Area Forecast Discussion Issued by NWS Salt Lake City, UT
000
FXUS65 KSLC 131004
AFDSLC
Area Forecast Discussion
National Weather Service Salt Lake City UT
404 AM MDT Wed Mar 13 2024
.SYNOPSIS...A storm system will continue to bring significant snow
to the higher terrain of Utah into Thursday. A period of
significant easterly downslope winds is expected to develop over
prone areas of the Wasatch Front and Cache Valley. Wrap around
precipitation is anticipated across southern Utah late Thursday
through Friday, bringing some of the most significant snow totals
of the season to that area.
&&
.SHORT TERM (Through 12Z/6AM Saturday)...The cold front that made
its way across northern and central Utah yesterday is now located
near a KCDC-KHVE-KRKS line this morning. Still seeing some
generally light precipitation focused along this boundary, but
associated frontogenetic forcing has decreased significantly from
what was seen earlier. Elsewhere, seeing a few showers here and
there but otherwise much of Utah and southwest Wyoming is
experiencing a lull in precipitation during this event. Radar
indicates a small lake band attempting to get organized along the
Tooele/Salt Lake county line. Conditions will remain favorable for
lake effect snow through at least mid-morning, and would not be
surprised to see some areas of slushy roads in the vicinity of
this band for the Wednesday morning commute.
The wave on the back end of the trough is currently centered near
the Idaho/Montana border as it starts to close off. This wave is
expected to slide south along the Utah/Nevada border, reaching the
Utah/Arizona/Nevada triple point by early Thursday morning. The
passage of this trough will bring a reinforcing shot of cold air,
with 700mb temps as low as -13C forecast to move into northern
Utah during the afternoon. In cold pool instability, will see
coverage of snow tend to increase, primarily over the mountains of
northern and central Utah. Accumulations for this part of the
system are expected to be on the light side, with the best totals
anticipated for the Upper Cottonwoods. The progress of the low is
expected to put the area under a northerly flow for Wednesday
night into early Thursday, with a notable pressure gradient
setting up over southwest Utah with a good amount of cold
advection. Thus, conditions will be favorable for the development
of gap winds in prone areas of Washington county, with high res
ensembles showing 60 to 70 percent of members with wind gusts
greater than 55 mph late Wednesday into early Thursday in these
prone areas. Earlier shifts held off on headlines for this area
given limited impacts and will continue this line of thinking.
As the low slides eastward into Arizona Thursday morning, the flow
over Utah and southwest Wyoming will develop a much more
pronounced easterly component, with flow aloft increasing with a
tightening pressure gradient, particularly between southwest
Wyoming and the Wasatch Front, where some modest cold advection
will be present. These ingredients make conditions rather
favorable for a significant downslope wind event impacting prone
areas of the Northern Wasatch Front and Salt Lake Valley, with
some potential for the Cache Valley and Eastern Box Elder county
to see some potent gusts as well. Latest guidance indicates about
60 percent of high res ensemble members showing wind gusts in
excess of 65 mph for the most prone areas of the Northern Wasatch
Front (and have found the guidance is generally underdone with
these events), and a 60 to 80 percent chance of wind gusts in
excess of 50 mph for a broader area of areas favored during
downslope wind events between Ogden and about Millcreek. The
signature across the Cache Valley looks less notable, with the
high res guidance indicating about a 30 percent chance of wind
gusts in excess of 40 mph. Of note is that the guidance this
morning indicates a peak in the winds late Thursday morning into
Thursday afternoon, which is several hours earlier than guidance
had been previously indicating. Now is the time to secure loose
objects and make other preparations if located in an area prone to
downslope winds. Given the relatively high level of uncertainty
that accompanies the events (as all the ingredients have to come
together just right to get these very significant gusts), have not
made any changes to the headlines at this time and will allow the
day shift to make upgrades with another suite of guidance
available to them. By Friday morning, the wobbling of the Desert
Southwest low will significantly decrease the flow and weaken the
pressure gradient over northern Utah. Though canyon winds will
persist at times Friday into Saturday, gusts will be much less
significant.
In addition to the winds, the position of the low is expected to
put the Uintas and southwest Wyoming late Wednesday into Thursday
in a favorable position for wrap around snow, particularly for
areas east of about Mirror Lake. Going Winter Weather Advisory
covers this threat well. By Thursday afternoon, will see the
central and southern mountains start to get in on this action,
with a particularly potent wrap around band expected to impact
southeastern portions of the area during the day Friday. These are
likely to be the highest accumulations of the season for the
central and southern mountains, with particularly notable
accumulations also expected for the adjacent national parks, Bryce
Canyon and Capitol Reef, as well as some valleys of south central
Utah, primarily Escalante and points eastward. Some uncertainty
still remains with regard to snow levels, which would have some
implications for accumulations in Capitol Reef and Escalante, for
example, something to keep an eye on moving forward.
