Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 011018
Area Forecast Discussion
National Weather Service Salt Lake City UT
318 AM MST Thu Dec 1 2016
.SYNOPSIS...A weak storm system will bring light snow to portions
of northern and central Utah through tomorrow as it slowly crosses
the region. Strong winds may develop outside of the St George area
later tonight through Saturday morning. A stronger and colder
storm system will impact the region early next week.
.SHORT TERM (THROUGH 12Z SUNDAY)...
Water Vapor Satellite shows a trough entering the interior west
with several embedded disturbances approaching from the
northwest. 400-200mb MDCARS wind observations place a 125-150kt
northwesterly jet well offshore of the west coast. GOES/SLC 00Z
RAOB/HRRR indicate that precipitable water values vary from
0.10"-0.20" mountains to 0.25"-0.30" valleys.
Next system is currently approaching from north central Nevada. On
IR Satellite, cloud tops are rather warm. This confirms the meager
reflectivity on Doppler Radar. In addition the 00Z KSLC RAOB shows
some dry air below 700mb that needs to be overcome.
As of this writing, only have seen sporadic visibilities from NWS
and UDOT sensors drop below 5 miles. So any snow reaching the
ground is very light. Nevertheless, the message this morning is
there will be areas of light to perhaps at brief times moderate
snowfall. However the roads are below freezing, and why they are
not abnormally cold due to the warm subsurface temperatures
resulting from a warm fall season, they will allow for
accumulation. Slick spots are the main concern for both commutes
Suite of high resolution guidance has best accumulations in Davis
and Weber counties with warm advection beginning to shift to cold
advection early this morning. Then with time the Salt Lake and
Tooele valleys fill in today. Do think these models are overdoing
the contribution from the Great Salt Lake as adjusted BUFKIT model
soundings for a 3.9C surface water temperature yields 100-200j/kg
CAPE and 9-12kft equilibrium levels. Still something to monitor
for potential periods of moderate snow later today.
There will be a slow improving trend with regard to snow shower
chances through tomorrow morning as energy splits southwest with
the arrival of the Pacific jet. This setup is conducive to strong
northeast wind gusts beginning Thursday night outside of St George
across Dixie given 25kt northeast flow at 700mb, a 5-8mb
northerly pressure gradient between Cedar City and the Grand
Canyon. The 700mb cold advection is quite strong later tonight
through at least Saturday morning. This tips the scale for more of
a low end warning event or high end advisory event. High
resolution models agree with gusts ranging from 40-60kts.
We issued a High Wind Watch for outlying Dixie, which does not
include St George itself, from midnight through 10am Saturday.
These winds affect downslope areas west of St George and the
Interstate 15 corridor from the town of Washington to the Iron
County line, and not St George itself. The Watch will allow the
next shift to assess model trends and coordinate with partners
before ultimately deciding on an advisory vs. a warning.
Ridging builds in from the west Saturday. Have kept slight chance
of snow for the northern mountains with warm advection developing.
.LONG TERM (AFTER 12Z SUNDAY)...
The shortwave ridge over the desert southwest on Sunday morning
begins to flatten through the day Sunday, as a cold trough pushes
through the Pacific Northwest. Ahead of this trough, moist warm
advection will likely produce precipitation and increased cloud
cover along the Utah/Idaho border through the latter half of
The cold front associated with this incoming trough should enter
Utah Sunday night, though the ECMWF remains about 6-12 hours slower
than the GFS in pushing this front through northern Utah.
Precipitation is expected along the frontal boundary, but models
have generally trended toward the idea of not producing much
additional precipitation in the cold sector behind the front, in
part because the low level airmass is so cold. Forecast POPs Sunday
night through Tuesday generally indicate a slow but steady
progression of the front through the state, with some broad brushing
to account for uncertainty in timing.
By Tuesday morning, the broad trough has completely taken over the
Great Basin, with 700mb temperatures as low as -18C. The ECMWF has
even hinted at 700mb temps as low as -20 or -22C over far northern
Utah by Tuesday night/Wednesday. Long story short- it`s gonna be
cold. Have maintained highs for Tuesday and Wednesday that are well
below freezing, except near the Arizona border. Have also kept lows
quite cold, with either Tuesday night or Wednesday night as the
coldest night of the week, depending on the evolution of the trough
and associated cloud cover.
Global models begin to show significant divergence late Tuesday, as
the GFS swings the trough out of the area by Wednesday morning,
while the ECMWF remains at least 12 hours slower. (The Canadian is a
compromise between the timing of these two models.) While ridging a
bit sooner would not have too many consequences, as valleys would
generally remain cold and inverted, the 00Z GFS has a wrinkle that
could make a difference- a moist and somewhat strong warm advection
zonal flow that develops across southern Idaho late Wednesday into
Thursday. If this happens, many northern Utah valleys could get a
relief from the cold air a day or two sooner. This said, the exact
evolution of Wednesday/Thursday is fairly low confidence at the
moment. Have kept temps for Wednesday and Thursday cold for now,
which leans toward the ECMWF solution, and also acknowledges that
there are more pattern possibilities that lead to continued cold
than lead significant valley warming.
Periods of MVFR or IFR conditions in snow showers are likely at
the SLC terminal through at least 20Z today, though some of the
individual showers may be fairly short-lived. Away from showers,
expect ceilings to still generally remain below 7000 feet. Winds
should switch to the northwest around 14-16Z, though winds could
be erratic at times near showers.
UT...High Wind Watch from late tonight through Saturday morning for
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