Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
337 am MST Monday Dec 29 2014
Synopsis...lake effect snow will end this morning across the
northern Wasatch front and Salt Lake Valley. Very cold air will
spill west then southwest across Utah Monday night through
Tuesday...causing strong winds across the northern Wasatch front.
The second cold storm system will intensify and impact southern
Short term (through 12z thursday)...
water vapor loop shows a cold trough across much of the west...with
a closed low over the Pacific northwest.
Amdar 400-250mb wind observations indicate a 125-165kt northerly jet
from British Columbia into far northwest Nevada. GOES/GPS/rap/00z
radiosonde observation indicate the precipitable water value ranges between 0.05"
northeast mountains...to 0.25" most valleys. Blended total
precipitable water product indicates a very dry airmass has settled
across the west.
Based on surface observations and radar...much of the activity
across the north has waned. The exception is a lake band which
developed across the northern half and has been affecting the
northern Wasatch front. Despite all guidance showing northwest winds in the
lower levels...radar had a warm advection look about it. Sure
enough...tslc VAD wind profile had southwest flow from 6-8kft. This
is likely in response to a wave ejecting east from Nevada into
central Utah per rap. Once it does so...expect flow to become northwest
again and the band or remnants of it...to shift south toward Salt
Lake City. Have cancelled all northern advisories except for Salt
Lake Valley and the northern Wasatch front.
Did hoist a Winter Weather Advisory across west central Utah and the
central mountains...along with beefing up probability of precipitation and quantitative precipitation forecast through the
morning. This is in response to lowering visibiltiies...web cams...
and cooler cloud tops on infrared satellite due to the approaching wave.
Expect all activity to become showery this afternoon as
destabilization increases per short range ensembles. This zone of
activity shifts southward through Tuesday night as the
aforementioned closed low sinks south along the West Coast and
The synoptic evolution favors an easterly downslope/canyon wind
storm across the north beginning later tonight and continuing into
Wednesday morning. This is supported by a very strong northeasterly
pressure graident (1052mb NE Wyoming to 1017mb SW ut). 700mb easterly
cold advection with -20c SW Wyoming to -15c Wasatch front Tuesday
morning. Local high resolution WRF models have been consistent
indicating warning level gusts across the northern Wasatch front and
advisory level gusts across the Cache Valley and eastern portion of
the Salt Lake Valley. Issued the warning and advisories on this
shift. Also concerned about blowing snow locally reducing
visibiltiies and causing slick roads. Not to mention very cold wind
chills as the Arctic boundary comes through with values between -20f
and -30f expected across northeast Utah and southwest Wyoming
forecast late tonight into Wednesday morning.
As the closed low makes its way from Southern California into
Arizona Wednesday night...isentropic upglide above the
boundary...along with increeased diffluence aloft and warm advection
will create a winter storm which extneds into southern Utah late
wendesday and continuing on into Thursday. This storm also latches
onto a Pacific moisture surge and advects it into southern Utah. The
cold conveyor belt looks to set up over southern Utah. There is no
doubt that it will be cold enough for all snow across Dixie. Expect
the day shift to issue a Winter Storm Watch.
Long term (after 12z thursday)...
closed low continues east across Arizona Thursday...keeping a
good amount of moisture and instability over southern Utah. Areal
coverage of precipitation will likely decrease during the day as the
center of the low moves east of the area...tapering off Thursday
night/Friday morning. Flow becomes more northeasterly during this
time...allowing any remaining canyon winds to taper off.
High pressure is prognosticated to build over the Pacific behind the
exiting storm system...keeping the forecast area under north/northwest flow
ahead of it. Much of the area will see a drying and warming trend
during this time but weak shortwave energy moving over the ridge
will keep a bit of moisture over far northern Utah...allowing for an
outside shot of precipitation...primarily over the higher terrain.
Temperatures are expected to approach seasonal normals for most
locations by day 7.
Aviation...light lake effect showers are expected to keep the slc
terminal in the MVFR range through between 13z and 15z before the
band weakens. There is a 20 percent chance of brief IFR conditions
during this time. VFR conditions will prevail after the band
weakens...with a 30 percent chance of showers in the vicinity of the
terminal decreasing conditions to MVFR through the early afternoon.
Southeast winds are expected to shift to the northwest between 17z
Utah...Winter Weather Advisory until noon MST today for utz002-003-015-
Wind Advisory from midnight tonight to 4 am MST Wednesday for
High Wind Warning from midnight tonight to 4 am MST Wednesday
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)