Scientific Forecaster Discussion
fxus65 kslc 101135
Area forecast discussion
National Weather Service Salt Lake City Utah
435 am MST Sat Dec 10 2016
Synopsis...a moist westerly flow aloft will remain focused on the
northern half of Utah throughout the weekend. High pressure aloft
will produce a short break from active weather early next week.
The next series of storms will arrive for the latter half of the
Short term (until 00z wednesday)...the leading edge of the
atmospheric river extending from southwest of Hawaii to the
western Great Basin will work into northern Utah early this
morning. Very light precip/virga exists across far northern Utah
and upstream across southern Idaho/northeast Nevada at this time.
Dynamic and thermally generated lift will remain fairly weak
during the morning, then steadily strengthen this afternoon and
evening as low-level warm advection switches to cold advection and
the mid-levels cool with the arrival of low-amplitude shortwave
late this afternoon and evening.
Peak precip intensities should occur tonight through early Sunday
morning under a cyclonic west-northwest flow and fairly strong
low-level cold advection. Orographics will be in play overnight,
with the heaviest accumulations in the Wasatch Range.
Southern and eastern Utah will for the most part miss out on
precip this weekend. The exception will be across west-central
and southwest Utah tonight. Low-level cold advection working south
through the area should be able to generate enough lift for
precip. The mid-level cold air and dynamic support will not be
following the low-level cold south, so any precip that develops
will likely remain on the light side.
Lingering mountain showers early Sunday will likely dissipate
during the day, with any remaining precip likely concentrated
near the Idaho border where low-level warm advection will
generate weak lift to support light precip. This area of precip
will likely persist into early Monday, then retreat north as a low
amplitude upper ridge develops across the Great Basin for Monday
night through Tuesday.
Long term (after 00z wednesday)...deep, moist, zonal flow will be
ongoing across the interior west to start the long term forecast
period. A mid-level baroclinic zone is expected to be stretched
across some portion of northern Utah or southern Idaho. This will
help to focus precipitation across the northern half of Utah Tuesday
evening. Zonal flow remains in place through at least Thursday
morning. Expect some fluctuation of the baroclinc zone but the
highest threat of precipitation will remain across northern Utah
through at least Thursday morning.
Model to model and run to run consistency is poor, especially
discerning the path of this trough and the response of the
baroclinic zone Thursday into Friday. Rather than narrowing the long
term pops in this portion of the forecast to a particular solution
generally kept elevated pops across much of the County Warning Area during the
period as a blend of the global guidance.
By Saturday, the back end of the trough is forecast to move
through the interior west. A very, very cold airmass is currently
forecast by much of the global guidance by Saturday with 700mb
temperatures between -20 and -24c across northern Utah. Experience
suggests the models will moderate these very cold temperatures
over the next couple days. Still, especially if any snow cover is
present, it appears a significantly colder than normal airmass may
build into the area by next weekend.
Aviation...northwesterly winds in response to an area of showers
crossing into northern Utah have developed across the slc terminal
at 11z. Have a difficult time expecting winds to shift back to the
southeast at this point, however, there is a 20 percent chance
southerly winds will redevelop between 12-15z. Ceilings will remain
or below 6000 feet above ground level after 12-13z. Do not expect any accumulating
snow through tonight.
Utah...Winter Storm Warning until 9 am MST Sunday for utz007>009.
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