NWS Salt Lake City, UT wrote its last area forecast discussion before the first event at Feb 13, 4:26 AM MST, 8 hours 34 minutes earlier.
Area forecast discussion, Feb 13, 4:26 AM MST, 8 hours 34 minutes before
Area Forecast Discussion
National Weather Service Salt Lake City UT
426 AM MST Mon Feb 13 2023
.SYNOPSIS...A storm system will cross the Desert Southwest through
Monday as Utah and southwest Wyoming stay mostly dry and stable. A
stronger storm system and attendant cold front will then move
through the area late tonight through early Thursday.
&&
.SHORT TERM (Through 12Z/5AM Wednesday)...Early morning
observations indicate a closed low centered near Yuma, AZ while
modest high pressure ridging is in place across northern Utah and
SW Wyoming. Finally, farther north, a low pressure trough with an
axis centered along the Canadian Pacific coastline is observed.
Heading through the period, NWP suite is in good overall agreement
regarding the progression of these large-scale features, with some
meaningful small-scale differences later in the forecast period
that we'll discuss shortly. Heading through today, the southern
closed low is forecast to gradually weaken as it crosses through
Arizona before opening up into a trough over Colorado and New
Mexico tonight. Light snow showers will rotate around the northern
periphery of the low into southern Utah this morning, with
observed precipitation largely confined to high terrain areas
like the Pine Valleys and Boulder Mountain vicinity this morning.
By this afternoon, model soundings do indicate low-level
saturation which will support precipitation reaching the surface
across Lake Powell/Glen Canyon and areas northward towards Goblin
Valley and the Western Canyonlands. This afternoon, snow levels
are forecast to range from 5,000 feet near Hanksville and Goblin
Valley to as high as 6,500 feet across south-central Utah and Lake
Powell/Glen Canyon. Precipitation amounts are forecast to range
from a Trace to 0.10" most areas, with high terrain areas
receiving as much as 0.25" near Canyonlands NP. NWP suite is in
good agreement in depicting this precipitation quickly diminishing
this evening. This break will be short-lived, however.
Meanwhile, northern Utah ridging will quickly give way to the
trough digging across the PacNW today. This trough will continue
to strengthen as it digs into the Great Basin tonight. This trough
is forecast to evolve into a closed low over SW Utah later
tonight before migrating into northern Arizona on Wednesday. As
this process evolves, a cold front will cross the area from
northwest to southeast, reaching extreme NW Utah just before
sunset this evening. The front will then quickly propagate
southeastward, having passed through the Wasatch Front and Dugway
during the evening. The front will eventually decelerate as it
reaches southern Utah on Tuesday. Ultimately, the front is
forecast to complete its journey into Arizona by late
afternoon/early evening Tuesday. Snow showers associated with the
front will swing across northern Utah tonight into Tuesday morning
before reaching the central mountains late tonight. Precipitation
will then quickly expand across southern Utah Tuesday morning,
especially by later in the morning. Going back to northern Utah,
most valleys are forecast to receive anywhere from a trace to 2
inches of snow, with the highest amounts relegated to Wasatch
Front benches. Across northern Utah mountains, favorable
orographics in W-WNW mid level flow will support steadier
snowfall from about midnight tonight through the day on Tuesday.
Presence of modest instability will support isolated to scattered
snow showers across northern Utah through the afternoon. Finally,
a modest trailing wave is reflected as snow showers in modeled
CAM reflectivity fields Tuesday evening, with a trailing area of
snow showers shown drifting southward across much of northern
Utah Tuesday evening. Given the presence of cold temperatures in
the post-frontal airmass, even light amounts of snow will be
enough to result in some slick spots on untreated surfaces.
Across southern Utah, precipitation will blossom along and behind
the front from late morning through afternoon Tuesday. Low to mid
level flow will transition from the WSW ahead of the front to the
NW-N behind it as we head through Tuesday afternoon. At the same
time, NWP suite indicates a period of impressive frontogenesis
across SW Utah. Digging further into the smaller-scale details, CAMs
indicate the presence of an associated narrow band of moderate to
perhaps heavy precipitation developing across extreme southern
Utah and northern Arizona from Tuesday afternoon through the
evening hours. Given the combination of dynamic cooling within
this band and cold advection behind the cold front, snow levels
will fall to valley floors across southern Utah. The exception is
across Lower Washington County where even though temperatures
will become cold enough for snow behind the front, by the time
temperatures get cold enough, precipitation will have largely
ended, leaving a dusting at the most. As far as the mesoscale
banding is concerned, there remain typical differences in terms
of the location of this band, and this will have implications
regarding final snowfall totals. Despite this, have enough
confidence that accumulating snow will fall across Lake
Powell/Glen Canyon that we have expanded the Winter Storm Watch to
include these areas. NBM probabilities of 1" of snow range from
~50% near Page, Arizona to 100% in higher terrain areas north/east
of Lake Powell. The mesoscale band then transitions to an area of
broader deformation precipitation Tuesday evening. As we head
through Tuesday night, a surface low tracks across extreme SE
Utah, with associated deformation precip just to the N and W of
the low track. Otherwise, northerly flow will mean a continuation
of orographically forced snow showers through Tuesday night from
Enterprise to Cedar City northward towards Cove Fort and Salina
Summit while precipitation shuts off across south-central Utah
given this region will reside within an increasingly downslope
dominated regime.
