Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 122312

Area Forecast Discussion
National Weather Service Salt Lake City UT
412 PM MST Tue Dec 12 2017

.SYNOPSIS...Strong and persistent high pressure aloft will remain
the dominant weather feature across the western states through the
end of the work week.


.SHORT TERM (Through 00Z Saturday)...Strong high pressure remains
over the Great Basin this afternoon, with persistent low level
inversions across Utah. Fog and stratus across the northern Utah
valleys have retreated toward the Great Salt Lake a bit this
afternoon, but will likely expand again through tonight. Flurries
will also continue to be possible, especially near the Great Salt
Lake. As such, have leaned fairly heavily on persistence for the
forecast the next 18-24 hours.

A weak shortwave trough is forecast to drop into Utah/Colorado
from the north Wednesday afternoon through Thursday morning. While
this wave still does not look strong enough to significantly
impact northern Utah valley inversions, models have trended
somewhat stronger/wetter with the wave in the last couple runs,
particularly in the 12Z ECMWF. Have increased POPs a bit across
the far northern and northeastern portions of the forecast area,
though any precipitation amounts/snow accumulation is expected to
remain pretty light. One other potential concern with this wave is
the possibility of freezing drizzle in some inverted areas, most
notably the Cache Valley. For now, have put a mention of freezing
drizzle in the Cache Valley for Wednesday afternoon/evening. That
said, confidence is not high, and any impacts would probably be
fairly similar to the freezing fog that we have seen the past few

After that wave exits to the south, high pressure re-builds for
Thursday and most of Friday. Assuming the previous disturbance
doesn`t surprise in its strength, this will mean a continuation of
the inversion event and poor air quality across northern Utah.
However, it looks like hope might be on the way starting Friday

.LONG TERM (After 00Z Saturday)...Global models are still hanging
onto the idea of bringing a shortwave trough through the forecast
area late Friday night into Saturday. The 12Z GFS and ECMWF are in
pretty decent agreement bringing -10C H7 temperatures into northern
Utah. This airmass, along with increasing winds behind a cold front,
looks sufficient to scour out most valley inversions. A quick look at
the 18Z GFS shows a trend towards a slightly stronger storm as the
trough carves a bit farther back into Utah, which is even more
promising. However, we must be careful and not wish for it to carve
too far back, as it could end up like the Canadian, which ends up
splitting this storm as it crosses the western CONUS, leaving the
colder air northeast of Utah. That being said, have trended the
Friday night/Saturday forecast more along the lines of of 12Z
GFS/EC, with temperatures trending towards better mixing, and some
precipitation across northern Utah and southwest Wyoming.

High pressure is forecast to return to the area by late Sunday and
persist through Tuesday. However, the ridge is not forecast to be as
strongly amplified as the present ridge, and H7 temperatures will be
slightly cooler. In addition, there could be some lingering mid/high
clouds across northern Utah which could be enough to keep
temperatures from plummeting as much. Still, inversions should
redevelop to some extent for the early part of next week. Looking
beyond Day 7, however, the ridge is forecast to break down during
the middle to latter part of the week, so this inversion event may
not be very long-lived.


.AVIATION...Fog and stratus will be the dominant weather concern
through the valid TAF period. IFR conditions will generally prevail
until around 00-01z when conditions are expected to trend more into
the VLIFR category where it will likely remain through the night.
Light snow will at times precipitate out of the fog/stratus possibly
creating light accumulations on untreated surfaces. Otherwise, winds
will remain light.





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