Winter Storm Warning 10, NWS Salt Lake City, UT

Beaver County, UT, Box Elder County, UT, Cache County, UT, Carbon County, UT, Davis County, UT, Duchesne County, UT, Emery County, UT, Garfield County, UT, Iron County, UT, Juab County, UT, Kane County, UT, Millard County, UT, Morgan County, UT, Piute County, UT, Rich County, UT, Salt Lake County, UT, Sanpete County, UT, Sevier County, UT, Summit County, UT, Utah County, UT, Wasatch County, UT, Washington County, UT, Wayne County, UT, Weber County, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
407 AM MST Thu Mar 9 2023

.SYNOPSIS...A southwest oriented and quite warm atmospheric river
will arrive in the region Friday and produce impacts from snow,
rain and wind through the weekend.

&&

.SHORT TERM (Through 12Z/5AM Saturday)...A departing shortwave and
associated cold front is currently exiting the area, with
lingering light snow showers in favored upslope regions of the
northern Wasatch and Bear River Range expected to linger into the
post-dawn hours before diminishing. Model consensus is then well
clustered around the idea of transient, modest shortwave ridging
translating across Utah and SW Wyoming this afternoon, bringing a
brief period of dry conditions.

Onset of warm advection then begins in earnest this evening thanks
to an incoming, inland-penetrating atmospheric river across
California. Deep-layer southerly flow ramps up quickly this
evening, spreading from west to east across the area. Model
soundings and plan views are centered around the idea of 700mb
S-SW winds increasing into the 45-55kt range by late this evening.
These gusty winds will translate down to the surface across wind-
prone areas. These winds are expected to continue through Friday
evening. With increased mixing during the day Friday through a
combination of increased lapse rates and enhanced momentum
transfer via convective elements, widespread south to southwest
wind gusts of 45-55 mph are expected to develop across western
valleys and the Tooele Valley on Friday, with locally stronger
gusts for wind-prone areas. A Wind Advisory may be needed to
account for these winds.

A lead shortwave will combine with ample moisture (IVT values as
high as 500 units) to initiate precipitation around midnight
tonight across northern Utah, particularly across the mountains
and lower elevation areas generally along/north of I-80, with
gradually increasing precipitation chances across northern
valleys as the night continues, probabilities gradually increasing
towards the Idaho border. Thermal profiles are cold enough to
support snow at the onset of precipitation across northern valleys
such as Eastern Box Elder county, the Cache, Wasatch Back and the
Bear River Valley. There is uncertainty regarding both the depth
and resilience of cold pools across these northern valleys,
leading to a variety of possible outcomes. Deeper, more stubborn
cold pools will favor a longer duration of snowfall and a delayed
transition to rain. With an anomalously deep snowpack in place
here, this could contribute to deeper, more difficult to mix out
cold pools. Highest confidence for a complete changeover to rain
exists across Eastern Box Elder county and the Cache Valley, where
the earliest transitions will occur, followed by the Wasatch Back
and Bear River valley. NBM precipitation type probabilities first
exceed 50% for rain across the eastern Box Elder County area
between 8AM-11AM Friday, followed by the Cache a couple of hours
later, with the transition beginning first across southern
sections of the valley. For the Wasatch Back, rain is favored by
shortly after 12PM Friday afternoon. Probabilities for rain/snow
top out around 50% across the Bear River Valley, where cold air is
most difficult to mix out. This suggests that the precipitation
type forecast could go either way by Friday afternoon. Reality
likely won't be so cut and dry, as multiple precipitation types
will likely be observed simultaneously in these valleys from one
location to the next.

It's worth mentioning that typically colder NAM thermal profiles
are cold enough for all snow for the entirety of the event in the
Wasatch Back and Bear River Valleys. This would be good news in
terms of reducing the rain on snow potential, but would likely
require winter headlines as this colder scenario would result in
several inches of snow (with as much as 6-9" per NBM 90th
percentile). Even the colder NAM warms the thermal profile enough
to support a complete changeover to rain in Eastern Box Elder
County and the Cache Valley by Friday afternoon. Given the wide
range in potential precip type scenarios, anywhere from 0.25-0.75"
of rain is possible in these above-mentioned valleys. Given the
cold nature of the snowpack, it should be efficient in absorbing
at least some of this water, but can't rule out localized sheet
flooding and enhanced runoff, especially if an earlier transition
to rain pans out. We also have similar uncertainty regarding
precipitation type across the western Uinta Basin and to a lesser
extent Castle County. NBM precipitation type probabilities are
similar across these areas when compared to northern valleys, but
the colder NAM thermal profiles would also suggest mainly snow for
the western Uinta Basin as well. Similar scenarios to our
northern valleys would play out here as well, with deeper, more
stubborn cold pools resulting in more snowfall, with earlier
eroding cold pools resulting in greater potential and duration for
rain on snow.

