Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 161545

Area Forecast Discussion
National Weather Service Salt Lake City UT
845 AM MST Thu Feb 16 2017

.SYNOPSIS...High pressure aloft will shift east of the area today
as the first in a series of weather disturbances reaches Utah
tonight. The active weather pattern will continue across the Great
Basin well into next week.


Water Vapor Satellite shows a ridge from southern California
into the Central Rockies. The next storm system is approaching the
Oregon coast. 400-200mb MDCARS wind observations place a
100-145kt cyclonic jet into northern California and the Pacific
Northwest. GOES/SLC 12Z RAOB/HRRR indicate that precipitable water
values vary from 0.05"-0.10" mountains to 0.20"-0.45" valleys.
Blended Precipitable Water product shows an atmospheric river into
the West Coast with origins from near Hawaii.

850-700mb Saturated Equivalent Potential Temperature and
Thickness place a cold front across western Nevada. This boundary
is expected to make its way into Northwest Utah by late day.
Resultant cold advection will destabilize the post frontal
environment enough for the development of isolated to scattered
rain and high elevation snow showers. The snow level is expected
to be between 5500-6000 feet.

Otherwise expecting an increase in mid and high clouds. Lowered
temperatures for the Bear River Valley as inversion remains
strong. Southerly winds will increase today, hopefully enough to
begin scouring the building urban haze.


The recent dry pattern is about to end as the upper ridge over
the eastern Great Basin shift east of the area later today. The
first shortwave to cross the region currently sits off the Pacific
Northwest coast. This feature will weaken a bit as it races east-
northeast across far northern Utah this evening. Precip associated
with this feature should remain close to the near 700mb
baroclinic zone which will settle into far northern Utah
immediately ahead of the shortwave. Precip for the most part will
remain light, and likely end quickly behind the shortwave late
tonight/early Friday.

A secondary area of light precip not associated with the above
mentioned shortwave could work into extreme southwest Utah late
tonight. Weak but somewhat moist low-level warm advection into
southwest Utah supported by a weak mid-level shortwave and the
exit region of the upper jet should be sufficient to generate an
area of light precip across the far south late tonight/early

The next disturbance to impact the region looks to be a much more
substantial system as it moves into the region for Friday
afternoon through Saturday. This next feature will deepen rapidly
off the California coast, with an eventual upper low forming and
tracking east along the Arizona/Mexico border late Saturday
through Sunday.

A deep layer moisture source tapped by this system will advect
into southwest Utah early Friday, then spread across the entire
region by Friday evening. Shortwaves ejecting out ahead of the
strengthening trough along with good upper diffluence should
support fairly deep synoptic-scale lift over the south beginning
Friday, then across portions of the north and east late Friday
afternoon and night.

Shower activity will remain loosely organized Saturday through
Saturday evening as dynamic lift looks to be weak but moisture
quite plentiful. The next system to move in off the Pacific will
reach Utah by Sunday afternoon. This feature will offer up the
best chance for widespread valley rain/mountain snow, especially
across the northern/central portions of the state. The ambient air
mass will still be quite moist as the near 700mb baroclinic zone
works into western Utah by early afternoon. Low-level cold
advection working into the northwest portion of the state along
with increasing dynamic lift from the advancing trough and
favorable jet support in the exit region of the jet all suggest
fairly strong synoptic-scale lift and the potential for widespread
precip. Snow levels will be coming down during the day, though
suspect that all but the highest valley locations will remain in
rain or a rain/snow mix. Mountain locations, especially across the
north will see several inches of new snow with this feature, with
most of the accumulation arriving after the short term forecast

A pattern change will be taking place during this extended period
as we go from a west southwest storm track into the western CONUS
early in the week to a northwest storm track by the end of the
work week. The GFS and EC have a similar 500mb southwest flow
across the region Monday through Wednesday but the GFS 500mb
heights are 120 meters lower than the EC while the GFS ensemble
mean heights are about 60 meters lower than the EC. The EC has
been been trending slower with lowering of the heights with the
GFS playing catch up. Therefore have based forecast on the EC

With the higher heights across the CWA have backed off on the PoPs
through Wednesday and raised the temperatures a few degrees as well.

By Wednesday night the trough originating in the northern Pacific
will drop southeast and move into the Great Basin with substantially
colder air than past several storm systems which had originated over
the central Pacific. The EC and GFS timing of this system with the
associated 700mb trough axis moving across the CWA Thursday are
remarkably close which added confidence to boost PoPs Wednesday
night into Thursday. With the 700mb temps lowering to -10C degree
and colder, snow levels will come down to most valley floors with
this system.


Surface winds at the SLC terminal will likely become gusty out
of the south through the day. However, there is still a 10 percent
chance that winds could switch to the northwest between 21Z and
23Z. Ceilings will lower through the day, but should remain above
7000 feet through at least 01Z.





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