have issued a wind advisory on this shift
for most valleys in the southern half of the forecast area. A
couple of areas that were left out- the Sevier and Sanpete
valleys, as well as Castle Country- may have to be added in later.
In addition, considered a wind advisory for Utah County, where
strong post-frontal winds may ramp up tomorrow afternoon/evening,
as the gradient with the cold front tightens in place over the
Wasatch Front. Have decided to hold off on this advisory for now,
but these winds are definitely worth keeping an eye on in this
prone area.

Behind the front, widespread precipitation is expected, with snow
levels eventually dropping to 6500-7500 feet across the northern
half of the CWA. While snow amounts in the mountains are not
expected to get out of hand (generally 4-8 inches above 7500 feet
expected), this storm should bring accumulating snowfall to high
elevation seasonal roadways, which all remain open. As such, have
issued a winter weather advisory for the northern mountain zones
to highlight this potential impact to hunters and other
hikers/sightseers/etc. who may be on the roads or in the highest
elevations.

After the best frontal precipitation Thursday afternoon through
Thursday night, precip is expected to become more showery through
the day Friday. That said, a fair amount of additional
precipitation may be possible, especially in the mountains. The
current end time for the winter weather advisory (18Z Friday) is
a bit of a compromise between the GFS, which winds things down
considerably after 12Z Friday, and the ECMWF which keeps decent
precipitation going through 00Z Saturday. Also behind the front,
temperatures get cold enough to produce lake effect precipitation,
but a lake band does not appear likely at this time, as the flow
direction (somewhat light and erratic) does not look favorable.
Precipitation in general looks to continue tapering off heading
into Friday night.

.LONG TERM (After 12Z Saturday)...The final piece of the northern US
trough begins to drop southward over the Great Basin and slowly
progress eastward during the day on Saturday. Although this general
evolution is portrayed by all guidance the speed and amplitude of
this feature varies with the EC being slightly more amplified and
thus about 6 hours slower than the GFS solution. The placement of
the closed low just upstream of the area will help maintain
extensive cloud cover over the northern three-fourths of the state
and Saturday/Sunday maxes will run 15-20 degrees below normal.
Although some showers are possible in the higher terrain of northern
Utah Saturday afternoon, more widespread precip looks to hold off
until the closed low enters southern Utah on Sunday. Although there
is a mentionable chance of precip across the eastern two-thirds of
the fcst.area suspect that the heaviest precip (snow above 7k feet)
will be focused in the northern/central mountains where a h700
baroclinic zone tightens on Sunday. As the closed-low moves east of
the area late Sunday/Monday some additional wrap-around precip will
be possible in central/eastern Utah.

Following the exit of the closed low on Monday, drier mid-lvl
northwesterly flow will develop over the area, marking the beginning
of a gradual warming and drying trend. Both the EC and the GFS do
drop a disturbance into the area in the Tue/Wed timeframe, however
the EC is much weaker/faster with the system while the GFS amplifies
it into a closed low over Arizona. Neither solution would produce
much in the way of sensible weather for most of the state apart for
some additional cloud cover when the wave passes or maybe a small
chance of showers in southern Utah if the GFS solution verified.
There is fairly good consensus between guidance that a ridge will
begin to develop over the Western US towards the middle of next
week. This will likely usher in a period of relatively pleasant
weather with temps rebounding to near seasonal normals and little
precip expected

&&

.AVIATION...VFR conditions are expected to prevail at SLC through
at least Thursday morning. However, CIGs will start to fall below
7000ft during the morning hours, most likely before 15z. Gusty
southerly winds will continue through late this afternoon. A shift
to northwesterly winds is expected beginning 23-01Z, but there is a
30 percent chance that this may be delayed until later in the
evening.

&&

.FIRE WEATHER...A cold front is entering far northern Utah this
evening, and will slowly but surely move through the northern and
western portions of Utah through Thursday. Behind this front,
widespread wetting rain is expected Thursday and Thursday night,
with accumulating snow in the mountains. Ahead of the front,
southwesterly winds will be strong and gusty through Thursday
evening. Friday will be showery and considerably cooler in most
locations, with lighter winds area-wide. These cool and sometimes
unsettled conditions will linger into the weekend before drying
for the beginning of next work week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from noon to 9 PM MDT Thursday for UTZ013-015-016-
     019>021.

     Winter Weather Advisory from 6 PM Thursday to noon MDT Friday
     for UTZ008-009.

     Winter Weather Advisory from noon Thursday to noon MDT Friday
     for UTZ007.

WY...None.
&&

$$

Schoening/Carr/Cheng

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visit...
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