Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 161637
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
937 AM MST Tue Jan 16 2018

.SYNOPSIS...High pressure will remain over the Great Basin into
tomorrow. A weak system will move through the high late today
through tonight bringing some clouds. The high then gives way to
a warm southwest flow on Thursday. A strong cold front will cross
Utah Friday through Friday night with a cold upper trough
remaining over the area Saturday.

&&

.DISCUSSION...
Water Vapor Satellite shows an amplified ridge over the Interior
West. 400-200mb MDCARS wind observations place a 120-140kt
cyclonic jet off the Pacific Northwest coast. GOES/SLC 12Z
RAOB/HRRR indicate that precipitable water values vary from
0.05"-0.15" mountains to 0.25-0.35" western valleys.

Surface ridge has moved to our east with the amplified mid level
ridge overhead. 12z KSLC RAOB shows several inversions below
700mb though gusty canyon winds owing to the easterly 10mb
pressure gradient helping to mix around the haze. Expecting the
west to increase from partly cloudy this morning to cloudy this
afternoon with the ridge axis shifting east. Removed all PoPs from
out west this afternoon given the rather dry mid level conditions
and decaying wave approaching from the west. Rest of the forecast
in great shape.

&&

.PREVIOUS DISCUSSION...
The mid level ridge is overhead early this morning, weakening and
shifting east as a short wave trof crosses CA and starts to move
into the Great Basin.

The easterly gradient across the CWA remains tight enough to
generate east winds at the surface early this morning with gusts
out of the canyons along the Wasatch Front from the Cache valley
south to Utah valley. Speeds maxed out in the low 40s earlier in
the night with low 30s now. Expect the gradient to relax thru the
morning with the winds diminishing.

The approaching short wave tro around the haze. Expecting the
west to increase from partly cloudy this morning to cloudy this
afternoon with the ridge axis shifting east. Removed all PoPs from
out west this afternoon given the rather dry mid level conditions
and decaying wave approaching from the west. Rest of the forecast
in great shape.f weakens considerably as it shifts
east across NV today and UT tonight with the ridge rebounding
quickly behind it tonight into Wed. The models have finally come
into agreement that this trof will be mainly clouds for the CWA.
They do generate a little precip over the far wrn CWA late today
shifting a little farther east this evening before the qpf
disappears after 06z. Have kept the slight chance mention of
precip in the grids in these areas thru 06z. Models do keep
somewhat higher RH at lower levels after 06z as the wave shifts
east and have left token low pops in the mtns for the 06-12z
period.

The area dries out Wed as the ridge rebounds. It shifts east
Thursday ahead of a deep cold trof advancing inland with
increasing swly flow over the CWA. These winds aloft should
surface across most wrn valleys Thu and much of the CWA overnight
into Fri morning as the associated cold front begins to move into
UT.

The EC and GFS are a little off on how fast and cold the front
will be as it moves thru nrn UT on Fri. The new 06z GFS is a
little slower than the 00z run but it is still faster than the EC.
The main result of this difference is precip turns to snow a
little sooner in the day if the GFS is right.

This still looks like a fairly significant snow storm for much of
the CWA with snow likely impacting travel in the mtns and valleys
across about the NW half of the CWA by sunset FRI, spreading south
and east overnight as detailed around the haze. Expecting the
west to increase from partly cloudy this morning to cloudy this
afternoon with the ridge axis shifting east. Removed all PoPs from
out west this afternoon given the rather dry mid level conditions
and decaying wave approaching from the west. Rest of the forecast
in great shape. below.

The cold trough and associated cold front remain on target for
moving through the central and southern portions of the CWA after
00Z Saturday. The EC has slowed the cold front progression down
some from its previous 12Z run. So just in case this solution is
more correct than the faster GFS have adjusted the weather portion
of the forecast between 00Z and 06Z Saturday to include mixed
precip across the southwest valleys. However, confidence remains
high that this system will be significant so did not touch the
PoPs in the first 2 twelve hour periods with mountains having
categorical PoPs Friday night and likely during Saturday, while
the western valleys south of I-80 will be mainly likely or
categorical Friday night and chance Saturday.

Models are a little stronger with the shortwave ridging Saturday
night through Sunday, although they still show a somewhat moist
NW-N 700mb flow over western Utah valleys and adjacent mountains.
Consequently, have backed off some on PoPs especially eastern and
southern areas for this period but left token PoPs for upslope
areas of western valleys and across the mountains Saturday night
then reduced PoPs even more on Sunday.

Warm advection develops Sunday night into Monday across northern
portions of the CWA ahead of the next trough. Lowered PoPs some
across eastern and southern portions of the CWA since they do not
normally due as well under warm advection. The EC and the GFS
diverge in the strength of this trough by Tuesday. Have leaned
toward the slightly less deep GFS trough solution which is more
like the 12Z EC mean ensemble. Nonetheless, another round of snow
is expected across the northern two-thirds of the CWA. Have raised
PoPs a tad for Monday night and Tuesday.

Cold temperatures will prevail during this extended period with
most places about 5 degrees below average, but will feel much
colder in comparison to the recent mild 10-15 degree above average
temperatures we have experienced lately.

&&

.AVIATION...
Little operational concerns at KSLC today as VFR conditions will
prevail with gradually increasing mid/high-lvl clouds. Light
southeasterly winds are expected to shift to prevailing
northwesterly around 18-19Z.

&&

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

$$

PUBLIC...10/Wilensky/Struthwolf
AVIATION...Carr

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