Scientific Forecaster Discussion
fxus65 kslc 201225
Area forecast discussion
National Weather Service Salt Lake City Utah
525 am MST Wed Nov 20 2019
Synopsis...the low pressure system over central California
this morning will track southeast, then turn northeast into
southern and central Utah by early Thursday. Rain and snow along
with colder temperatures will impact much of the area through
Thursday evening. High pressure aloft will work east across the
Great Basin this weekend.
Short term (through 00z sunday)...evolution of the storm appears
to be on track this morning. The now well defined upper low over
central California will continue on a southeast track into
Southern California later today. Ahead of the cali upper low, the
old upper low off the northern Baja Coast has been absorbed with a
residual circulation moving northeast through southern Arizona at
The main cali low will begin to take a turn to the northeast by
this evening. This feature will reach southwest Utah by early
Thursday, then slowly weaken as it drifts east across southern and
central Utah through Thursday night.
Widespread moderate to heavy precip that developed across southwest
Utah last evening was likely driven by an ejecting shortwave from
the parent upper low with good divergence aloft to support deep
lift. The tropical air mass that advected in ahead of the low
fueled the heavier precip and helped keep snow levels fairly high
Precip has reached into northern Utah, but so far is not
generating more than light precip as the dynamic/thermal support
for lift is somewhat less and terrain influences not as pronounced
as they were across southern Utah. Anticipating some increase in
precip intensity across northern Utah during the morning as the
mid-level baroclinic zone tightens up along and south of the
deformation axis across far northern Utah.
Looking a short break in the widespread rain/snow across southern
and central Utah this morning. This activity should expand fairly
quickly and turn more convective this afternoon and evening as
the upper low approaches and the associated colder air aloft
brings about increasing instability. Lowering snow levels with
the arrival of the colder air will change the precip type to all
snow for the mountains, with pockets of snow developing in the
higher intensity convective events throughout the higher valley
Rain and snow aren't the only issue today/tonight. A developing
low-level cold advection easterly flow out of southwest Wyoming
in response to the approaching upper low will impact the
northern Wasatch front with easterly canyons wind later today.
Deepening of the easterly flow and the associated cold advection
tonight, along with an increasingly favorable surface gradient
across Wyoming could bring canyon winds close to advisory levels
along the northern Wasatch front overnight. A this point an
advisory looks marginal, so will hold off on highlights at this
Fairly widespread convective precip will persist with the passage
of the upper low Thursday. Far northern Utah/southwest Wyoming
will likely miss out on the additional showers as the drier and
more stable easterly flow aloft will inhibit additional showers.
The gradual weakening of the upper low Thursday afternoon/evening
along with the development of a trailing anticyclonic north-
northeast flow should bring an end to most, if not all, of the
remaining shower activity by late Thursday night or early Friday.
High pressure aloft expanding east across the Great Basin to begin
the weekend will bring dry conditions and a gradual warming trend
to the forecast area.
Long term (after 00z sunday)...Utah will remain under a
northwesterly flow aloft to start the long term forecast period.
Upstream, an upper low will remain off the Southern California
coast. This low is now forecast by global deterministic models and
ensemble means to remain west of the Continental U.S. And, along with high
pressure to its north, will set up a blocking pattern off the West
Coast for the first half of next week. This block looks to remain
sufficiently west of the area such that storms will be able to drop
into the eastern Great Basin in the northwesterlies.
The deterministic GFS and ec are coming together towards a solution
that will bring a storm more typical of late November to the area on
Monday. This storm, as it stands now, is progged to track right
through Utah with a fast-moving but solid cold front and a period of
precipitation that would bring snow to the valley floors across much
of Utah. However, the trough is less amplified in the gefs and eps
ensemble means. Although this less-amplified solution would still
bring a cold front to the area, precipitation would be confined more
to northern Utah.
There looks to be a brief break in the weather on Tuesday before
models try to bring another storm to the area Wednesday. Here, there
is more run-to-run spread which increases uncertainty, but
deterministic GFS/ec show a decent chance for another shot of snow
down to the valley floors. The 00z eps is leaning a bit towards the
deterministic runs, although the gefs is less amplified and overall
less enthusiastic regarding the strength of this system. Regardless,
it looks like the large-scale pattern is shifting to one where the
longwave ridge will remain over the eastern Pacific with more of a
mean trough developing over the Great Basin and Rocky Mountain
region. In short, this means winter might finally be here to stay
Aviation...rain will continue to fill in at the slc terminal this
morning, with ceilings lowering below 6kft in conjunction. Rain will
continue on and off through the afternoon, then taper off for the
evening. There is a 30 percent chance for brief periods of MVFR
conditions in moderate rain. Winds are expected to shift to
northwest after around 14-16z, but may be erratic at times through
Utah...Flash Flood Watch through this evening for utz016-019-020.
Winter Weather Advisory until 4 PM MST Thursday for utz009-010-
Winter Storm Warning until 4 PM MST Thursday for utz518.
Wyoming...Winter Weather Advisory until 11 am MST Thursday for wyz021.
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