Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 130315
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
815 PM MST Fri Jan 12 2018

.SYNOPSIS...A strong ridge of high pressure will build in across
the region this weekend through early next week. An active weather
pattern will develop from the middle to the end of next week.

&&

.DISCUSSION...
Water Vapor Satellite shows a ridge centered off Baja California
up the West Coast. 400-200mb MDCARS wind observations place a
100-130kt anticyclonic jet down the Rockies. GOES/SLC 00Z
RAOB/HRRR indicate that precipitable water values vary from
0.05"-0.15" mountains to 0.30-0.40" valleys.

High pressure has built in, with a 10mb northerly gradient across
the state per HRRR. Models dont show much in the way of strong
winds for outlying Washington county owing to weak cold advection
and very weak flow at 700 mb.

GFS indicates strong height rises and warming aloft ongoing
overhead downstream of the ridge. 00z KSLC RAOB confirms this
with several subsidence inversions between 720mb and 570mb. We
currently have some PoPs in the mountains near the Idaho border.
When looking for a mechanism, it would have to be shallow under
the aforementioned inversion. Under inspection of the 285-295K
isentropic surfaces, one could suppose enough low level lift may
occur, with models indicating Condensation Pressure Deficit
between 40 and 60 mb. So really not a great shot of seeing
precipitation, but going PoPs, are only between 20-30% in the far
northern Wasatch Mountains. Wont update though, due to the fact
that it matches our neighboring office quite nicely and cant be
fully ruled out.

With that in mind, dont see a reason for an evening update.

&&

.PREVIOUS DISCUSSION...
A somewhat moist northwest flow will remain in place across the
northern portions of the CWA overnight with nothing more than
scattered clouds and possible snow flurries over the mountains.
Temperatures are expected to remain several degrees above average
across the north due to these clouds but farther south where
mainly clear skies are expected temperatures will be near normal.

A strong ridge developing along the West Coast tonight will
gradually work its way in across the Great Basin this Holiday
weekend bringing mild and dry conditions to the entire area. Most
areas will be about 10 degrees above normal each day but some
areas could push towards 15 degrees above normal by Sunday and
Monday.

With this high pressure prevailing through the weekend we are
expecting the air quality along the Wasatch Front and the Cache
Valley to deteriorate due to an building inversion.

The high-amplitude ridge over the area Tuesday morning will shift
east by the afternoon, opening the door for a weak shortwave
trough to cross the area. Given that this is the first trough to
break into the ridge, inter-model and run-to-run consistency has
been low. The 12Z EC came in a bit faster and stronger with this
system, although it is sort of an outlier compared to other
operational and ensemble model runs. Still, given the uncertainty,
have increased PoPs into the slight chance category in several
zones Tuesday. Otherwise, precipitation looks a bit more likely
Tuesday night. Although the airmass will be relatively mild, the
stable airmass ahead of the storm will result in valley inversion
which are not expected to fully mix out prior to the arrival of
the storm, so some colder valleys could see snow or a rain/snow
mix while most others should see all rain.

A shortwave ridge will rebound late Wednesday into Thursday, but
will not reside long across the area as the pattern looks to remain
fairly progressive. The trough axis will shift east of the area
Thursday afternoon allowing for increased southwest flow ahead of
the next storm system. Global models agree that southerly winds will
increase Thursday afternoon across the western valleys which should
result in good mixing. Temperatures may well end up warmer than
currently forecast, as a more conservative approach was taken with
the event still several days out. The trough will then push a cold
front through Utah Thursday night through Friday, bringing the
potential for widespread precipitation to the area. With
temperatures progged to fall as low as -12 to -14C across a large
portion of the area late Friday into Saturday before the storm winds
down, valleys will have the potential to see accumulating snow.
Although this storm is still way out in the Day 7 period, models are
in relatively good agreement for the time being...

&&

.AVIATION...
Light southeasterly winds are expected to become established at
the SLC terminal by 04-05z. Otherwise, VFR conditions should
prevail under clearing skies.

&&

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

$$

PUBLIC...10/Struthwolf/Cheng
AVIATION...Cheng

For more information from NOAA`s National Weather Service visit...
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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