Scientific Forecaster Discussion
fxus65 kslc 180341
Area forecast discussion
National Weather Service Salt Lake City Utah
841 PM MST Wed Jan 17 2018
Synopsis...an upper level trough currently over the eastern
Pacific will move into and across the Great Basin Friday through
early Sunday. Precipitation will develop across northern Utah late
Friday, then spread across most of the region for Friday night
through Saturday night. High pressure will return to the area late
Discussion...a positively tilted mid level ridge axis extends
across the forecast area this evening, while a canopy of cirrus
spills overtop the ridge axis and into northern and western Utah.
This ridge axis will slowly shift east during the day Thursday,
eventually allowing southerly flow to increase across western
Utah. Temperatures aloft are expected to warm, however mid and
high level cloud cover may limit heating somewhat, but will likely
still approach or exceed record Max temps at many locations. Going
forecast has this well in hand and no updates are planned this
Previous forecast discussion regarding the weekend storm system
remains valid and is included below...
The cold core upper low currently
centered near 135w will gradually evolve into an open trough as it
moves inland late Thursday night/early Friday. This now open
trough will reach the western Great Basin Friday evening, then
continue east across the remainder of the basin Saturday through
The downstream upper ridge will shift east into and then across
Utah late tonight through Thursday morning. Once the ridge axis
moves off to the east, an increasingly strong and warm southwest
flow aloft will set up across western Utah Thursday afternoon,
then across the entire area Thursday night. Record or near record
warm temperatures are possible across northern/western Utah Thursday,
with the additional records possible across southern and eastern
Utah ahead of an approaching cold front Friday afternoon. Breezy
southerly winds are expected across western Utah Thursday, with
substantially stronger pre-frontal winds across southern and
eastern Utah Friday afternoon and evening.
The aforementioned cold front associated with the advancing upper
trough will settle into far northwest Utah Friday morning. The
trailing near 700mb baroclinic zone will be close on the heels of
the surface front, arriving over the northwest corner around mid
to late morning. Convergence into this boundary will initially be
weak with limited light precip developing by late morning.
Convergence into the baroclinic zone will increase during the
afternoon, generating significantly stronger low-level forcing.
This forcing supported by a series of weak mid-level shortwave
ejecting out ahead of the main trough and the positioning of the
right entrance region of the jet on top of the baroclinic zone
will lead to increasingly strong lift near the boundary as it
presses southeast across northern Utah. Moderate to heavy precip
will develop along the boundary by late afternoon, with a quick
changeover from rain to snow due to the strong vertical motion and
rapidly cooling temps near 700mb. Anticipating significant travel
impacts across northern Utah heading into the evening hours.
The biggest concern in terms of heavy snow will focus across
central through southwest Utah late Friday night through Saturday.
The near 700mb baroclinic will slow considerably, or even stall,
across that area for much of Saturday. A similar synoptic
situation will exist with dynamic support for lift from the
advancing upper trough and the still present entrance region of
the jet overhead. Between the quasi-stationary nature of the
baroclinic zone and fairly strong synoptic-scale lift, would fully
expect some areas of central/southwest Utah will receive heavy
snow by the time thing begin wind down heading into Saturday
For now will hold off on any winter weather highlights, though
suspect an advisory-level event is in store early in the storm for
the north, with winter storm conditions possible across central
and southwest Utah during the latter half of the event.
Long term (after 00z sunday)... global models continue to trend to
slow downstream trough evolution Saturday night as the upper low
closes off over central Colorado. Expecting in this scenario a
continuation of snow showers will occur along the mtn spines along
the I-15 corridor and points east, with areas who produce better in
northwest flow having the highest concentration. Bumped pops fairly
notably, especially central/eastern mtns. Heights will begin to rise
more significantly Sunday bringing an end to snowfall for most all
areas during the morning hours.
A fairly active period will continue thereafter through the long term
period, though nothing stands out as overly noteworthy at this time.
A weak wave within a west-northwest warm advection environment remains on track
to clip the northern third of Utah late Sunday night into Monday,
with the potential for light snow greatest in the northern mtns and
northern Wasatch front. This said, timing in globals remains
slightly off due to the progressive nature of the ensuing pattern
and the low amplitude nature of the weak wave. At this time it does
not look overly significant but will continue to monitor. In wake,
mid level ridging will build yielding dry and slightly below
seasonal conditions Tuesday into Wednesday.
The progressive and somewhat moist west-northwest flow at the mid/upper levels
remains in place thereafter in globals, with the next upstream
trough encroaching on the area by Thursday. Not focusing on too many
details with this as of yet due to the nature of model performance
this far out in such a progressive low amplitude pattern, but
potential exists for another modest snow producer late week.
Aviation...south winds and VFR conditions will prevail at the
kslc terminal through the overnight hours and into the morning
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