St. George, Utah 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

231
FXUS65 KSLC 212311
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
411 PM MST Sat Dec 21 2024

.SYNOPSIS...High pressure will give way to a series of
disturbances that will begin to impact the area later Sunday into
Monday, continuing through the week and into next weekend.

&&

.SHORT TERM...(Through 00Z Thursday/5PM Wednesday)...Warm,
southwesterly flow will give way to cooler temperatures gradually
filtering in from the west overnight tonight, marking the very
beginning of a more active pattern. This afternoon and evening,
scattered to broken mid- to high-level clouds will continue to
overspread much of the forecast area, likely attempting to limit
fog development overnight and acting to slightly weaken valley
inversions. Valleys are more likely to mix out gradually late
Sunday into Monday as a couple of weak waves move across northern
Utah. These waves, taking advantage of increasing moisture, will
produce periods of light snow across the northern mountains
through Monday evening. Due to lack of forcing, accumulations will
be relatively minor, around 2-6 inches. Valley precipitation type
will be rain due to antecedent warmth and the mild nature of this
system.

Tuesday will be mainly dry as high pressure briefly resides
across the area, though western valleys could be a bit breezy with
increasing southwesterly flow ahead of the next storm system.
Gusts could briefly approach advisory criteria in a few isolated
areas in southwestern Utah (25% chance), though most western
valleys are expected to see gusts around 25-40 mph.

Precipitation will then begin late Tuesday as an upper-level
trough moves into the southern Great Basin. Model guidance has
honed in on a more southerly track, with the base of the trough
moving through northern Arizona/southern Utah. As this trough
begins to close off as it reaches out area, this means the best
dynamics will be located further south, thus northern Utah QPF has
decreased compared to what we saw a few days ago. Snow totals are
still looking to be sub-advisory, largely around 2-6 inches across
Utah`s mountains, with the highest amounts in northwest-flow
favored areas such as the Upper Cottonwoods and Tushar Mountains.
In these areas, there is still a low (25%) chance of seeing
upwards of a foot if forcing comes together in the right way. Valleys
will receive mainly rain, though the big question has been if
there`s a chance of a white Christmas. Higher elevation valleys
will likely transition to snow by Wednesday morning, though
accumulations will remain minor, around an inch above 6000 ft.
Lower elevation valleys will be slower to transition, with a much
lower chance of seeing an inch of snow...still around 10 percent
to quantify it. Precipitation will gradually taper off Wednesday
afternoon and evening.

.LONG TERM (After 00Z Thursday/5PM Wednesday)...Synoptically, we
will remain in a period of active westerly flow from Christmas Day
through approximately the end of the year, though there are details
to be ironed out in terms of amplification for any one of the
troughs embedded in the westerly flow. As a whole temperatures will
remain largely above average through the period, so precipitation
type will continue to be a question mark for the Utah valleys west
of I-15.

After Christmas, focus quickly turns to the next weather disturbance
on the heels of the Christmas trough. Ensembles show a variety of
amplitude differences, which vary from a grazing trough impacting
only far northern Utah and southwest Wyoming to an area-wide
precipitation event with an atmospheric river component in the north
sneaking around the northern end of the Sierra. Terrain favored in
westerly flow from around Salt Lake County northward stands the best
chance of appreciable precipitation in either scenario, with a floor
(10th percentile 48-hour total NBM) of a few tenths of liquid and a
ceiling (90th percentile 48-hour total NBM) of 1.5". Similarly, the
current 10th-90th spread of 700MB temps is -10C to -4C, highlighting
that valley precipitation type remains a question mark. In summary,
those with travel plans over mountain passes in the day or two
immediately after Christmas should be prepared for winter driving
conditions, and those hoping for a white Boxing Day on the Wasatch
Front have at least some chance, though still marginal. Finally, far
southern Utah from St George to Lake Powell has the least chance of
precipitation, currently in only the 10-30% range.

Additional ripples in the westerly flow look to bring continued
mostly cloudy conditions and additional mountain snow / valley rain-
snow into the post-Christmas weekend, with one such impulse
currently shown in deterministic models late Friday into Saturday
and another towards the end of the month. The general synoptic
regime (low-amplitude ridging over Northern California) could allow
for inland-penetration of Pacific moisture around the northern end of
the Sierra Nevada, though it also would likely bring mild
temperatures for this time of year. Thus, the same themes remain:
precipitation type outside of the mountains remains a question, and
precipitation chances are generally higher farther north and in the
mountains, with southern Utah being on the fringe of any chances.

Looking way far out in time to the beginning of the new year, a
majority of ensembles lean back towards drier conditions (roughly
60%), and cluster analysis indeed shows that ridging is the slight
preference (with the remaining membership showing a continuation of
westerly flow into the Intermountain West).

&&

.AVIATION...KSLC...HZ with VIS teetering between VFR/MVFR will
continue to prevail into the evening. Overnight there remains
roughly a 25% chance for denser FG to develop near the Great Salt
Lake and yield LIFR conds at the terminal. On Sunday, CIGS lower to
likely result in mountain obscuration, with a gradual increase in
rain shower chances through the day. Rain chances further increase
by Sunday evening. Winds expected to stay light (less than 10 kts)
with diurnally normal to at times variable direction.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conds largely persist at
the majority of area terminals, though inverted sheltered valley
terminals (such as LGU) will see a ~25% chance of denser FG
development and LIFR conds. Ahead of a weak system, CIGS will
gradually lower, resulting in likely mountain obscuration at many
mountain adjacent terminals. At northern terminals, scattered precip
chances gradually increase through the day Sunday, maximizing by
late Sunday evening into the overnight hours. Broadly, snow levels
throughout most of Sunday will be in the 6 to 7.5 kft MSL range,
with precip type and severity of any potential categorical
reductions impacted accordingly. Winds generally to remain light
with diurnally normal to at times variable directions.


&&

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

$$

Cunningham/Van Cleave/Warthen

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