Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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641
FXUS65 KSLC 162237
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
337 PM MST Sat Dec 16 2017

.SYNOPSIS...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 (THROUGH 00Z WEDNESDAY)...
Water Vapor Satellite shows a splitting system to our west with
the norther branch energy crossing the Northern Rockies and the
deepening southern energy cutting off over Southern California.
400-200mb MDCARS wind observations place a 110-145kt northerly jet
on the backside of the trough from British Columbia into Central
California. GOES/SLC 12Z RAOB/HRRR indicate that precipitable
water values vary from 0.35"-0.50" northern and western valleys to
0.10"-0.20" central and southern mountains.

Roads warmed up through the day as expected given increasing solar
and low snow rates. Will begin to lose the solar impact in a
couple hours so some canyon and higher elevation roads especially
may become slushy or snow covered. Accumulations will be light
with northerly low level flow and a shearing trough passing
overhead. Added some fog tomorrow morning for the Cache Valley
due to recent snowfall and anticipated clearing.

Height rises and warming aloft will end the threat of precipitation
tomorrow morning. Low amplitude ridging will build Sunday night
and Monday. Today`s guidance have backed off on the southern
extent of the Pacific jet, now along the Canadian border.
Therefore low level warm advection is weaker, thus lowered PoPs
along the Idaho border Monday.

Given recent pattern, bias corrected temperature/RH guidance
should have a good handle on valley conditions. Did lower
temperatures and raise RH Odgen northward due to today`s
snowfall. Think enough mid and high clouds stream in to not
mention fog for Logan after tomorrow morning, but its something
to monitor.

Utah DEQ sensors indicate strong improvement during the last few
hours Salt Lake northward, owing to precipitation and increase in
northerly flow, as well as cooling aloft.

.LONG TERM (AFTER 00Z WEDNESDAY)...
Focus in the long term shifts towards a somewhat better trough
passage (relatively) later Wednesday through Thursday, and the
potential for a significantly colder airmass encompassing the
region from the north next weekend. Higher confidence in the
former than the latter attm.

Regarding the midweek trough, globals continue to depict a somewhat
consolidated trough diving SE along the downstream side of the ridge
and into the eastern Great Basin Wednesday into Thursday, then it`s
evolution (downstream or closing off near the 4-corners) becomes a
lower confidence outcome. Solid agreement does exist that a cold
front with good baroclinicity will move across northern Utah
Wednesday afternoon, then into southern Utah during the evening
hours. Strong CAA in wake of the front coupled with a period of
decent upper jet support will yield a period of precip, but all
guidance supports a less than modest event. This said, the trailing
airmass will be cold enough for low elevation snow. Latest GEFs
plumes for KSLC suggest a most likely outcome of about .20" of water
(highest outlier only pushing .40") and up to a few inches of snow,
heaviest Thursday evening and extending almost to KCDC. Surrounding
mtns shouldn`t see much more than .30-.50" water/4-8" snow by
Thursday morning. Depending on it`s evolution and whether or not
the system closes off Thursday, amounts shouldn`t be much more than
that unless things change.

Thereafter, globals continue to flirt with an arctic intrusion by
the weekend. This will depend on the orientation of the upstream
ridge just off the Pac coast, and enough strength from any short
wave energy descending the backside of this that would allow enough
amplification of the long wave trough this far west. Ensemble mean
H5 progs do support enough westward amplification of the long wave
trough for the cold intrusion, so have leaned that direction at
least for now. This said, feel it won`t be as strong as the current
deterministic EC forecast yields (H7 temps of -26 C by Sunday
evening). Time will tell.

&&

.AVIATION...
At the SLC terminal...VFR conditions are expected to prevail
through the evening, with periods of low-end MVFR to high- end IFR
conditions possible in light snow this evening, primarily between
03Z and 06Z. Northwest winds are expected to continue through the
evening.

&&

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

$$

SHORT TERM...10
LONG TERM...Merrill
AVIATION...Traphagan

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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