Scientific Forecaster Discussion
fxus65 kslc 171158
Area forecast discussion
National Weather Service Salt Lake City Utah
458 am MST Fri Nov 17 2017
Synopsis...a cold storm system will cross the region today. The
associated cold front will move from northern Utah this morning
and be across southern Utah by early this evening. High pressure
will build in over the weekend with a gradual warming trend.
Short term...moist southwest flow will continue across northern
Utah this morning with warm advection expected through about 15z
then gradually become cold advection by midday. The 500mb cold
core is still over the pac northwest this morning and the axis is not
expected to cross northern Utah until about sunset this evening.
During the afternoon the associated surface cold front combined
with an increasingly unstable air mass will bring a burst of
precipitation across the County Warning Area this afternoon as it moves
southeastward. Rain will change to snow with this front across
the Cache Valley and extreme northern Wasatch front this
morning and other northern valleys early this afternoon and
across west central valleys this afternoon. There is a slight
chance of thunderstorms with this front.
Snow accumulations of 6-12 inches are expected across the northern
mountains and 4-8 inches in the tavaputs and less than a couple of
inches farther south across the central and southern mountains.
Strong southwest winds will prevail across southern and east
central valleys ahead of the cold front today. A Wind Advisory is
in effect from 9am until 9pm in these areas.
In the wake of this 500mb cold trough a strong ridge will build in
over the region tonight through this weekend. Temperatures will be
the coldest of the season so far tonight through Saturday night as
the air mass moving in is cold and Bone dry (pw's less than 0.15
of an inch) allowing for significant radiational cooling.
Temperatures in the mountains will moderate rapidly between
Saturday and Sunday as an inversion is expected to prevail in the
valleys of central and northern Utah. Southern valleys should see
temperatures back to slightly above normal by Sunday while
remaining below normal across the valleys of the northern half of
the County Warning Area.
Long term (after 12z monday)...in general, forecast certainty
remains low next week with the exception of a midweek ridge. Global
deterministic models continue to indicate a weakening shortwave
trough swinging southeastward from the Pacific northwest through
central/northern Utah late Monday into Tuesday. Pwats from the GFS
show a decent moisture tap ahead of the trough despite the typically
dry nature of a trough taking this track. However, the GFS ensemble
reveals that this shortwave trough has a fair bit of uncertainty
associated with it as its ensemble members are all over the place
with the amplitude (in fact, some have a shortwave ridge over Utah
instead during this period). If the trough materializes as the
deterministic models suggest, would expect showers over central and
especially northern Utah and southwestern Wyoming, with snow levels
in the 6-8 thousand foot range.
Models are in good agreement towards the middle of the week in
building a high-amplitude ridge axis over the Great Basin, bringing
warmer temperatures (although a few valleys may struggle to realize
the warming aloft as reduced daylight has pushed US towards
inversion season). Meanwhile, a healthy moisture tap advecting
around the periphery of the ridge could keep cloud cover over
northern Utah, and perhaps even a few showers over the higher
Forecast certainty quickly dissolves again for the latter half of
the week, including the Thanksgiving Holiday, as models struggle to
handle the break down of the midweek ridge. With low certainty, have
kept the forecast near climo, meaning a chance of precip for the
higher terrain and near normal temperatures. Those with Holiday
travel plans should stay tuned for future forecast updates as the
forecast comes into better focus.
Aviation...at the slc terminal this morning ceilings will drop
below 7000 feet at times as rain showers continue. Winds will be
northwesterly in general, though there is a 30% chance of a Switch
Back to southerly from 14-18z. Low-level wind shear is a concern
as winds at 2000ft above ground level remain southerly. A cold front will push
through between 17-19z, bringing gusty northwest winds and 40%
chance of MVFR visibilities and ceilings with brief moderate
rain/snow. A few graupel showers are possible in the afternoon.
Utah...Wind Advisory from 9 am this morning to 9 PM MST this evening
Winter Weather Advisory until 5 PM MST this afternoon for
long term...Van Cleave
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