Area Forecast Discussion
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FXUS65 KSLC 282232

Area Forecast Discussion
National Weather Service Salt Lake City UT
432 PM MDT Tue Mar 28 2017

.SYNOPSIS...High pressure aloft across the Great Basin through
Wednesday will give way to the next cold Pacific storm system
late in the week. High pressure aloft will return late in the
upcoming weekend.


.SHORT TERM (Until 00z Saturday)...A low amplitude upper ridge
currently along the west coast will move east across the Great
Basin on Wednesday. The mean layer flow aloft will be dry, but
remain a bit north of west Wednesday. As a result temps will
increase a bit, but likely stay close to seasonal norms Wednesday

A shortwave in the western Gulf of Alaska near 160W will be the
feature that develops into a cold and energetic trough over the
Great Basin during the latter half of the week. This shortwave
will continue on a easterly track across the gulf, reaching the
Pacific Northwest coast late Wednesday. A strong upper jet on the
back side of this feature will turn this shortwave southeast and
begin a rapid intensification into a closed low over Nevada late
Thursday. From there this newly formed low will turn more to the
east and track near the Utah/Arizona border Thursday night through

A small difference in the trajectory of the upper low could have
a significant impact on sensible weather, especially for northern
Utah. The latest versions of the ECMWF and Canadian models put the
trajectory through northern Arizona, while the GFS remains farther
north over extreme southern Utah. Inclined right now to follow the
more southerly track of the ECMWF/Canadian owing to the strong jet
on the back side of the trough and significant upstream ridging
into western Canada.

The surface cold front associated with this deepening storm will
enter western Utah Thursday afternoon. the trailing near 700mb
baroclinic zone will reach northwest Utah late in the afternoon,
with the more energetic southern portion driving hard into
southwest Utah late in the day. Anticipating that entire length of
the baroclinic zone will be convectively active, with the
strongest convection over southwest Utah. Strong low-level thermal
advection along greater instability with the arrival of the mid-
level cold air supported by strong dynamic lift and the upper
divergence along the nose of the upper jet should support deep
convection. This strong convection should turn northeast into
central Utah where the deep layer cold advection migrates.

Up north the baroclinic zone is progged to tighten near the urban
corridor during the early evening. Looking at a short period where
convection could enhance where upper divergence increases in the
northeast quadrant of the upper low.

Precip will become more stratiform and widespread Friday as the
upper low tracks across northern Arizona. Moist deep layer warm
advection along the north and west flanks of the upper low will
create fairly strong synoptic-scale lift, which will produce
significant valley rain/mountain snow, especially across the
central/southern mountains and the west-central and southwest

Northern Utah will be a challenge due to the deep layer east-
northeast flow aloft around the upper low. Initially the fairly
strong warm advection pattern should generate widespread precip.
Over time with the loss of the strong advective component this
flow will become downslope off the Wasatch and bring an end to
much of the precip across northwest Utah late in the day Friday.

.LONG TERM (After 00Z Saturday)...The late-week closed low over the
Four Corners region will pull east of our area by early Saturday,
though wrap-around precipitation should continue over central and
southern Utah Saturday. The main effects will be continued below-
normal temperatures (especially over southern Utah), and a soggy
start to the day with continued low snow levels the farther
southeast you go in Utah. Much of these details will come down to
the track the low takes, which is still somewhat uncertain.

Another potential impact on the back side of this system is easterly
flow over northern Utah leading to a gap wind event for parts of the
Wasatch Front. That said, tools to analyze the gap wind potential
based on local research are indicating a very limited chance of it
occurring at this point.

While global models disagree on the closed low track, they are in
agreement with the low existing into the Plains by Saturday night,
and thus a break for our area later Saturday and Sunday as brief
ridging moves in under northwest flow aloft.

Global models have struggled the last few days with the period from
early next week onward. The general theme of today`s 12z runs is a
West Coast ridge with a few shortwave troughs riding through the
eastern periphery of the ridge and impacting our area with cool
temperatures and chance of showers. The GFS in particular has a more
energetic wave train than the ECMWF. But, looking to past runs, this
is fairly different than recent solutions. Suffice to say that
confidence is low. The going forecast is thus near-normal
temperatures and chances of precipitation from late Sunday onward.


.AVIATION...Increasingly dry/stable northerly flow aloft will
bring VFR conditions under mostly clear skies through at least
Wednesday morning. North winds will gradually decrease this
evening, then gradually turn to a light southeast drainage late





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