Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 021020
Area Forecast Discussion
National Weather Service Salt Lake City UT
320 AM MST Fri Dec 2 2016
.SYNOPSIS...A cold upper trough will remain overhead today. High
pressure will move over the region tonight into Sunday. A colder
upper trough will slowly cross the area through the first half of
.SHORT TERM (THROUGH 12Z MONDAY)...
Water Vapor Satellite shows a trough overhead, with 2 pieces of
energy. The first main energy is trying to close off over Las
Vegas, with energy from the old northern trough over northern
Utah. 400-200mb MDCARS wind observations place a 145-165kt
northerly jet over the Pacific Northwest and California. GOES/SLC
00Z RAOB/HRRR indicate that precipitable water values vary from
0.05"-0.15" mountains to 0.20"-0.30" valleys.
Two main weather impacts currently underway. First, we have a wind
advisory for Dixie outside of St George. These canyons and
downslope areas will likely see the strongest northeasterly wind
gusts near 60 mph this morning, with the remaining period through
tomorrow morning with gusts of 40-55 mph. This is supported by 25kt
northeast flow at 700mb, a 5-8mb northerly pressure gradient
between Cedar City and the Grand Canyon. Also 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.
Ridging builds in from the west tomorrow. Have kept slight chance
of snow for the northern mountains with warm advection developing
to our north.
Global models continue to advertise the coldest storm system of
the season early next week, with 700mb temperatures falling to
-19C to -23C Tuesday into Wednesday, and 500mb temperatures
ranging from -34C to -38C. There has been good consensus amongst
the models regarding the large scale pattern with the left exit
region of a strong Pacific jet traversing over the state Monday
and Monday night. Global models also suggest a strong connection
to a very moist atmospheric river.
Differences still remain regarding cold frontal timing, where
exactly it stalls before making its way through the forecast area.
Given the confidence of the storm, we bumped PoPs into the likely
range late Sunday night for many valleys and definite range for
the adjacent northern mountains. Right now the Monday morning
commute looks nasty. In addition to the synoptic forcing, there
will also be additional lake effect contributions.
.LONG TERM (AFTER 12Z MONDAY)...
The cold front continues to push through the northern half of the
CWA through the day Monday, though there still remains some
timing/structural differences between the GFS and ECMWF. The GFS
is a little faster with the front, and has a stronger baroclinic
zone, resulting in a little more precipitation. On the whole,
models have trended a little slower in pushing the cold air into
southern Utah, so have warmed temperatures a bit in the southern
third of Utah Monday-Tuesday.
Timing/placement discrepancies for the front linger into Tuesday as
well, but both models have the idea of a secondary shortwave coming
down the backside of the broader trough. This lights up
precipitation along the front over central and/or southern Utah
through the day Tuesday, and pushes even colder low level
temperatures into the CWA. By 12Z Wednesday, models continue to
promote a very cold airmass over the entire CWA, with 700mb
temperatures over northern Utah dipping as low as -19 to -22C.
Wednesday will likely be the coldest day across much of the CWA,
with either Wednesday morning or Thursday morning as the coldest low
The general agreement in temperatures between the global models
masks significant differences starting around 00-12Z Wednesday.
Models have remained fairly stubborn the last few nights, with the
GFS as the fastest to push the trough out of the area (with ridging
building Wednesday morning), while the ECMWF is significantly slower
(ridging not building until Thursday afternoon). As is common, the
Canadian remains a friendly compromise between these two.
Because of these differences, anything beyond Wednesday morning
(including the day 5-7 forecast) is low confidence. Have not made
many changes to the extended because of this, but did generally
trend Wednesday night and Thursday a little colder to bring in a
little more ECMWF influence.
There is a 40 percent chance of ceilings below 7000 feet developing
sometime between 11Z and 16Z, with brief periods of MVFR or IFR
ceilings not out of the question. Winds this morning may be light
and variable at times, before likely increasing from the north
again after 15Z.
UT...Wind Advisory until 10 AM MST Saturday for UTZ019.
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