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fxus65 kslc 232144 
afdslc

Area forecast discussion
National Weather Service Salt Lake City Utah
344 PM MDT Thu Mar 23 2017

Synopsis...a Pacific storm system will move downstream of the
area this evening. High pressure will return for Friday, followed
by another storm system this weekend.

&&

Short term (through 00z monday)...the broad upper low over the
eastern Great Basin closed off today and remained essentially
stationary over the last 12 hours. Strong mid level convergence
has continued to exist along the northwestern flank of this low
aiding tight near 700 mb thermal packing within a near stationary
axis, this driving steady heavy precip focused along the I-15
corridor. Presence of a stalled and shallow cold frontal boundary
just southeast of the slc valley earlier also drove strong low
level speed convergence from roughly Ogden south through Point of
the Mountain aiding intense valley precip. Only moderate precip
was noted above the boundary in the mtns. These high pi rates in
the valley Drew snow levels down to the valley floor midday (near
isothermal below 8000ft), where in some places they remain due to
dynamical cooling from the sustained precip this afternoon.

Several records were broken today at kslc precip wise. The two
most notable were breaking the all time wettest day in March, and
moving into the 7th all- time wettest day in history. We are
still measuring.

Though focused precip has occurred along the I-15 corridor today,
have noted scattered convection within the core of the low from
Castle country south to the Powell vicinity. There, snow levels
largely have run between 6-7000ft, but convective showers have at
times lowered those levels down to 5000ft with presence of
graupel bringing brief and slushy accums at times. Areal coverage
is beginning to wane however as the instability axis is beginning
to shift east. This trend will continue over the next few hours.

Do see downstream evolution developing at the mid and upper levels
of the trough currently, and anticyclonic flow has begun to nose
into western Utah over the last hour. Warming cloud tops have
become evident across the west owing to high confidence in a rapid
diminishing of precip early this evening, with a clearing trend
from west to east overnight. Hi-res models as well as medium range
agree.

High pressure will build overhead tomorrow driving a brief but
rapid warming trend across all areas, but the next upstream trough
will be racing in off the Pacific come early Saturday. Good
agreement exists in medium range guidance in track and timing of
this feature bringing a cold front east across the area Saturday
morning, a mid level axis passage Saturday afternoon, then
transitioning into a somewhat moist and unstable northwesterly
flow for Saturday evening. Modest precip totals expected, with
snow levels potentially lowering to 5000ft once again. Rapid
downstream progression of this trough Saturday night will allow
another low amplitude ridge to build in for Sunday.

Long term (after 00z monday)...the active weather pattern looks to
continue through the long term period, as a Pacific trough will
begin to impact the state during the day on Monday. Although all
guidance depicts this feature, there are significant differences in
its amplitude, projected track/speed and consequently its impacts on
sensible weather. The GFS is the most amplified and slowest
solution, first transitioning the Pacific wave into a closed low
early Mon and then digging it down to the Arizona/Mexico border
tues/Wed. This solution would likely result in the best precip being
focused over southern and central Utah during the Tue/Wed timeframe,
although some precip is also possible on Monday in western Utah near
a stalled mid-level boundary and over the central Utah mountains due
to moist southwesterly flow. Furthermore with the GFS solution the
coldest temps would be pinned much further south, and northern Utah
would experience a period of warm advection tues/Wed raising temps
to near climatology.

The ec and (especially) the Canadian are less amplified and faster
with this feature driving a cold front through northern/central Utah
late Monday/early Tuesday. This would result in more precip for
northern Utah and also significantly cooler temperatures. Even with
this cooler solution, snow levels would likely remain above valley
floors in northern Utah, although the Canadian solution would
potentially bring slow down to the benches (i.E. 5000-6000 foot
zone). For this fcst package leaned towards the ec/Canadian solution
and consequently cooled temps a bit, raised pops for the Mon-tues
timeframe, and backed off some on pops for the tues/Wed timeframe.
The caveat being that due to the aforementioned differences in
guidance significant modifications to these fcst parameters may be
necessary in subsequent updates depending on model trends.

After the first system moves out (either Wed or thurs depending on
which solution verifies) there will be a brief respite until another
Pacific trough begins to deepen just upstream of the fcst area.
Although a cool down and precip appear likely with this system,
details are tough to nail down at this point, thus went with a
simple model blend for pops and went just below climatology for late
week temps.

&&

Aviation...for the kslc terminal, low end VFR and high end MVFR
conditions are expected to persist with showers at or near the
terminal into the early evening. Cigs should lift above 5000 feet
and visibility improvements to VFR or better by 03z. Winds
anticipated to switch to southerly after 09z.

&&

Slc watches/warnings/advisories...
Utah...Winter Weather Advisory until 4 PM MDT this afternoon for
utz007>010-517-518.

Wyoming...none.
&&

$$

Merrill/Carr/Dewey

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