High Wind Watch 2, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
445 AM MST Thu Feb 29 2024

.SYNOPSIS...After another warm, spring-like day Thursday, a
powerful winter storm will bring strong winds and widespread snow
to the region Friday into early next week.

&&

.SHORT TERM (Through 00Z Monday)...A lot to talk about in this
short term forecast discussion, with the potential for the wind
event of the winter Saturday across much of the CWA and widespread
heavy mountain snow.

Early morning upper air and satellite analysis indicates a
strengthening upper level trough over the Gulf of Alaska, with a
pronounced 140kt+ jet max approaching the Pacific Coast. Broad
upper level diffluence associated with jet max is shifting into
the Northwest. A weakening upper level embedded in the southern
jet is now over New Mexico and shifting east away from the region.
Upper level ridging continues to shirt east into the central
Rockies and eventually High Plains.

For today, continued modest warm air advection will bring another
warm spring-like day to the region, with temperatures along the
Wasatch Front in the mid to upper 50s.

The previously mentioned upper level trough will continue to shift
south and east into the Pacific Northwest through Friday
associated with a stout northern jet. With favorable jet dynamics
and increasing mid-level moisture, expect precipitation to
develop across the northern Utah mountains late tonight into
early Friday. With relatively dry air in valley locations, expect
the heaviest precipitation to be constrained to the mountains for
this portion of the event.

Meanwhile, 700mb winds will increase this afternoon to around 30
to 40 kts. With a well mixed boundary layer, expect gusty winds
across the western Utah valleys this afternoon and evening.
HREF probabilities of wind gusts in excess of 45 mph (advisory
criteria) average around 75% to near 100% across the southwest and
west central Utah valleys and the Tooele/Rush Valley. Wind
advisories are in effect for the western valleys mainly near and
west of I-15 and south of I-80. Current guidance suggests a <30%
chance of wind advisory criteria in the Salt Lake Valley, but
something to monitor.

Moving into Friday, a strong upper level trough remains off the
Pacific Northwest Coast. 700mb winds increase around 5-10 kts,
reaching around 40-50 kts across western Utah. Ensemble
probabilities for wind gusts up to 50 mph are widespread at or
above 90% across the western valleys up to Tooele Valley by late
Friday evening. 700mb flow doesn't increase to what would
typically be associated with high wind warnings levels (58 mph or
higher) until after 00Z Saturday. By 03Z Saturday, guidance
suggests 700mb winds will be in excess of 50-60 kts (potentially
higher) across nearly the entire state. The 00Z EFI (EPS based
Extreme Forecast Index) is nearly maxed out after 00Z Saturday as
well across most of the state.

Looking at the ensemble probabilities of wind gusts greater than
60 mph, expect southwestern and west central Utah to see winds at
or above 60 mph by Friday evening (>90% chance, especially in more
wind-prone locations). This also coincides with an increase in
threat for the Tooele/Rush Valleys. Battling decoupling, the rest
of the state will stay windy, but is unlikely to realize winds
above 58 mph until Saturday morning.

Saturday morning is looking like the beginning of the wind event
for the year. A cold front will shift south and east into
northern Utah Saturday afternoon. Areas ahead of the front will
see the strongest winds, right now, looking like Tooele Valley and
south. Salt Lake and Utah Counties are more questionable right
now as frontal timing will be key. For the remainder of western
and central Utah, however, ensemble probabilities are >80-90% for
wind gusts over 60 mph. Particularly telling is 40-50%
probabilities of wind gusts in excess of 70 mph across portions of
western Utah and even southern Utah Saturday afternoon into early
Sunday (in areas remaining ahead of the front). Will there be wind
gusts over 70 mph? Something to monitor with trends moving
forward.

Wind headline-wise, for Thursday, added additional western valleys
south of I-80 and near and west of I-15, though left the Wasatch
Front aside from the Tooele Valley out of the headlines for now.
Continued the wind advisories for the same locations into Saturday
morning. A High Wind Watch remains in effect for west central and
southwest Utah Friday morning through Saturday evening. As
mentioned above, the probability of winds above 58 mph remains
<50% until Friday evening, so left the High Wind Watches as is. If
current trends hold, the wind advisory for Thursday could be
extended into Friday afternoon, with any High Wind Warnings
starting Friday evening. Added High Wind Watches to the Great Salt
Lake Desert, Tooele Valley, eastern Millard and Juab Counties and
the greater Richfield area for Saturday. Given the latest
guidance, these High Wind Watches may need to be extended across
the bulk of central and southern Utah south of the cold front
Saturday afternoon and evening.

If you are still hanging with me, now for the winter side of this
storm. This is messy and I'll attempt to describe what the current
suite of guidance suggests is the likely evolution of this portion
of the storm. By Saturday afternoon, a broad upper level trough
will be in place across the West. A strong jet max will be
shifting into the Great Basin. Several shortwave troughs will be
ejecting ahead of the main trough across the region Saturday into
Sunday. This is going to be the key factor in the forecast and in
potential valley snow accumulations.

By 00Z Sunday, a strengthening cold front, with a tight 700mb
temperature gradient will be stretched across the northern
portion of the state. This period of frontogenesis will continue
for a time as the front continues south and east. Once the
shortwave trough associated with this period of enhanced
frontogenesis begins to shift away from the region, the front is
expected to nearly stall and weaken around 03Z-06Z Sunday. The
location of this stalling front ranges from north of Ogden to
central Utah. But, wait, that's not all. Another shortwave trough
will eject ahead of the main trough and may push the front
further south and east (and potentially have another round of
frontogenesis). Another, stronger shortwave trough with a
reinforcing cold front is forecast to push through the area Sunday
morning into Sunday afternoon.

