Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 161128

Area Forecast Discussion
National Weather Service Salt Lake City UT
428 AM MST Sat Dec 16 2017

.SYNOPSIS...A weakening Pacific storm system will cross the area
today bringing light precipitation to northern and western Utah.
High pressure aloft will build back into the area beginning
Sunday. A new and substantially colder storm system will arrive
in Utah for the middle of the upcoming week.


.SHORT TERM (Until 00z Wednesday)...The approaching upper trough
extending from western Montana through central California will
continue to split and weaken as it advancing into Utah this
morning. The surface cold front out ahead of this trough stretches
from southwest Wyoming through west-central Utah at this time.
This front will strengthen a bit as generally weak low-level cold
advection catches up with this feature this afternoon.

Precip so far remains focused along and west of the near 700mb
baroclinic zone. Frontogenic forcing, such as it is, will likely
peak this morning across northwest Utah. This low-level forcing
along with the dynamic support from the advancing/weakening mid-
level trough will place the best chance for precip across northern
through west-central Utah today through this evening. Precip-type
for the most part will be snow, though can not rule out a brief
period of rain or rain/snow mixed this morning as the trapped
cold air in the valleys mixes out and precip intensities remain on
the light side.

Air quality should show a marked improvement as the arrival of
much colder air aloft breaks down the strong valley inversions.
While air quality will improve, generally light northerly winds
will struggle to advect out the thick haze layer in place.
Suspect that some hint of visible haze will be present, but
considering conditions of the past several days the improvement
will be more than welcome.

The location of the upper ridge across the eastern Pacific will
result in a downstream anti-cyclonic west-northwest flow across
the Great Basin beginning Sunday and continuing through early next
week. Across far northern Utah/southwest Wyoming the moisture
embedded within the mean layer flow will keep the area under some
form of cloud cover through early next week. Shallow low-level
warm advection could be sufficient to generate enough lift for
some light precip along the higher terrain near the Idaho border
Monday/Tuesday. The cloud cover will also serve to keep temps in a
somewhat narrow range and valley inversions from strengthening
again early in the week.

.LONG TERM (After 00z Wednesday)...Nearly zonal flow and warm
advection in place across Utah as the long term begins will break
down as the trough moving southeast from the Pacific Northwest
further digs through the Great Basin. The main trough axis swings
through Utah Wednesday with a fairly decently packed baroclinic zone
depicted at 700mb. A good frontogenetic forcing along with
substantial moisture and fairly strong northwest flow behind the
front should lend to a good precipitation producing cold front.

Extended models continue to be in rather good agreement in time and
spatial coverage with this system, and confidence is increasing.
Another bump to increase PoPs with this package, as well as some
areal coverage as well. This storm looks to be a good snow producer
for our area, finally. Forecast models do hint at a
splitting/pinching nature to the system as it rotates overhead, but
has the cut off low forming over the Utah/Arizona border late
Thursday. A chance for some wrap-around moisture is possible
Thursday along the spine of central and southern Utah.

As the flow becomes north-northeasterly Friday, drying conditions
will develop, though still under a rather cold airmass. With
elevated winds, feel that the cold trough will suffice to clear out
valley inversions along the Wasatch Front through the following
weekend and the the threat for rebuilding is very low.


.AVIATION...Anticipating the onset of snow with IFR visibilities
at the SLC terminal this morning, by 15z. Northerly winds already
in place should continue through the day. Visibilities expected to
improve to MVFR by 20z - 21z with continued MVFR cigs through
most of the day.





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