Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 010002
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
502 PM MST Thu Feb 29 2024

.SYNOPSIS...A powerful winter storm will bring increasingly strong
winds through the weekend along with widespread snow Saturday into
early Monday.

&&

.SHORT TERM (THROUGH 12Z MONDAY)...A closed low is noted on current
water vapor imagery centered off the BritCol coast while the
associated mean trough churns into the PacNW and NorCal. Rounding
the base of the trough is a near 140kt upper jet punching into the
NorCal coast. Downstream of this trough, southwest flow is
increasing over Utah. Models indicate H7 flow of 20-35kt with a
strengthening south to southeast surface pressure gradient. This is
aided by a weak disturbance ejecting out along the Canadian border,
which will aid in pushing a weakening cold front into southern Idaho
tonight. Current surface obs show widespread wind gusts 25-35 mph in
the western Utah valleys and portions of the Wasatch Front, with
peak gusts so far today topping 40 mph at some sites. A Wind
Advisory is already in place for the western Utah valleys which will
continue overnight into tomorrow morning. Due to excellent mixing in
these areas, temperatures have also trended warm, reaching the upper
50s to low 60s.

As the trough continues to slowly come ashore tomorrow, southerly
winds are only going to increase. By 00z Saturday (late tomorrow
afternoon), widespread H7 flow of 35-45kt (locally 50-55kt across
west-central and southwest Utah) is noted in the global
deterministic models, with an even tighter southerly surface
pressure gradient. The ECMWF Extreme Forecast Index (EFI) shows some
climatologically anomalous winds, but the signal is not especially
strong. Diving more into the probabilistic data, HREF ensemble mean
wind gusts reach advisory criteria, but there is a generally low
chance of reaching warning criteria except in isolated spots where
probability exceeds 50% through tomorrow evening. We are continuing
to advertise warm temperatures tomorrow (afternoon maxes 5-10
degrees above climatological normals) due to good mixing, since
clouds are not expected to be particularly thick except across far
northern Utah.

As the storm system makes its way inland, several disturbances
embedded in the flow will round the base of the trough and eject
across Utah. The first of these will cross the area Saturday late
afternoon/evening. This will push a rather strong cold front into
the area. The front is still progged to cross northern Utah Saturday
afternoon. South winds will continue to increase ahead of the front,
as strong pressure falls drive an even tighter southerly pressure
gradient across the area. In addition, H7 winds are forecast to
exceed 50kt across a large area Saturday afternoon, peaking at over
60kt in some spots. The ECMWF EFI has a stronger indication of
highly anomalous winds for Saturday. This is supported by the NBM,
which even the ensemble min wind gusts are reaching warning criteria
across a portion of west-central Utah. The NBM probabilistic guidance
shows a greater than 90% probability of warning criteria winds
Saturday afternoon across a good portion of central and southern
Utah. While the NBM has a tendency to overdo the winds, this is
still a strong signal to support strong winds on Saturday. The cold
front is expected to stall across central Utah as the initial
shortwave disturbance exits the area, and strong prefrontal winds
are forecast to continue across south-central and southeast Utah
into Sunday as a result, especially as the cold front redevelops
across northern/central Utah Sunday morning before sweeping through
the remainder of the area through the day Sunday and into Sunday
night. Otherwise, winds across most areas are expected to start to
drop off shortly after the passage of the cold front. With the
winds, some concern exists for blowing dust. Soil moisture is
relatively dry across portions of western Utah where some of the
main sources of dust exists. Winds as strong as what is forecast
will have a good chance of transporting this dust especially across
the western valleys, adding to the impacts.

So, given all that, have made some changes to the going wind
highlights. Mainly have upgraded existing Watches to Warnings, while
adding the Salt Lake Valley. Have also pushed the start time up to
tomorrow night as there is some potential for the stronger wind
gusts (isolated over 75 mph) to downslope off the north end of the
Oquirrhs and Stansburys Friday night or Saturday morning. Also added
Wind Advisories to Utah Valley and the Sanpete Valleys for Saturday.
Have opted for High Wind Watches for the south-central and southeast
valleys at this time, since these winds occur later in the forecast
period. Models are strongly favoring warning criteria winds, and the
ECMWF EFI is also indicating fairly anomalous winds. However, some
of these areas do have a history of winds being over forecast by the
NBM.