Notably at this point, a portion of the guidance shifts the trough
through the region and brings drier air into the state late Sunday
into Monday morning. Other guidance, for example the 06Z GFS,
stall the front across southern Utah, with additional
precipitation across the state into Tuesday morning (see the long
term forecast discussion).

As noted, this is messy. If the front remains in the same
relative location for a long duration, this would result in
heavier snow totals for the valley locations underneath the area
of precipitation. If the front slowly chugs south and east with
time, this may spread the snow accumulations out over a broader
area.

At some point, winter headlines will be needed for the
valleys of most of the state, but sniffing out the locations with
the highest probabilities is difficult at best right now. The
mountains on the other hand, are a different story. There is a
high probability (>80%) of winter storm warning amounts for the
mountains of northern Utah north of I-80. Given the residence time
of moderate to heavy mountain snow is longer for this portion of
the mountains, upgraded to a winter storm warning. With a later
onset for the remainder of the northern mountains and Uinta
County, WY, issued a Winter Storm Watch for these locations (>50%
chance of 18+ inches for the Wasatch, 12+ inches for the western
Uintas and Book Cliffs and 6+ inches for Uinta County, WY. Look
for additional winter headlines over the next few forecast
packages as this is a very cold system and there won't be any
p-type questions post- frontal with 700mb temperatures near
-12 to -14C.

.LONG TERM (After 00Z Monday)...Long term forecast period begins
with the base of a broad scale trough settling across the area. By
this time, a strong cold frontal boundary looks to be largely
through the forecast region, though some guidance has a portion of
the front stalled out across southern Utah before becoming more
frontolytic. Deep moisture will be much more limited by this time,
but with decent synoptic jet support ongoing, more modest
isolated to scattered snow showers will remain possible as Monday
progresses. The continued stronger deep westerly flow will also
continue to promote enhanced orographic snow showers across much
of Utah's mountains. Mountains will be able to pick up some
additional modest accumulations on the order of an inch or so,
with any additional valley accumulation likely pretty light. That
said, GFS derived lake effect guidance does show ~20-30% chance to
see some lake effect snow showers downstream of the Great Salt
Lake. If these were to carry some higher rates, or be timed near
commute hours, could see some minor impacts accordingly.
Otherwise, will see cold temperatures around 5F-15F below
climatological normal Monday. With some light winds Monday
morning, current forecast carries wind chill values in the 10F-15F
range for the Wasatch Front, and low 30s for Lower Washington
County.

By Tuesday, general consensus would support the trough axis and
coldest air beginning to lift out of the region. That said, still
a pretty sizable spread in 75th/25th percentile 700mb temps
(around - 6C/-14C) indicating some uncertainty in how quick and to
what degree the cold air retreats. Most guidance supports some
lingering trough effects to zonal flow, though around 28% of
ensemble members show potential for some quick shortwave ridging
which would yield a bit milder/drier solution. In any case, should
at least see some sort of warmup in comparison to Monday, with
precipitation chances fairly low and more limited to the higher
terrain.

Wednesday onward, something of an active pattern looks to
redevelop, though a good bit of uncertainty is noted on how
exactly the greater flow pattern evolves. The rough agreement is
that some sort of Pacific trough will begin to penetrate inland by
midweek, but quite a bit of variation/spread is noted in how much
moisture and cold air this feature will carry, in addition to
more specific timing. As such, at this time forecast just includes
a gradual ramp up in precipitation chances with temperatures
remaining on the cooler side of things for early March.

&&

.AVIATION...KSLC...A strong system will begin its approach
towards the region. VFR conditions expected to prevail, with a
gradual increase in mid/upper level cloud cover. Initially,
biggest impact from the system will be winds. Southerly winds will
increase, especially right off the surface, with moderately
strong gusts (~20-35 kts) mixing down through the day. Biggest
question comes overnight in regards to if surface can
decouple/stabilize at times. If it does, surface winds will
decrease, but introduce low level shear concerns given strong
southerly flow ~30-45 kts remaining ~750+ ft off the surface. If
it stays mixed, gusty winds persist. Low confidence.

.REST OF UTAH AND SOUTHWEST WYOMING...A strong system which will
span several days will start to sink into the region through the
TAF period. Initially, biggest impact will be increasingly strong
low level southerly flow, which will result in increasingly gusty
conditions at area terminals. Some question on if/where surface
may decouple overnight. If it does, surface winds/gusts will
lessen, but low level wind shear concern would increase. If it
does not, would expect gusty conditions to persist. Low confidence
is noted at this time. Otherwise, will largely just see a gradual
increase in mid/upper level cloud cover, with maybe some low end
precip chances creeping in at far northern terminals by Friday
morning.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 11 AM this morning to 8 AM MST Saturday for
     UTZ101-102-116.

     High Wind Watch from Saturday morning through Saturday afternoon
     for UTZ101-102.

     Winter Storm Warning from 11 PM this evening to 11 PM MST
     Saturday for UTZ110.

     Winter Storm Watch from Saturday morning through Sunday
     afternoon for UTZ111>113.

     Wind Advisory from 11 AM this morning to 11 AM MST Friday for
     UTZ115-122.

     High Wind Watch from Friday morning through Saturday evening for
     UTZ115-122.

     High Wind Watch from Saturday morning through Saturday evening
     for UTZ116-119.

WY...Winter Storm Watch from Saturday morning through late Saturday
     night for WYZ021.

&&

Kruse/Warthen

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