Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 172316

Area Forecast Discussion
National Weather Service Salt Lake City UT
416 PM MST Tue Jan 17 2017

.SYNOPSIS...High pressure aloft will remain over the Great Basin
through midweek. This high pressure will give way to a series of
weather disturbances beginning late in the week.


.SHORT TERM (Until 00Z Saturday)...High pressure aloft, the axis
of which extends from the eastern Pacific through the northern
Plains will sag south into Utah for tonight/Wednesday. Extensive
areas of stratus in the Uinta Basin and Salt Flats/northwest
desert will persist, if not expand a bit tonight/Wednesday. The
stratus through the Sevier/Sanpete valleys, and across eastern
Millard county will likely form up across these areas again
tonight, then dissipate again during the late morning hours

The upper ridge will yield to a series of upper level troughs off
the Pacific beginning Thursday. Model guidance continues to show a
pronounced split with each of these features as they enter the
western Great Basin. This splitting will sap a good portion of the
energy from these dynamic features, though not so much that
widespread precip is expected beginning with the first trough fro
late Wednesday night through early Friday.

Precip-type will once again be an issue as the lack of a well-
defined surface front will serve to keep colder air trapped in the
lower northern/eastern valleys. The first concern will be for
freezing rain across the Salt Flats with the initial surge of
precip into western Utah. Considering that this precip will be
light and surface mixing poor, the chance for freezing is
sufficient to include a mention in the forecast. Areas farther to
the east may see just enough mixing ahead of the precip to keep
precip as snow or a rain/snow mix through the urban areas
Thursday. The rain/snow line could end up below the benches/foothills,
so will keep the possibility of accumulating snow for those areas
late Thursday/Thursday night.

Southern Utah will have a somewhat better chance at mixing Thursday,
with most of the very cold air already scoured out as the precip
reaches the area. Will start precip as mainly snow, then
transition to a rain/snow mix during the day Thursday.

a shortwave ridge immediately ahead of the next splitting trough
will produce a briefly lull in precip late Friday morning/early
afternoon. This second trough, though splitting, will come into
the western/southern Great Basin as a negative-tilt feature with
solid jet support. strong synoptic-scale lift will generally
concentrate on Arizona, with still significant amount of lift and
the resultant precip across southwest Utah beginning late Friday.

.LONG TERM (After 00Z Saturday)...Powerful jet nosing into southern
California will bring another somewhat splitty trough in across the
Great Basin Saturday. The baroclinicity at 700mb is minor but there
is good cooling at 500mb which will help destabilize the air mass
through the day Saturday. In addition have raised PoPs along the I-
15 corridor and adjacent mountains from Ogden to Cedar City to
account for the moist northwest flow Saturday afternoon as the jet
buckles along the west coast. The 700mb temps of -8 to -10C support
snow except the Dixie area of UT.

This buckling of the jet will serve to create a short wave ridge
over the CWA late Saturday night into Sunday. As a result the chance
of measurable precip between midnight Saturday and midday Sunday is
minimal at best. What appeared to be a dry day ahead for Sunday from
yesterday`s models now does not look as promising as some moisture
will spread in the across the western portions of the CWA by
afternoon. However, the GFS looks to be too aggressive with this
moisture at 700mb and QPF and sided with the EC which may also be a
tad too fast since the ridge axis at 500 mb is just barely moving
overhead. Have backed off slightly on the PoPs for Sunday.

This storm system developing for Sunday night through Monday has
much more dynamical support as well as colder air as the jet buckles
even more into a full latitude trough. Have raised the PoPs for
these periods but didn`t go overboard as the strength of this system
still varies between global models as the GFS is very sharp with the
trough and progs a good baroclinic zone at 700mb moving across the
CWA while the EC is more broad with the trough and has less
baroclinicity. These differences impact the weather for Tuesday with
the EC lingering much more weather due to the broad scope of the
trough vs the GFS which is faster with the ridge moving in behind.


.AVIATION...Borderline VFR/high end MVFR in visbys will continue
at the SLC terminal through early this evening then high end MVFR
visbys expected after 04Z. There is a 20 percent chance of dense
fog forming from 09Z through 15Z. Light northerly winds will shift
to southeast about 04Z tonight.





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