* Key Message for Winds: Winds will continue to increase areawide into
  Saturday. Widespread 45-55 mph wind gusts can be expected across
  western Utah tomorrow with isolated gusts to 60 mph. Widespread 50
  -60 mph wind gusts with isolated gusts over 70 mph are expected
  Saturday, mainly from I-80 south. Strong winds will be confined more
  to south-central to southeast Utah Saturday night into Sunday.

Now, on to the snow...

...Snow Through Saturday Morning
* Key Message: Accumulating mountain snow is expected to remain
  limited to the near the Idaho Border in the Bear River Range, with
  highest totals 12-18 inches near Logan Summit and Tony Grove.

A somewhat tricky forecast along the Idaho border through Saturday
morning, owing to some uncertainty in the exact location of a
stalled frontal boundary. Models have trended slightly farther north
with the location of the boundary tonight through Friday, having it
spend more time on the Idaho side of the border. However, a few
ensemble members are insistent on keeping the boundary farther south
over the Bear River Range. The snowband object tracking tool over
all the high resolution CAMs, primarily keeps the band located
across southeastern Idaho through Friday, before increasing
probability that it shifts southward across northern Utah by Friday
night through Saturday morning. Thus largely expect lighter, more
occasional snow tonight through the day on Friday before
increasingly persistent snow later Friday night into Saturday.
Forecast QPF amounts have accordingly decreased with this forecast
update, with has resulted in a decease in snow totals in the Bear
River range through Saturday. However, will have to keep an eye on
these totals should one of the low probability scenarios of the band
staying farther south evolve. Thus, the Winter Storm Warning that is
in place for the Northern Utah Mountains starts overnight tonight,
and have left that in place, however expect the main impacts with
that warning will hold off until later Friday into Saturday.


...Snow Saturday afternoon through Monday Morning
* Key Message 1: Significant statewide mountain snow accumulations
  of 1 to 2 feet with locally higher amounts up to 3 feet possible.
* Key Message 2: Valley locations will see a sharp transition to
  heavy snow Saturday afternoon and evening. Snowfall accumulations
  of 4 to 8 inches are possible, with locally higher totals on
  benches, and into central Utah valleys where the front may briefly
  stall into Sunday morning.
* Key Message 3: There is about a 50% chance of a second period of
  heavier snow across southern Utah (outside of St George) later
  Sunday night into Monday, which depends on the track and magnitude
  of a reinforcing storm system

The main snow event will arrive Saturday afternoon as a sharp cold
front boundary pushes across the state. Dynamically speaking, this
cold front passage really has it all: left jet exit region aloft
along with positive vorticity advection, strong surface
frontogenesis that maximizes over northern into central Utah, and
ample Pacific moisture to work with, all the making of strong
vertical motions to support efficient, widespread precipitation.
This will set the stage for precipitation to go straight to snow
with the cold front Saturday afternoon, progressing from northwest
Utah midday, through the Wasatch Front by mid-afternoon and on into
central Utah Saturday evening where it will stall across
south/central Utah Sunday. The main question for valley snow
accumulations remain the forward speed of the front. Areas across
northern Utah may see the quickest frontal passage speed, before we
see a slowing of the frontal progression into central Utah before it
stalls. Currently the highest probability area of exceeding warning
snow amounts in the valleys (6+ inches of snow) falls into central
Utah around Nephi, and over into the Sanpete Valley, with probs
decreasing north and south of that area. Have opted to start with
Winter Storm Watches for much many of the valley locations to
highlight this potential, and as confidence in this maximum in
snowfall increases these watches can be appropriately converted to
Warnings or Advisories.

Expect lighter snow to linger across much of the state through
Sunday as we remain under the unstable westerly to northwesterly
cyclonic flow aloft with ample moisture still in place. Subtle
shortwaves moving through the mean flow aloft will keep areas of
snow continuing through the day on Sunday. The main forecast
question comes Sunday night through Monday. About 50% of ensemble
solutions advertise a potent trailing vort max that carves out to
our west again and reinvigorates precipitation along the frontal
boundary that is stalled out across southern/central Utah. Should
this scenario pan out, a second period of heavy snow could impact
areas across southern Utah (largely outside of St George). The other
half of ensemble solutions do not carve this secondary trough to our
west, and instead build heights over the forecast area resulting in
a period of drying out later Sunday into Monday.

