725
FXUS65 KSLC 231058
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
358 AM MST Mon Dec 23 2024

.SYNOPSIS...A series of weak weather disturbances will cross the
area through Monday, with brief high pressure for the first half
of Tuesday. Potentially stronger systems will cross the region
over Christmas, as well as late in the upcoming week.

&&

.SHORT TERM (Through 00Z Thursday/5PM Wednesday)...Early Monday
morning, low stratus clouds were in place across southwest Wyoming
and northern Utah, particularly near the Idaho border, with low
clouds extending southward along the terrain into central Utah.
Stubborn valley inversions were still largely in place across
northern Utah valleys, with attendant haze remaining. Model
forecast soundings continue to indicate some semblance of mixing
this afternoon, but confidence has decreased as to whether or not
sufficient mixing will materialize to realize ventilation of
haze. With a ridge of high pressure moving across the area for the
first half of Tuesday, it is plausible that valley inversions and
haze may linger into the first half of Tuesday, with high
confidence that these will mix out with the arrival of our
Christmas system and associated wind later on Tuesday.

Mid clouds could be seen streaming into the area from the west
early Monday morning, associated with a weak wave of low pressure
and modest mid-level theta-e advection. Model consensus tracks
this weak wave eastward across Oregon this morning and southern
Idaho this afternoon, with the southern extent of the wave
brushing across extreme northern Utah near the Idaho border. As a
result, light precipitation (in the form of valley rain and
mountain snow) will be confined to the northern Utah mountains,
and northern Utah valleys, primarily north of I-80. Ensemble mean
QPF is less than equal to 0.10" in valleys, and highest across the
Bear River Range, with amounts around 0.50" near the Idaho
border. Roughly double these amounts for the 90th percentile
values, although given both weak forcing and modest moisture with
this wave, think these values will be quite difficult to attain.

Regarding the much-discussed Christmas system, model consensus
has diverged significantly over the last 24 hours. Latest
consensus digs a trough into the Western Great Basin during the
day on Tuesday, in line with previous model runs, but this is
where the consistency with respect to previous days model runs
comes to an abrupt end. As we head through Tuesday night, latest
guidance closes off a low near Las Vegas, NV, and tracks the low
eastward near the UT/AZ border through Wednesday morning and early
afternoon. This results in two large changes to the forecast: 1)
A reduction in both valley and mountain snow and precipitation
amounts across northern Utah, with a period of light easterly flow
late Tuesday night/early Wednesday morning 2) An increase in
precipitation amounts across southern Utah. Mean QPF has dropped
across the northern and central mountains to 0.50" or less, and
increased across the southern mountains to around 0.50" for the
Brian Head vicinity. For a point in the Upper Cottonwoods, for
example, most ensemble members constrain QPF in the 0.25"-0.50"
range, or about half of what we were seeing 24 hours ago. Valley
snow potential has dropped to a T-1", with a T-2" on benches.
Still envision a period of favorable moist, upslope flow in a cold
advection regime Wednesday afternoon, from the Wasatch south to
the Tushars. Certainly, best dynamics and large-scale lift has
shifted into southern Utah with this latest run of models, a
fairly dramatic shift from 24 hours ago. Despite this latest
shift, those with travel plans, especially over mountain passes,
should be prepared for winter driving conditions late Tuesday
through Wednesday.

.LONG TERM (After 00z Thursday)...An increasingly progressive
onshore pattern will occur post Christmas on through the long term
period anchored by a low amplitude ridge over the southeastern
Pacific, and a deep long wave trough in the Gulf of AK. Although our
local area will reside on the more southern fringe of of the mean
storm track, and a little too far interior for deeper atmos river
moisture, still looking a net positive and near daily precip
potential in the north (periodic in nature), but southern portions of
the area will still remain a bit too far south in latitude for
tangible precip, which is needed. I-70 is a fair demarcation for
these differences in latitude attm.

Embedded within the progressive onshore flow are two notable short
waves, Thursday afternoon and again Friday. A mini wave train over
that period with each tied to at least some remnant atmos river
moisture rounding the northern Sierras prior to advecting interior
across the northern Great Basin/Rockies regions. With a further
southerly track of the Christmas storm (as latest guidance
portrays), these subsequent waves are exhibiting a slightly more
positive translation across northern Utah, and blended qpf for this
period has shown a net uptick in latest runs. More maritime than
not, primarily looking at snow impacts to occur above valley floors
and focused in local passes/canyons, and mountain valleys east of
the I-15 corridor, though a mix or brief changeover in the lower
valleys remains possible within the cold sector of each wave. Still
spread in these details that will be hammered out in time as we get
closer.

Thereafter the progressive onshore flow will be retained across the
northwestern conus, but majority of solutions shift the mean storm
track a skosh more north due to subtle easterly shift of the ridge
axis, and slight amplification locally. Subtleties that have a large
impact on significance of subsequent short waves later in the
weekend into early next week. A variety of outcomes remain, but a
few takeaways favor the UT/ID border region for additional precip
(on a tangible level), and warmer than normal conditions limiting
valley snow impacts. Spread in guidance has been anomalously high
regarding details, even relatively close in time of late, so
certainly not hanging a hat on anything more than northern Utah
remaining on the southern fringe of active La Nina`ish storm track
as we bring on the new year.

&&

.AVIATION...KSLC...Weak wave passage this morning will bring a small
potential for light rain showers, but high confidence mountain
obscuration conditions within the 14-22z window. Southerly winds
holding through a near normal northwesterly shift within the 18-20z
timeframe with normal diurnal trends continuing thereafter. VFR
cig/vis conditions expected with this wave passage, with thinning
clouds late day.

.REST OF UTAH AND SOUTHWEST WYOMING...Another weak wave will bring
light precip to the northern terminals focused within the 14-22z
timeframe today, generally north of the Salt Lake area. Mild
conditions in place will retain snowlevel impacts and higher vis/cig
reductions elevations above 6500ft (IFR conditions), though
widespread mountain obscuration can be expected. Most northern
valleys still sitting 30-40% chance for brief MVFR conditions,
otherwise VFR conditions are favored as the norm. Light
winds (largely <10 kts), with diurnal directional changes are
expected.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

ADeSmet/Merrill

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