Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 171217
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
517 AM MST Wed Jan 17 2018

.SYNOPSIS...High pressure will build over the area today through
tonight then shift east Thursday as a warm southwest flow
develops. A strong cold front will slowly cross Utah Friday
through Saturday morning with a cold upper trough remaining over
the area into Saturday night.

&&

.SHORT TERM (Thru 12Z Saturday)...The weak system that moved
through the ridge over the west is moving out of our CWA at this
time. The weak wave left behind patchy stratus clouds over the
north and sern CWA with just some high thin cirrus over the
remainder.

Expect the stratus to dissipate this morning altho the area over
the north may actually expand and linger into the afternoon.

The ridge rebounds over the Great Basin today and the airmass
aloft warms but increasing stability could limit mixing and kept
temps near persistence for today.

The ridge shifts overhead tonight and off to the east Thursday as
the next deep cold trof in the Gulf of Alaska starts to shift
inland.

Models continue to be quite consistent indicating this trof moves
into the Great Basin Friday and across UT Saturday. The associated
cold front moves slowly thru nrn UT on Fri then on south and east
thru more of the CWA Fri night.

Altho the srn end of the upper trof does deepen as it moves
inland, this is a good deep progressive wave with good dynamics
throughout and expect a solid band of precip to accompany the
front thru the entire CWA. If anything, the trof deepening as it
moves into the Great Basin will slow down the frontal band
allowing for greater snow accumulation.

Valley precip should switch over to snow fairly quickly once the
front passes, with this occurring at SLC a little before 00z Sat.
Expect enough valley accumulation to impact travel, north of SLC
during the day and across much of the nrn and wrn CWA Fri night.
Heavy snowfall is expected in most mountains. Winter Weather
highlights will likely be needed for this event, and these
decisions will probably be made today after coordination with
partners.

South winds ahead of the cold front start to pick up Thu aftn and
temps could get quite mild as these winds spread thru the wrn
CWA. The winds will get locally strong on Fri and wind highlights
may be needed for swrn UT. Winds behind the front do not look like
they will be as strong.


.LONG TERM (After 12Z Saturday)...The timing of the cold front
passage is slightly different between the GFS and EC. The GFS has
it located at 12Z Saturday from about Saint George to Green
River vs the EC which has it near Milford to the western Uinta
Basin. Both have the front exiting the CWA by Saturday evening.
Used a blend of the two forecasts to create a PoP forecast which
resulted in categorical PoPs for the I-15 corridor from Provo to
Cedar City as well as in the mountains from the southern Wasatch
Mountains southward to the Pine Valley mountains due to a moist NW
flow and cold 500mb trough axis aloft. This is a progressive
trough and much of the accumulating snow will end by Saturday
evening, although the new 06Z GFS run now shows QPF hanging out
through 12Z along the I-15 corridor from Cedar City to Provo which
is new and may need to be addressed in the next package.

A shortwave ridge moves in Sunday with mainly dry conditions
except for the northern mountains where some snow showers
continue. Temperatures will remain near or slightly below normal
Sunday, especially over western valley and mountains where snow
cover is likely. Another shortwave trough will move in across
northern Utah with warm advection precip Sunday night then will
transition to scattered showers by Monday as cold advection sets
in. Have removed PoPs south of about Provo as models no longer
support precip.

Another ridge builds back in Monday night into Tuesday with
another round of warm advection precip ahead of the next trough
across the northern half of the CWA. Have backed off on the PoPs
in most areas as this trough appears to be associated with very
little cold air aloft. What is interesting is that all of the PoP
forecasts appear to be way overdone from Tuesday through
Wednesday. Could not find any model that had dynamics or thermal
fields that would support these PoP forecasts so used scratch
grids and added minor PoPs in the northern mountains and along the
Idaho border for consistency purposes, but feel they still may be
overdone.

&&

.AVIATION...The only operational weather concern for the SLC
terminal is a 30 percent chance that cigs below 6000 ft AGL will
occur at times through 19Z. Winds will be light southeast this
morning becoming light northwest after 19Z this afternoon.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

SHORT TERM...Wilensky
LONG TERM/AVIATION...Struthwolf

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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