Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 230945
Area Forecast Discussion
National Weather Service Salt Lake City UT
345 AM MDT Sun Apr 23 2017
.SYNOPSIS...A series of progressively colder and energetic storm
systems will cross the region beginning today, and continuing
through the upcoming work week.
.SHORT TERM (Through 12Z Wednesday)...Overnight satellite imagery
and objective analysis show a long fetch of zonal flow extending
across a considerable swath of the Pacific, with the leading edge
of this belt of westerlies currently impinging on the Pacific
Coast. A shortwave trough at the forefront of these westerlies is
forecast to translate inland and through the Great Basin today,
pushing a cold front through northern Utah this afternoon.
Moisture will be limited with this feature, however will likely
see a band of weak convection along this baroclinic zone, where
the models indicate weak instability will be present. Could see
microburst winds where this convection coincides or outpaces the
surface boundary, with measurable precip expected to remain
minimal as a whole. Have added a mention of thunder to the
forecast for the afternoon and evening hours.
Further south, increasing southwesterly flow coupled with pressure
falls associated with the approaching trough will yield a windy
afternoon across central and southern Utah, however it appears
winds should remain sub-advisory with 700mb flow expected to
remain in the 30-40kt range.
A stronger jet and associated moisture plume currently further
offshore is forecast to push inland during the day Monday,
reaching far northern Utah as early as Monday morning. An embedded
wave associated with this feature is expected to cross the
northern Great Basin Monday afternoon/evening pushing another
baroclinic zone southward through northern and central Utah by
Monday evening, bringing a better chance for measurable
precipitation to portions of northern and central Utah through
Monday night. A second trailing wave will maintain valley rain and
mountain snow through the day Tuesday. With the strength and
orientation of the flow aloft, most eastern and southern valleys
may remain dry through this period, however all areas will see a
cooling trend through the first half of the week.
.LONG TERM (After 12z Wednesday)...A series of storm systems will
move through the Great Basin during the period resulting in a
stretch of unsettled weather from mid week into the start of the
weekend, especially across northern Utah. The period begins with a
trough axis centered over the southern plains and a broad flat ridge
off of the west coast. This places Utah under a prolonged period of
moist northwest flow.
A shortwave will move across Utah during the day on Wednesday.
Northern Utah is positioned beneath the left exit region of a
narrow, but vigorous, upper level jet streak as this wave moves
through which promotes a broad area of upper level divergence.
Anticipate widespread valley rain and mountain snow with this wave
Another system quickly moves into the region late Wednesday night
into Thursday. Once again jet dynamics will assist with large scale
lift associated with this feature. This system has a better mid
level baroclinic zone which is slightly frontogenetic as it moves
into northern Utah. In addition, the GFS indicates that a period of
convective instability across northern Utah on Thursday as theta-e
is decreasing with height. This should support more vigorous
response to the given forcing and also supports the threat of
thunder. The chance of precipitation may need to be increased if
subsequent runs support this evolution. Anticipate a period of
decent orographic precipitation on Thursday in the Wasatch Mountains
with deep neutral stability, and even some convective instability,
and persistent northwest flow. This moist northwest flow would also
support orographic response along the I-15 corridor from Fillmore
more to Cedar City.
This system ushers much colder air into the region and the ECMWF and
GFS are now in good agreement on the evolution of the pattern late
this week after the EC moved strongly toward the GFS. The 500 mb low
closes off over Utah Thursday night and Friday continuing the cool
unsettled weather across the northern portion of Utah. With the
deeper cold air in place during this window snow may lower to the
valley floors across northern Utah Thursday night into Friday
morning. Precipitation chances begin to decrease on Saturday as the
upper low begins to move east and then an upper level ridge builds
into the west late in the weekend to finally dry things out.
Temperatures will remain below normal statewide through the period.
.AVIATION...VFR conditions will continue at the SLC terminal
through the TAF period. Light south winds this morning will become
and increase after 1200 UTC. Winds are expected to switch to the
northwest between 1700-1900 UTC although there is a 20 percent
chance that the switch will occur before 1700 UTC.
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