Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 090314
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
814 PM MST Thu Dec 8 2016

.SYNOPSIS...A persistent and quite moist westerly flow aloft will
generate significant precipitation for northern Utah and southwest
Wyoming through the upcoming weekend. Southern Utah will remain
south of the storm track through Saturday then become unsettled
into Sunday. Monday into Tuesday a temporary ridge is expected.

&&

.DISCUSSION...
Water Vapor Satellite shows a deep closed low over the Gulf of
Alaska with zonal flow into the interior west from the eastern
Pacific. 400-200mb MDCARS wind observations place a 105-125kt
northwesterly jet over the Pacific Northwest into northern Utah.
GOES/SLC 00Z RAOB/HRRR indicate that precipitable water values vary
from 0.15"-0.30" mountains to 0.35"-0.50" most valleys. Blended
Precipitable Water product shows an atmospheric river from near
Hawaii nosing into the Great Basin.

Several factors make this setup a low confidence forecast:

1) The speed of weak waves in zonal flow makes them hard to time.

2) Precipitation type is difficult since model BUFKIT forecast
soundings vary significantly when it comes to maintenance of low
level cold air.

3) Varying precipitation rates due to terrain which factors in
not only how much QPF to expect but also precipitation type as well.

First weak wave passed to the north late this afternoon, helping to
touch off the first area of light to infrequently moderate snow SLC
northward. 700mb warm front progresses through northern Utah
tonight, with overrunning providing another shot of moderate
precipitation across the north after midnight.

Surface warm front will take much longer to erode cold air in closed
valleys of the Wasatch Back, Cache Valley, and north of Ogden as
well. This may drag out into early afternoon tomorrow. BUFKIT
forecast soundings vary between eroding these areas late tonight all
the way until tomorrow late afternoon. We are leaning toward the
colder solutions due to experience of prior storms like these.

Roads are quite cold in the same areas listed above, generally lower
20s. Any snow will quickly accumulate on roadways causing winter
driving conditions.

More confidence on changing to rain late tonight across much of the
Salt Lake and Utah Valleys. Though pockets even in these areas will
hold onto cold air longer allowing for more snowfall. Since
impacts are now expected to be isolated, have cancelled the winter
weather advisory for these areas.

Other change was to utilize the combination of observations and
LAMP guidance for the T/Td/RH curve overnight through tomorrow
morning. Show rising temperatures after midnight.

&&

.PREVIOUS DISCUSSION...
A moist westerly flow associated with a near zonal jet is
penetrating the northern Great Basin and northern portions of our
CWA this afternoon. Weak disturbances within this flow will cause
variable intensities of precipitation tonight through Saturday.
The first disturbance is associated with the warm front aloft
moving northeast across the region this afternoon has been the
reason for the general light snow. Temperatures behind this front
will gradually warm through tonight with pockets of cold air
likely remaining into early Friday. This complicates forecast with
mixed precipitation likely developing across the southern Wasatch
Front late this evening and working its way northward through the
night reaching Brigham City about mid morning Friday.

BUFKIT sounding for Wendover shows that a period of freezing rain
is possible between about 8 pm and 2 am tonight. This looks
reasonable as surface gradient supports a low level northeast flow
keeping a shallow cold layer in place while warm air overrides.
The rest of the forecast soundings along the Wasatch Front shows
the lowest boundary layer warming therefore the threat of freezing
rain minimal but that is not to say that pockets in certain
protected valleys couldn`t see a period of freezing rain.

Behind this warm front there is a hiatus in precipitation as seen
on the radar but this should fill in as another disturbance over
north central Nevada races eastward bringing in a better threat of
widespread precipitation after midnight. The valley temperatures
at the time that this second disturbance arrives will be critical
in the amount of snow that may occur along the Wasatch Front and
Mountain Valleys.

Temperatures will warm into the 40s along the Wasatch Front from
about Ogden southward on Friday while farther west have kept a
colder pocket in place over the west desert as it should be hard
to erase the cold air there. Snow levels expected to rise to near
7000 ft by afternoon. By Friday evening most of the precipitation
should be confined to areas north of I-80, in the northern
mountains and southwest Wyoming.

By Saturday morning the jet buckles again and another disturbance
brings cold advection in across northern portions of the CWA. Its
questionable if temperatures will get cold enough for snow in the
northern valleys other than perhaps the Cache Valley. So have kept
temps on the warm side with rain and prevailing precip in the
valleys and more snow in the mountains.

Breezy conditions may develop over southwest valleys Saturday ahead
of this northern Utah disturbance along with mild temperatures.

The cold front trailing the short wave that is forecast to ripple
past on Sat drops swd into the CWA Sat night potentially flipping
precip back to snow along the Wasatch Front and keeping heavy snow
going in the mtns. The med range guidance starts to diverge at
this point with the EC not as cold and it also stalls the front
across central UT instead of the srn border by Sun morning. Leaned
towards the colder GFS and kept valley snow in the forecast for
Sat night.

Looks like a break Sun aftn thru Mon except over the far north
then guidance brings another flat wave thru the Great Basin on Tue
with widespread precip along the baroclinic zone stretched across
the CWA thru Tue night. EC and GFS are in a little better
agreement for Wed both showing this zone lifting nwd as a warm
front thru Wed night stalling across the n Thu with most of the
CWA in the warm sector. Both then lean towards a trof deepening
over the western states Thu night into Fri with precip becoming
widespread across the CWA again.

Confidence in the details of the forecast is low but trended pops
towards these latest ideas. Bottom line is continued unsettled
thru the end of next week.

&&

.AVIATION...
The KSLC Terminal may see MVFR conditions develop after midnight,
but this should be short lived as precipitation should change to
rain and thin out during the morning. Winds will continue to be
light and variable as southeast drainage works against northerly
influence due to precipitation to the north.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until noon MST Friday for UTZ010.

     Winter Storm Warning until 4 PM MST Saturday for UTZ007>009.

     Winter Storm Warning until 3 PM MST Friday for UTZ001-002-005-
     006.

WY...Winter Weather Advisory until 3 PM MST Friday for WYZ021.

&&

$$

PUBLIC...Rogowski/Struthwolf/Wilensky
AVIATION...Rogowski

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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