Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 102218

Area Forecast Discussion
National Weather Service Salt Lake City UT
318 PM MST Sun Dec 10 2017

.SYNOPSIS...The strong and persistent high pressure aloft will
remain the dominant weather feature across the western states
through at least the middle of this week.


.SHORT TERM (Until 00z Thursday)...The short term forecast period
will remain under the influence of the strong and persistent high
pressure aloft across the western CONUS.

Temperatures will remain static or only show a slight upward trend
at the higher elevations and across the southern valleys the next
couple of days. Any increase will be driven by slightly warmer near
700mb temps and continued clear to mostly clear skies. Also looking
for a continuation of fairly large diurnal trends in those areas
owing to the very dry air mass place, especially across southern and
central Utah.

Fog/low clouds will persist in the Cache valley and over/near Bear
Lake into Wednesday morning. A couple of areas where fog/low clouds
have shown an increase this weekend have been along and west of the
GSL and to a lesser degree over Utah Lake. Anticipating a further
expansion of the fog/low clouds south and west of the GSL the next
couple of nights/mornings, with some impact along I-80 in the
eastern half of Tooele county.

The latest runs of the GFS/ECMWF in general agreement in
retrograding the upper ridge back to the west coast by late
Wednesday. Shortwaves cresting this ridge will now find a track
across the northern Rockies and will graze western Wyoming/far
northern Utah beginning late Wednesday. This first shortwave will
likely not be able to breakdown the valley inversions as this
feature will show little surface reflection and the best low-level
cold advection will remain over southwest Wyoming. At this point the
best to hope for looks to be a reduction of fog/low clouds over Bear
Lake and possibly over the Cache valley.

.LONG TERM (After 00z Thursday)...The latest 12z EC has trended
farther east and weaker with the Thursday wave that was advertised
just 24 hours ago to drop south through Utah, glad I didn`t lean
on that for yesterday`s forecast. The ridge begins to weaken and
flatten with the anticipated trough moving through Friday and
Saturday, however models disagree yet again.

The 12z version of the GFS has slowed somewhat and drastically
weakened the system, pushing it farther north and east. The EC has
even more so weakened the trough to almost a weak shortwave. A
bit disappointing to see. Confidence is once again on the low side
with these recent changes. The GFS brings down a -8C 700mb front
while the EC remains at -2C. With the lower confidence in the EC
(more drastic changes model run to run), leaning towards the GFS
even though it weakens the trough more. With this arrival of
colder air, this would greatly help to erode away the inversion in
the Cache and Salt Lake valleys.

Opted to leave in the fog mention for the Cache Valley through the
extended just for sake of persistence, given the idea that if the
models continue to trend inconsistent the fog likely will
continue. Also of note, if the GFS plays out, we could see some
valley snow showers late in the extended, though given the
wobbling of temperatures some rain-snow mix is likely as well if
the moisture remains consistent. All in all, confidence remains
low on any appreciable precip in the extended but more confidence
lies in the possibility of breaking and eroding the inversion.
Continuing to cross my fingers.


.AVIATION...At the SLC terminal...VFR conditions are expected to
prevail through the early evening before visibilities deteriorate to
the MVFR range for periods during the late evening and overnight
hours. Northwest winds are expected to shift to the southeast
between 03Z and 05Z.





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