NWS Salt Lake City, UT wrote its last area forecast discussion before the first event at Dec 26, 3:36 AM MST, 1 hour 24 minutes earlier.
Area forecast discussion, Dec 26, 3:36 AM MST, 1 hour 24 minutes before
Area Forecast Discussion
National Weather Service Salt Lake City UT
336 AM MST Thu Dec 26 2024
.SYNOPSIS...Additional storms through Monday will bring periods
of heavy snow and wind to the northern mountains thanks to
moisture transport streaming in from the west and northwest.
Valley snow is expected at times Thursday before a warming trend
into the weekend raises snow levels well above valley floors.
&&
.SHORT TERM (Through 12Z/5AM Saturday)...Active period of winter
weather will be well underway during the short term period of the
forecast, starting with the emergence of a band of snowfall that
will migrate eastward across northern and central Utah this
morning through early afternoon. This is associated with a wing of
moist/warm advection along and head of an attendant warm front.
Heaviest snow in lower elevation areas associated with this front
will fall across the Northern Wastach Front and Eastern Box Elder
County, as well as for the extreme northern portion of the Wastach
Back. For valley locations, snowfall will be relatively brief,
falling in a period of 3-5 hours or so.
After a brief lull in the action during the early to mid
afternoon hours, models are locked into an approaching trough and
associated cold front/baroclinic zone reaching the Idaho border
region late this afternoon, before moving southward across the
Wastach Front late this afternoon and evening. Mid level
instability release can be seen on forecast soundings associated
with this feature, with attendant ensemble mean SBCAPE on the
order of 10-50J/kg, along with modest frontogenesis and relatively
steep low to mid level lapse rates. When we stir all of those
ingredients together, we get scattered rain and snow showers (odds
for snow highest as one gets closer to the Idaho border, with all
snow from about Ogden northward). An examination of snow squall
potential indicates a low to medium chance for a couple of snow
squalls associated with this activity, best chances from the
Northern Wastach Front northward as well as along the adjacent
terrain, particularly in the 4PM to 7PM timeframe. Farther south,
forecast soundings support mainly rain across the Salt Lake,
Tooele and Utah Valleys, but could envision moderate to heavy
graupel accompanying strongest cells with rates driving a mix of
rain and snow. In addition, there is a 10-20% chance of lightning
with the strongest cells. This activity has the potential to
impact the evening commute. Drivers should be prepared for rapidly
changing conditions during the evening commute period. Snow
showers will gradually wind down overnight. Highest valley
snowfall accumulations through Friday evening are from the
Northern Wastach Front northward, with 1-2" in valleys, and 1-4"
for benches. Amounts steadily increase farther north, with 3" near
Tremonton and 3-6" towards Snowville, as well as the northern
portions of the Wastach Back.
On Friday, the next wave of precipitation will overspread central
and northern Utah in association with a trough and the eastward
extension of an Eastern Pacific atmospheric river. Given the
maritime origins of this wave, expect rising snow levels with warm
advection through the day, with rain favored in valley locations
from late morning onward. An exception may be across Eastern Box
Elder County, the Cache Valley and especially the Bear River
Valley, where modest cold pools will take more effort to mix out.
Here, a longer period of snow will result. For the Wasatch Front,
only about a 10% chance for snow exists at the onset Friday
morning. For the Northern Wastach Front, there is about a 40%
chance for snow at the onset, primarily during the morning
commute, before warm enough air forces a changeover to rain. With
30-40kts of wind indicated at 700mb Friday afternoon and early
evening, expect surface gusts of similar magnitudes across Uinta
County, WY and favored terrain, with accompanying blowing snow at
times. Friday's snowfall totals are generally in the 6-12 inch
range for the northern mountains, with more to come through the
weekend and into early next week.
.LONG TERM (After 12z Saturday)...Hints in guidance showing a bit
more form regarding differences between previous EC/GFS ensembles
for areal coverage and extent of significant precip across the
northern half of the area this shift. This especially holds true
regarding the Sat-Sun timeframe, the period of best potential IVT
penetration into the central/northern mountains. Trends are favoring
the previous EC ensemble solutions, that now look to hold the
strongest weight.
High confidence for significant SWE in the central/northern
mountains remains, but pulling back totals for areas south of I-80 a
skosh vs previous forecast. The northern Wasatch, and especially
Bear River Range, remain near status quo however. Again, it all
comes down to the low amplitude ridge placement/amplification in
where the greatest breadth of moisture overtopping the ridge will
exist (both in residence time and extent). With a slightly more
interior locale of the ridge, and slightly more amplification now
dominating solutions, have shifted both breadth/residence time of
the moisture plume slightly more north this forecast. In scenarios
like this, 50-100 miles in latitude are significant to outcome.
Regarding the Sat-Sun period in a bigger picture...still looking at
primarily an upper elevation snow impact. High confidence exists for
lower valley rain within a pseudo maritime environment, and
significant shadowing remains present downstream of the crests in
the northern mountain valley areas where snow will fall (low end
snowfall numbers and periodic frequency exists attm). Ongoing
headlines reflect this thinking.
The caboose of the onshore wave train remains poised to cross the
eastern Great Basin region Sunday into early Monday, and though very
progressive in nature still, consensus continues to indicate enough
short wave amplification to displace the ridge further south than
the previous waves. As such, areal extent of precip will once again
shift further south in latitude through the central mountain
regions, but not as far south as the southern mountains. Although
the cold advection component of the wave will be the strongest of
the bunch, consensus maintains the remnant IVT within the warm
sector, with rapid drying as the CAA trends cold enough for much in
the way of tangible valley snowfall. Still something to watch
however.
Bottom line for the long term...significant mountain snow/SWE in the
northern third of the area remains a high confidence outcome (1-3"
SWE). Largely dry conditions in the southern mountains/valleys
remains a high confidence outcome. Northern valley snow impacts
remain tied to Monday morning, but is far from a confident outcome
attm.
&&
.AVIATION...KSLC...Southerly winds in place are expected to prevail
through the daytime hours today. Another unsettled round of weather
is encroaching on the terminal however, with the first wave of light
snow likely yielding MVFR snowfall during the 16-20z window (minimal
accums, not expected on pavement). A secondary wave will bring
another round of MVFR cigs (rain showers most likely, but graupel
possible), and an 80% chance of a WNW wind shift early this evening,
focused within the 00-05z timeframe.
.REST OF UTAH AND SOUTHWEST WYOMING...A somewhat active TAF period
exists moving forward as the first of series of disturbances is
rapidly encroaching the airspace. MVFR/IFR snowfall is expected to
move into the northern/central portions during the 14-20z period,
followed by a more energetic wave returning MVFR/IFR cigs/vis
periodically in the 22-04z window. Gusty and erratic winds are
possible across northern areas with this latter wave, as are a
periods of west-northwest winds in the north vs lighter southerly
winds which are expected to prevail elsewhere.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 8 PM MST this evening for UTZ103.
Winter Storm Warning until 11 PM MST Saturday for UTZ110>113.
Winter Weather Advisory until 11 PM MST Friday for UTZ117.
WY...None.
&&
ADeSmet/Merrill
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