.LONG TERM (After 12Z/6AM Saturday)...Main adjustment to the long
term portion of the forecast was to maintain residence of the
Arizona closed low and associated precipitation chances going across
southern Utah through Monday, with a few showers even lingering into
Tuesday. Ensemble and deterministic solution space are now in good
agreement in this idea through Monday, with about 50% of members
keeping the low around through Tuesday. The low is forecast to
undergo a gradual weakening trend, however, especially after
Saturday. Despite the weakening trend, prolonged easterly flow will
be juxtaposed with lingering upper diffluence and weakening
deformation/TROWAL structure to keep occasionally moderate
precipitation ongoing Saturday across portions of central and
southern Utah, with terrain perpendicular to the easterly flow
favored for the highest amounts. With heights gradually rising and
warmer air wrapping around the low, snow levels are expected to
gradually rise through the day Saturday to 6,000-6,500 foot
elevation after starting the day off some 500 feet lower. During the
day Saturday, an additional 2-6 inches of snow is likely for places
like Fremont Junction, Fish Lake and the Boulder Mountain and Brian
Head locations.
With the low lingering around, this will maintain easterly
700mb winds in the 25-35kt range going into Sunday across SW
Wyoming and wind-prone areas of northern Utah. By this time, the
worst of the downslope wind event will have ended, but wind-prone
locations are expected to continue to gust in the 25-35 mph range
with locally higher gusts through Saturday.
Meanwhile, a mature omega block with a strong ridge of high pressure
will extend from the western Canadian provinces southward into the
PacNW into early next week. Northern Utah and SW Wyoming will reside
on the southern periphery of this ridge, allowing for dry and
steadily warming conditions from central Utah northward into early
next week. Across southern Utah, showers will linger but
temperatures will steadily climb through midweek. Warmest
temperatures are expected Tuesday, with highs 0F-10F above normal
across central and southern Utah, and 5F-15F above normal across
northern Utah. Beginning Wednesday, model consensus quickly breaks
down, with two different scenarios emerging. One scenario keeps
ridging in place over the area (albeit in a gradually weakening
state) in place through nearly the end of the workweek. By Thursday,
this scenario is represented by 25% of ensemble members. The other
75% of members quickly erode the ridge and replace it with a
deepening trough located somewhere over the western CONUS, with the
trough placement stretching from from off of the California coast to
the Northern Rockies to the Northern Plains. Official forecast will
show a gradual cooling trend by Wednesday with a reintroduction of
shower chances across the north. With so much uncertainty developing
by this period, this forecast is subject to change.
&&
.AVIATION...KSLC...An area of low pressure within a strengthening
trough will move through northern Utah late this afternoon through
this evening, maintaining unsettled conditions. Light, variable
winds early this morning will become NW between 11-12Z and remain NW
through tonight. Winds occasionally gusty this afternoon between 20-
25kts. CIGs will alternate between MVFR and VFR through 00Z-02Z this
evening when VFR conditions will take hold. Lake effect snow showers
expected to develop SE of the Great Salt Lake by 12Z this morning.
These are largely expected to remain over the west side of the Salt
Lake Valley over towards Tooele, but snow showers could occasionally
brush over the terminal. This activity diminishes early afternoon
with VCSH into 02Z this evening.
.REST OF UTAH AND SOUTHWEST WYOMING...Area of low pressure will drop
into Utah from the north tonight and reach the Las Vegas, NV area by
the end of the period. Lingering low stratus with mountain
obscuration will gradually lift this morning. Isolated to scattered
snow showers persist into early this evening, mainly near/over
terrain. Light snow within strengthening easterly winds will
overspread SW Wyoming between 00Z-06Z and remain in place through
the rest of the period. Strengthening northerly winds will
overspread western Utah by afternoon, with occasional NE gap winds
getting into SGU, especially after 02Z with occasional gusts 20-
30kts overnight here. Locally strong terrain-induced downslope
easterly winds will develop shortly after the period Thu morning
across the Northern Wastach Front as well as over the Book Cliffs
and Price areas with local gusts to 50kts.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Watch from late tonight through Friday morning for
UTZ103>105-107.
Winter Weather Advisory until 6 PM MDT this evening for UTZ110-
111-113-117-125.
Winter Weather Advisory until 6 PM MDT Thursday for UTZ112.
Winter Storm Watch from Thursday morning through late Friday
night for UTZ117-125-127>129.
WY...Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Thursday for WYZ021.
&&
$$
SHORT TERM...Traphagan
LONG TERM/AVIATION...ADeSmet
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