Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 130503

Area Forecast Discussion
National Weather Service Salt Lake City UT
1003 PM MST Tue Dec 12 2017

.SYNOPSIS...The strong and persistent upper ridge will retreat to
the west coast over the next couple of days. Weather disturbances
cresting this retreating ridge will begin to erode the strong
valley inversions, and possibly bring some precipitation to the
area by the weekend.


.SHORT TERM (Through 00Z Saturday)...The long dominant upper ridge
across the region will retreat back to the west coast for
Wednesday/Thursday. East of the ridge a shortwave digging
southeast through the northern Rockies/northern Plains will spread
mid and high clouds across northern/eastern Utah later tonight
through early Wednesday. At best will see some reduction in the
areal extent of dense fog and slightly warmer overnight lows,
otherwise little impact from this passing feature.

Fog/haze should remain fairly widespread under the still strong
valley inversions over northern Utah Wednesday. The next shortwave
scheduled to track south through the Great Basin Wednesday night
may have some impact on said inversions. Near 700mb temps ushered
in by this feature are progged to cool temps anywhere from 6 to 9
degrees C by late Wednesday night. This cooling should weaken, and
possibly eliminate some of the strong inversions across the far
north for Thursday.

Increasing low-level warm advection northwest flow Friday will
make temps a bit of challenge in terms of which valleys warm and
which remain stable and cold. Suspect that cool temps will hover
near the GSL during the day, with most other areas running at or
above normal ahead of the next strong upper trough scheduled to
arrive early in the weekend.

.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
at the SLC terminal through the valid TAF period. LIFR conditions
will likely remain in place through the night, although occasional
but very brief improvements to IFR may be possible. Light snow will
at times precipitate out of the fog/stratus possibly creating light
accumulations on untreated surfaces. Otherwise, winds will remain





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