The highest snowfall amounts with this system will occur in
orographically favored areas like the Northern Wasatch and
typically-favored areas within a northerly flow regime across
southern and central Utah from near Enterprise to Cedar City to
Cove Fort and Salina Summit, including Brian Head. Here, 6-12
inches with locally higher amounts looks like a reasonable range.
Greatest potential for amounts to exceed our current forecast will
be across southern and SE Utah where the mesoscale and deformation
banding is forecast to set up. Potential for some areas to
overachieve forecast amounts is a phenomenon that is common in
these setups. How high might snowfall totals get? We examined the
ensemble max to tease out a potential high-end scenario. In doing
so, we find up to 12 inches across the Vermilion Cliffs area
between Kanab and Big Water, with up to 15 inches across Fiftymile
Mountain and nearby higher terrain areas. Have decided not to
upgrade existing winter headlines at this point given uncertainty
in the position of the expected banded precipitation and to allow
additional coordination with transportation partners.
Additionally,widespread blowing/drifting snow will develop
especially in the post frontal regime across SW, southern and SE
Utah courtesy of widespread wind gusts in the 30-45 mph range,
with locally higher amounts expected in wind- prone areas. Gusty
winds will not be confined to just southern Utah, however. NW-N
winds of 30-45 mph are expected to develop across NW portions of
the state from Park Valley to Wendover along/behind the cold front
this evening, with the strongest winds expected from ~8PM-5AM.
There exists a ~75% chance for winds to reach 45 mph in downslope-
favored areas along I-80 near Wendover, but probabilities drop to
25% or less along the remainder of I-80 through the Salt Flats.
Given this, opted not to join our neighbors in issuing a Wind
Advisory for NW Utah, but should future forecasts indicate higher
wind potential, a headline may be needed.
Temperatures are expected to fall 10-15 degrees between today and
Tuesday in the wake of the cold frontal passage, with wind chills
crashing into the single digits and teens across northern and
western valleys by Tuesday morning. By Wednesday morning, wind
chills are forecast to drop into the single digits across the
Wasatch Front and as cold as -20F in Bryce Canyon.
.LONG TERM (After 12Z/5AM Wednesday)...Remnant snow showers are
possible over the Central and Southern Mountains to start the long-
term period as a low pressure system exits to our east. A ridge is
then expected to build into the area through at least Thursday and
Friday, with another storm system on the way moving into the
weekend.
Lingering snow showers will gradually diminish on Wednesday, lasting
as late as Wednesday evening across north-facing slopes. In addition
to precipitation, strong northerly flow will continue through the
day on Wednesday, especially across western valleys, the Book
Cliffs, Uinta Mtns, and higher terrain in southern Utah. Gusts to 40-
50kts are likely across the Book Cliffs and along the lee-side of
the central Uintas on Wednesday morning.
A ridge is then expected to build into the Great Basin, bringing a
warming and drying trend to our forecast area for the end of the
work week and likely into Saturday. This will bring maximum
temperatures to around 5-10 degrees below normal, which is in
contrast to our temperatures on Wednesday near 15 to 25 degrees
below normal.
The next storm system features a trough with northeastern Pacific
origins, that is expected to slide down the west coast and likely
deepen into a closed low. There is still some uncertainty in exactly
how and when this closed low forms, however clusters agree that it
will indeed form a closed low roughly off the SoCal coast.
Uncertainty in this system is two-fold: 1) the track of the closed
low and 2) the track of its parent trough.
Models disagree on how far south this closed low will move, and
whether or not it shifts inland late in the weekend. Because the
anticipated track is already so far south, any further shift south
will greatly limit precipitation chances across southern Utah if it
moves inland. Additionally, this closed low may stall just off the
coast, impacting timing of any potential precipitation. In fact, 44%
of ensemble members suggest the low stalls until at least late
Monday, with 8% even suggesting the formation of a cutoff low away
from the mean flow.
Additional uncertainty lies in the progression of the parent trough,
likely a broad feature that slowly pushes south into the
Intermountain West. This could supply dynamics for precipitation
across northern Utah early next week, if it pushes down west of the
Continental Divide.
In summary, we are likely to remain in an active pattern for the
time being despite a brief break in the action later this week. The
CPC 8-14 Day Outlook continues to favor below-normal temperatures
and above-normal precipitation for our forecast area.
&&
.AVIATION...KSLC...Light southerly flow will likely continue through
17-18z, with periods of variable directions possible. There is a 60%
likelihood of MVFR VIS due to haze through the morning. Otherwise,
VFR conditions will persist until a frontal passage brings light
snow, MVFR conditions, and northerly gusts to 20kts around 06z. IFR
conditions have a 20% likelihood after 06z.
.REST OF UTAH AND SOUTHWEST WYOMING...Fog and LIFR conditions will
likely continue at KLGU through 17-18z, though brief periods of IFR
conditions are possible. Otherwise, VFR conditions are likely to
continue through the day for most locations. A band of light-
moderate snow will likely develop over the San Rafael Swell between
18-00z, push northward. A strong cold front will also start to push
into far northwest Utah by 03z from the northwest, reaching the Salt
Lake Valley by 06z. This will be accompanied by a transition to
gusty northerly flow, light to moderate snow, and MVFR-IFR
conditions. Higher impacts are expected across southern Utah, with
the front reaching KCDC by 12-13z Tuesday.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from late tonight through Wednesday afternoon
for UTZ116-117-122-124-125-128-131.
WY...None.
&&
ADeSmet/Cunningham
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