Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 170937
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
337 AM MDT Mon Apr 17 2017

.SYNOPSIS...A weather disturbance will pass mainly to the north
today, followed by a stronger storm system tomorrow into tomorrow
night. A colder storm is possible late Thursday into Friday.

&&

.SHORT TERM (THROUGH 00Z THURSDAY)...
Water Vapor Satellite shows a small closed low over Oregon ahead
of a deeper more expansive closed low over the Eastern Pacific.
400-250mb AMDAR wind observations places a 90-125kt cyclonic jet
from the Eastern Pacific into Oregon. 00Z KSLC RAOB/GOES/HRRR
indicate precipitable water values range from 0.15"-0.20" central
and southern mountains to 0.25"-0.40" most valleys. An atmospheric
river with origins near Hawaii has penetrated the Sierras into
Idaho.

Regional radar currently places a band of precipitation across
north central Nevada into western Idaho. High resolution models
are bullish for precipitation near the Idaho border this morning.
However quite a bit of dry air to overcome per 00Z KSLC RAOB and
dewpoints in the upper 30s. So only bumped PoPs up about 10% into
the low likely category for valleys along the Idaho border.
Upstream precipitation reports across Nevada have been lighter
than guidance would indicate.

Overall not bullish on today`s system. Height perturbations pass
well to our north coincident with the track of the closed low and
associated mid level cold pool. A shallow cold front will
propagate across northern Utah this morning. Feel this will be the
best chance of seeing precipitation as its coincident with 700mb
cold advection.

Late today and tonight get into a warm advection setup as the more
significant system makes landfall. Model QPF fields vary
significantly regarding the amount of QPF in this period as well
as coverage across the north. Did not make any drastic changes to
the going forecast but its something to monitor as the day
progresses.

Strong Pacific jet will impact the area tomorrow. Models indicate
an opportunity for jet coupling by afternoon across the north
which could enhance developing precipitation. The left exit region
of the main jet structure punches into north central Utah as the
afternoon progresses, shifting into central Utah during the
evening before progressing eastward. This is the period of
greatest QPF threat. SREF indicates enough instability to
advertise thunder. Confidence is not very high yet as to where
exactly the heaviest QPF will reside across north central or
central Utah, which depends greatly on the track of the upper
level jet.

Cold front by 00Z Wednesday is expected to propagate into central
Utah. Went below most guidance for high temperatures Tuesday in
the cool sector. Gusty pre-frontal southwest winds are expected
across southern Utah owing to the 40kts of flow at 700mb and
5mb/3hr pressure falls during the afternoon. However, feel that
winds will remain below wind advisory criteria.

Snow Levels remain quite high for tomorrow`s event, generally
near 10kft tomorrow, 8.5-9kft tomorrow evening (with much of the
precipitation occurring), and finally falling to 7-8kft overnight.
Accumulating snowfall is expected for the highest elevations,
with potential for spotty moderate snowfall at high elevations due
to convective elements.

Shortwaves associated with the upper level jet will progress to
our east by Wednesday morning, allowing shortwave ridging to build
in during the day Wednesday. However, based on instability
parameters from SREF and remnant low level moisture, feel there
is a bit more of a chance of re-developing terrain circulation
showers and perhaps a storm or two than models currently indicate.
Bumped up PoPs about 10% to account for this.

.LONG TERM (AFTER 00Z THURSDAY)...
High pressure over the forecast area is expected to shift east
Monday night/early Tuesday ahead of the next approaching Pacific
storm system. This system is progged to bring a cold front through
northern and central Utah Thursday afternoon and then through the
rest of the state Thursday evening/early Friday as the main
trough slides across. Models have come into good agreement with
the track of the system and are showing better moisture and more
favorable dynamics than models runs yesterday. As a result, have
raised POPs a bit for northern and central Utah, particularly for
the Thursday afternoon period.

Latest GFS is trending a bit colder with the system, indicating 700
mb temps in the -7C to -9C range by Friday morning, which would
support snow down to northern Utah valley floors. The EC is
continuing to show 700 mb temps in the -4C to -6C range, which is
what previous GFS runs had been indicating. At this time, have kept
snow levels around 5000 feet at the lowest for northern Utah, but
will monitor model trends. Some precip is expected to linger,
primarily over the higher terrain, through Friday afternoon before
tapering off Friday evening.

Global models continue to indicate that ridging will move in behind
the exiting storm system for late Friday into Saturday. While
guidance yesterday was indicating this ridge would remain dominant
into early next week, EC/GFS are now indicating a storm system
moving across northern Utah on Sunday, bringing a shallow cold front
through along with a bit of precip. Have introduced some slight
chance POPs through early Monday to account for this solution.
Models then show high pressure rebounding for day seven.

&&

.AVIATION...
The SLC terminal will see VFR conditions through the morning with
cigs expected to remain above 6000 feet AGL. Southerly winds are
expected to shift to the northwest between 14Z and 16Z with the
passage of a cold front. There is a 30 percent chance this occurs
earlier than expected.

&&

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

$$

SHORT TERM...Rogowski
LONG TERM/AVIATION...Traphagan

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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