Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 062208
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
308 PM MST Tue Dec 6 2016

.SYNOPSIS...A cold storm system will bring light snow and the
coldest temperatures of the season to most of Utah and southwest
Wyoming through tonight. This cold snap will be short lived, as a
relatively mild but active westerly flow develops late in the week
through the upcoming weekend.

&&

.SHORT TERM (THROUGH 00Z SATURDAY)...
Water Vapor Satellite shows the next cold storm system approaching
from the Pacific Northwest and Idaho. Meanwhile another storm
system is crossing the Eastern Pacific. 400-200mb MDCARS wind
observations place a 120-160kt northwesterly jet from well off the
Pacific Northwest coast into northern California and western
Nevada. GOES/KSLC 12Z RAOB/HRRR indicate that precipitable water
values vary from 0.05"-0.10" mountains to 0.15"-0.25" most
valleys. Blended precipitable water product shows an atmospheric river
from near Hawaii into the aforementioned Eastern Pacific storm
system.

TSLC VWP indicates moistening of the lower levels is occurring,
with webcams indicating snow on the benches. Only a matter of time
until it fills in across the valleys. Snow should fill in this
evening as warm advective southwest flow becomes cold advective
northwest flow. Best precipitation coverage is expected this
evening.

The toughest part of the forecast for this first system is the
potential for lake effect. 700mb temperatures will falls to about
-18C by morning. When modifying BUFKIT forecast soundings with a
lake temperature of 3C, it yields about 400j/kg of CAPE with
equilibrium levels near 15kft. So instability is not lacking. Low
level RH (850-700mb), a significant factor in lake effect
potential, does however vary in the models from 65-85% tonight.
This variance is enough to widen confidence interval from 33% all
the way up to 75%. The high resolution models all attempt to
formulate a lake band into the Salt Lake Valley later this
evening, progressing toward Tooele overnight. For now have
highlighted this area with enhanced PoPs. Next shift will have to
monitor radar trends for impacts south of the GSL tonight.

Next several nights will be quite cold especially in areas that
are able to remain mostly clear to partly cloudy.

Warm front moves in from the west Thursday with the arrival of a
strong Pacific jet Friday. This system has a strong moisture tap
which penetrates into the interior west. Colder sheltered valleys
including the Cache, Wasatch Back, Uinta Basin, along with areas
north of Ogden will potentially see significant snowfall as the
colder airmass becomes overrun by the warmer airmass. Cant rule
out a period of light freezing rain or sleet in these areas. Other
valleys will turn to rain quickly, or see a period of rain/snow
mix. Northern mountains stand to pick up a lot of heavy wet
snowfall.

.LONG TERM (AFTER 00Z SATURDAY)...
An active pattern is expected to occur over at least northern
Utah through the long term. In the big picture a modest
atmospheric river will be continuing to attempt to penetrate the
interior mtn west through Saturday potentially providing
significant precip to at least northern Utah. Amplification of the
pattern on Saturday, especially regarding an upstream short wave
aiding to drive this penetration differs slightly in detail within
globals. This said, all point towards at least modest precip
extending as far south as central Utah, but snow levels, P-type,
and rate potential remain a very challenging forecast.

Erosion of low level cold air left over from midweek should occur
most valleys due to the breadth of WAA at the mid/upper levels with
this surge, so most valleys should be in rain vs. snow Saturday.
More protected valleys such as the Cache and Ogden valley in the
Wasatch Back may remain nearly isothermal as snow or a mix however
if rates remain high enough for long enough. This remains low
confidence at this time, and dependent on PI rate driven by pattern
amplification and strength of forcing supplied as such. The southern
periphery and penetration of moisture interacting is not high
confidence attm either.

A brief break remains on tap at some point Saturday night, but more
likely Sunday as deeper moisture shifts east coincident with CAA in
wake of wave passage. Any lingering precip looks showery at best,
but would support light snow showers down to the lower valleys
across the north.

A second, and potentially more impressive atmospheric river event
remains possible early next week, with better penetration potential
into our area given current guidance. Have continued to bump PoPs up
a bit those periods, but with details lacking in the first river
event, can not be overly confident yet in details regarding the
second. Just bank on active weather for the north and possible
central portions of the area at times, most likely beginning Monday
night.

&&

.AVIATION...
Low-end MVFR to high-end IFR conditions are expected to continue
at the KSLC terminal at times through 03z as a weak storm system
moves across northern Utah. Patchy light dustings of slush are
expected a on untreated surfaces through that time with a threat
of light icing becoming increasingly likely after 01z as temps
drop. Snowfall is expected to taper thereafter, though a 30
percent chance exists periodic snow showers will continue to
develop over and off the lake through 12z tomorrow morning.

&&

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

$$

SHORT TERM...Rogowski
LONG TERM/AVIATION...Merrill

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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