.LONG TERM (After 12Z/5AM Monday)...A storm system bringing
significant impacts to the forecast area will depart the region at
the beginning of the upcoming work week. That said, there is still
some uncertainty amongst the models in how far south the parent cold
front will progress and how quickly it does so, and how quickly the
trough lifts out of the area. These uncertainties could bring
potentially significant changes to the extended forecast, however, a
majority of ensemble members favor the waning of heavier
precipitation before the crack of day on Monday. There are still
~10% of ensemble members (including the deterministic GFS) that show
potential for heavier precipitation remaining in place across
central and southern Utah through Monday evening but, again, this
only represents a small minority of ensemble members. At the very
least, should continue to see upslope snowfall continue in the
mountainous terrain along the I-15 corridor through the remainder of
the day on Monday, with additional accumulations of up to 2 to 4
inches expected in favored areas (i.e. upper Cottonwoods, Tushar
Mountains, Brian Head area, etc.).

Moving into Tuesday, zonal flow is expected to develop across the
region, bringing only slight stabilization to the environment. While
valley areas west of the I-15 corridor will likely be free of any
more shower activity, mountainous terrain across northern Utah and
adjacent leeward valley areas will see the best chances for
instability driven snow showers. Little in the way of accumulating
snowfall is to be expected with these showers given the limited
dynamic forcing and moisture availability. Temperatures at the
surface will warm under this zonal flow regime, though highs are
still expected to remain below normal through the week (~5-10 below
normal).

Moving through the mid to late week period, focus begins to shift
toward our next potential storm system. Ensemble mean model
forecasts of the mid-level pattern from all 3 main ensemble suites
support a mid-level trough developing over the Great Basin region
through the latter half of next week. That said, when breaking down
into clusters of the 4 most likely solutions we see that there is
some uncertainty revolving around the overall timing and depth of
the trough. The two of these uncertainties will influence the onset
of precipitation, precipitation intensity, and areal extent of the
precipitation. About 60% of ensemble members are placing the axis of
the trough over the central to eastern Great Basin region by
Thursday evening, meaning that odds are favoring Thursday to be the
day where we see precipitation spreading across Utah and southwest
Wyoming. The other 40%, however, keep the trough farther to the
west, potentially delaying the onset of precipitation.

&&

.AVIATION....AVIATION...KSLC...Strong southerly winds with gusts
near 30 knots are forecast through the remainder of the evening. Low
level wind shear is possible after 02z this evening, at around 35
knots at 2,000 ft AGL, through approx 06z. Thereafter, a strong
southerly wind is expected throughout the night and into tomorrow.
VFR conditions are anticipated under high based cloud decks.

.REST OF UTAH AND SOUTHWEST WYOMING...Low level wind shear will
become increasing likely for northern Utah valley locations near and
after sunset tonight. Shear of 40 to 50 knots are anticipated.
Similar conditions are forecast at BCE. Winds headed into the
daytime hours are expected to gust in excess of 30 knots for many
terminals, with the exception of ENV and SGU.

VFR conditions are anticipated with high based cloud cover with mid
level clouds arriving overnight or tomorrow morning.


&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory until 11 PM MST Friday for UTZ101-102.

     High Wind Warning from 11 PM Friday to 5 PM MST Saturday for
     UTZ101-102-105.

     Winter Storm Watch from Saturday morning through Sunday
     afternoon for UTZ103>105-107>109-111>113.

     Wind Advisory from 8 AM to 8 PM MST Saturday for UTZ106-118.

     Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for UTZ106-116-118.

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

     Wind Advisory until 11 AM MST Friday for UTZ115-122.

     High Wind Warning from 11 AM Friday to 11 PM MST Saturday for
     UTZ115-122.

     Wind Advisory until 8 AM MST Saturday for UTZ116.

     High Wind Warning from 8 AM to 8 PM MST Saturday for UTZ116-119.

     Winter Storm Watch from Saturday afternoon through late Sunday
     night for UTZ117-125.

     High Wind Watch from Saturday morning through Saturday evening
     for UTZ120-121.

     High Wind Watch from Saturday morning through Sunday evening for
     UTZ127>131.

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

&&

$$

Cheng/Church/Webber/NDeSmet

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