Outside of the above-mentioned valleys, snow levels are forecast
to quickly rise on Friday, surging to 8,000-9,000 feet across
northern and central portions of the area, to as high as 10,000
feet across southern Utah by Friday evening. For areas above
these elevations, all snow is likely through the duration of this
event. The combination of falling snow and windy conditions in
wind- prone higher terrain areas will lead to locally significant
blowing snow with dangerous winter conditions. The period of
highest precipitation rates and strongest winds across the region
appears to be centered around Friday evening through early
Saturday morning.

The next phase of this system gets underway after midnight Friday
night, as a cold front drops into northern Utah. Latest
indications suggests the front will make it as far south as the
I-80 corridor by 5AM Saturday. This will force any lingering
precipitation to change back to snow. A period of dry advection is
favored along/north of the front, which may actually force
precipitation to briefly end across northern Utah and SW Wyoming
Saturday morning. An exception may be along the Wasatch and Bear
River Ranges, where light orographically-induced snow is favored
to continue.

.LONG TERM (After 12Z/5AM Saturday)...By daybreak Saturday morning,
the bulk of the atmospheric river will have shifted south of our
area and weakened, while the baroclinic zone on its northern end
will have shifted southward into central Utah. Widespread
precipitation (though not as heavy) will continue into the afternoon
from around Soldier Summit southward, with only a few showers
lingering over northern Utah and generally in areas favored by
westerly flow. Guidance has trended colder behind the baroclinic
zone (ensembles now averaging around -7C at 700MB over northern
Utah) suggesting the possibility of snow for mountain valleys as
precipitation winds down Friday night and snowflakes mixing in for
valley benches. Again, any amounts would be light at this point as
the focus of the moisture tap will have shifted out of our area.
Another implication of this cooler trend is weekend temperatures now
forecasted to run 10ish degrees below normal in the north and near
to slightly below normal in the south, which suggests more measured
melting for our low-elevation snowpack vs the melting potential
suggested just a few days ago.

Low-amplitude ridging builds into the Great Basin over the weekend,
though the potential for ongoing showers remains with a cool and
generally unstable air mass remaining in place. Monday finally
offers a break in the precipitation chances, though this break is
short lived as the next AR event has already reloaded and is moving
onshore into California around that time ahead of a trough moving
into the Pacific NW. Forecast certainty wanes at this point as
global ensembles have varying ideas in terms of how quickly the
aforementioned trough slides inland. One aspect of the forecast with
higher confidence is that warm temperatures will manifest once again
in advance of this next AR event, with 700MB temperatures of around -
2C and relatively little ensemble spread by late Monday into
Tuesday. We should clarify that "warm" in this case means near to
slightly above seasonal norms, however this is noteworthy given how
persistently cold we have been this winter.

As mentioned above, there is uncertainty in the details and timing
of the next trough moving through our area around midweek. That
said, there is near certainty in ensembles and their related
deterministics that the trough axis will push through the Great
Basin midweek. It is also worth noting that the general synoptic
pattern in advance of the trough could favor an AR route around the
southern end of the Sierra and into our area, allowing for more
action in southern Utah in comparison to this week's AR event. 25th-
75th percentile NBM numbers suggest another quarter inch to inch of
liquid in the valleys and half inch to 2 inches SWE for the
mountains, with a wide spread as expected due to the uncertainties
mentioned earlier. Even the lower end numbers would be noteworthy
given everything we've already received recently. Also, to give an
idea of the timing/amplitude uncertainties, global ensemble 700MB
temperatures over northern Utah range from around 0C to -15C by late
Wednesday, which of course has wide implications for precipitation
type in valleys. The mean of all these solutions is represented in
our deterministic forecast and suggests a return to snow potential
for most valleys again by later in the work week.

&&

.AVIATION...KSLC...Predominantly VFR are anticipating through the day, with
ceilings below 7000ft early this morning giving way to higher clouds
for the remainder of the day. There is a low (<30%) chance of MVFR
ceilings early this morning from those lower clouds. Southerly winds
will increase through the day, becoming gusty by late tonight ahead
of the next period of wet weather Friday.

.REST OF UTAH AND SOUTHWEST WYOMING...Any lingering snow showers
over northern Utah and southwest Wyoming will quickly end by mid
morning, with a few pockets of MVFR ceilings and mountain obscuration
in its wake, generally from KPUC northward. High clouds will move in
by late in the day along with increasing southerly winds ahead of a
period of wet and warmer weather arriving Friday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 5 AM MST early this morning for
     UTZ108-110>112.

WY...Winter Weather Advisory until 5 AM MST early this morning for
     WYZ021.

&&

ADeSmet/Van Cleave

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

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