Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 151202

Area Forecast Discussion
National Weather Service Salt Lake City UT
502 AM MST Mon Jan 15 2018

.SYNOPSIS...High pressure over the Great Basin will start to
weaken today. A weak system will cross the area late Tuesday
through early Wednesday. A strong cold front will cross Utah
Friday with a cold upper trof remaining over the area Saturday.


.SHORT TERM (Thru 00Z Friday)...The mid level ridge remains
centered over NV early this morning with a continued dry and mild
nwly flow over UT. Variable amounts of cirrus cover the CWA and
patchy fog and stratus has formed in the Bear River valley as well
as in the Cache valley.

A short wave dropping south around the cold plains upper low is
pushing an arctic boundary wwd across WY. This will push into UT
today but will modify as it does with the most cooling over our WY
zone and in the nern mtns. Temps across the rest of the CWA are
only expected to cool a few degrees or stay about the same as
yesterday. Part of that cooling will come from an increase in high
clouds as the NV ridge weakens.

The easterly gradient tightens across nrn UT this aftn thru
tonight in the wake of this boundary. This should generate local
gusty canyon winds but peak speeds are not expected to get much
above 40 mph, if that.

A deep trof in the ern Pac upstream from the ridge is forecast to
reach the coast today then lift north and minor out as a colder
system develops upstream from it in the Pac. The srn portion of
this trof shears and splits as it moves inland over the ridge with
a very weak system crossing our CWA Tue aftn and overnight.

Models, particularly the EC, have been struggling with the amount
of moisture that accompanies this system. The EC has over the
last several runs gone from a fair amount of QPF, to none, back to
some, then back to essentially none with the latest 00z run. The
00z GFS was also quite dry altho it did indicate there will be a
little precip, and the 00z NAM was the wettest. The latest 06z
runs of the GFS and NAM are wetter again, with the NAM still the
wettest, and slowest.

Kept mostly chance pops in the forecast for Tue aftn/night but
did adjust them downward somewhat based on the 00z output,
especially after midnight. However, the significant lack of run
to run consistency in the models make this a particularly low
confidence forecast, even this close to the event.

Ridging briefly returns for Wed then shifts east Thu as the next
deep cold low nears the coast. South winds increase Thu aftn ahead
of this trof and the airmass warms. The south winds should mix out
most valleys and temps will warm back up one last day.

.LONG TERM (After 00Z Friday)...Ahead of the strong trough moving
into the Pac NW southerly winds will increase over western valleys
Thursday night. This will make for a mild night but the
associated cold front will arrive across the central Wasatch Front
by about mid morning and to the western Uinta Basin-Cedar City
line by late Friday afternoon. Rain will occur ahead of the cold
front but should turn to snow within an hour or so of the passage
of the cold front. The latest 06Z GFS now shows an earlier timing
so if the 12Z GFS and EC follow suit, the weather grids will need
to updated to have the change over to all snow earlier.

Since the models have been consistent with the strength of the
trough which has good 500 thermal structure, 300mb jet and 700mb
frontogenesis across northern Utah have boosted PoPs to categorical
for the Wasatch front and northern mountains. Also increased PoPs
farther south Friday afternoon and night as the cold front
progresses southward.

Winds will be quite strong ahead of this cold front across
southwest and west central Utah Friday and then behind the cold
front in downsloping areas of central Utah between Price and
Fremont Junction.

The global models differ in how far south the core of the cold air
at 700mb will go by Saturday morning. The EC is a little slower
and deeper with the system which causes the 700 mb winds over
northern Utah to become more northerly to even northeasterly which
would cut off the snow in the northern mountains. However, the
GFS is more progressive with the southern end of the trough which
will keep a moist northwest flow going into Friday night across
northern Utah. Have leaned toward the GFS but the latest 06Z GFS
is now not as emphatic with the NW flow so perhaps the EC has a
better handle. Wait and see what the 12Z guidance shows.

A cold air mass will prevail across the region Saturday into
Sunday under a shortwave ridge. Have trended down on the PoPs
for Saturday night into Sunday but they may need to be further
trimmed or removed as the latest 06z GFS showing a sharper ridge
over Utah. Warm advection begins Sunday night across northern
portions of the CWA but then transitions to cold advection as the
GFS has now trended toward the EC which has a broad trough moving
in Monday.


.AVIATION...No weather concerns for the SLC terminal. Winds will
shift from southeast to northerly about 18-19Z.




SHORT TERM...Wilensky

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