Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 292229

Area Forecast Discussion
National Weather Service Salt Lake City UT
429 PM MDT Wed Mar 29 2023

.SYNOPSIS...An area of moderate to heavy precipitation will spread
through the area overnight, bringing mountain snow and mixed
valley precipitation across Utah. Thursday will feature colder
temperatures and potential for moderate to heavy valley snowfall
for portions of the Wasatch Front. Another area of snowfall
develops Thursday night through Friday morning before a brief
break in conditions heading into the weekend.


.SHORT TERM (Through 12Z/ 6AM Saturday)...A deep offshore low off
the central California coastline continues to pump moisture into
the eastern Great Basin region this afternoon while maintaining
strong southwesterly flow over the forecast area. Afternoon wind
gusts across the western half of Utah have been observed anywhere
from 45 to as high as 60 mph. The trough associated with the
offshore low will push inland overnight, allowing the associated
low level baroclinic zone to push into Utah and the wind maxima
ahead of the weakly defined boundary to shift eastward. Most of
the strong winds associated with afternoon mixing should subside
shortly after sundown, however, localized areas near terrain will
likely still see stronger wind gusts due to mountain wave
breaking. Additional strong winds are anticipated as the area of
moderate to heavy precipitation focused along the low level
baroclinic zone, but should be short lived.

Touching more upon the precipitation aspect of the forecast, the
aforementioned low level baroclinic zone will really serve as an
area where precipitation will be focused as it pushes through the
region. That said, most of the support for moderate to heavy
precipitation will be found in the upper levels as a jet maxima
shifts overhead. The low level boundary will really serve to kick
off lower level lift whereas the upper support will help maintain
more organized precipitation and intensify upper divergence. As
this low level boundary collides with the terrain through the
center of Utah, thinking that we`ll see some of the highest
precipitation intensity. Areally, most of the heavy precipitation
will be confined to the higher terrain, but areas directly
adjacent will see periods of moderate to heavy rates. The upper
support for this band of precipitation runs eastward by the mid-
morning hours, allowing precipitation to taper off... at least
temporarily. As of 4PM Wednesday, radar data is already showing
areas of showers over the West Desert of Utah.. coincident with
where winds are already shifting to a westerly direction (i.e. the
surface front). This area of showers and the coincident surface
boundary will continue to push eastward this evening, reaching the
I-15 corridor by 8-9PM, intensifying over the mountainous terrain
around 10PM and persisting through around 2-3AM Thursday for the
northern mountains and 5AM for the central and southern mountains.
Snow levels are likely to start at around 7000ft MSL during the
onset of precipitation, dropping to most valley floors by the
morning horas. There is certainly potential for accumulating
snowfall in the morning for the northern Wasatch Front, however,
snow will have a difficult time sticking to warm paved surfaces.
The most likely higher population area that will see more than 2
inches of snowfall by the morning will be the Park City area. Up
to 4 inches can be expected by 5AM, but minimal impact should be
brought to the morning commute. All the above is part 1 of this
storm system.

Part 2 of the storm begins tomorrow. As mentioned, a brief break
in heavier precipitation is expected early Thursday morning as the
upper support shifts eastward. A secondary frontal boundary, aided
by a trailing mid-level disturbance, will push into the region
during the afternoon and overnight hours Thursday into Friday and
kick off another round of precipitation for the northern half of
Utah and southwest Wyoming. With colder temperatures already in
place, snow will be the main precipitation type. The secondary
frontal boundary will help focus another area of snowfall across
mainly the northern Wasatch Front, however, hi-res guidance does
show some discrepancy on exactly where this band sets up and how
long it persists. The timing of this snowfall really looks to
start up during the late morning to early afternoon hours... so
we`ll be combating solar insolation on roadways. With that in
mind, not thinking we`ll see significant impacts in the valleys,
but that will change as the sun sets.

The aforementioned trailing mid-level disturbance will begin to
move overhead during the overnight hours into Friday, bringing an
increase in instability and mid-level forcing for precipitation.
Given the colder surface temperatures and loss of solar
insolation, thinking that this period will bring the highest
potential for accumulating road snow. Cold northwesterly flow will
remain in place through Friday morning, which, when combined with
disturbed mid-level flow and a warm Salt Lake, could help kick off
a period of lake effect (or at least lake enhanced) snowfall over
Salt Lake, Davis, and/ or Weber Counties. Hi-res guidance is
hinting at this potential, but we`ve only got a few models
reaching this lead time. Bottom line, Thursday afternoon through
Friday looks to be the most impactful period with this storm

By Friday evening, weak subsidence will begin building into the
region and allow precipitation to taper off through the evening
and overnight hours. This area of subsidence will be relatively
short-lived before another storm system arrives at our doorstep
late in the weekend.

.LONG TERM (After 12Z/ 6AM Saturday)...Saturday looks to offer a
quick break from the active weather with limited precipitation
chances and temps around 5-10 degrees below climatological normal,
but pattern quickly becomes unsettled once more. On Sunday, a potent
shortwave digging southward from the PacNW will help result in
precipitation chances increasing throughout the northern half of the
forecast region, though temps change only minimally. Into Monday,
the shortwave trough digs further into the Great Basin, resulting in
increasing upper level support in addition to forcing a strong
baroclinic zone / cold front through the forecast region.
Precipitation becomes widespread as these features move through.
Ahead of the baroclinic zone, snow levels remain high enough to
yield rain in most valley locations, with the exception being some
of the far northern Utah valleys in addition to southwest Wyoming.
Following the passage of the baroclinic zone through the day Monday,
700mb temperatures quickly fall to around the -12C to -16C range,
and in turn snow levels lower enough to support a transition to snow
(aside from far southern Utah low elevation valleys such as the St.
George area).

Precipitation chances begin to decrease behind the front overnight
into Tuesday, but remain non-zero as the base of the trough begins
to shift overhead and keep the pattern unsettled. In addition, cold
air associated with the trough will knock temperatures back down to
around 10 to 25 degrees below climatological normal following the
front. From later Tuesday onward through the remainder of the
forecast, confidence decreases as deterministic and ensemble
guidance diverge on the evolution/migration of the trough. While
around 60% of members keep some amount of broader scale troughing
over the area, around 20% keep a deeper lingering trough
(colder/wetter), and around 20% slip the trough further east
(somewhat warmer/drier). For those hoping for a more substantial
warmup in the extended time frame, unfortunately the 8-14 day CPC
temperature outlook highlights the region with around a 55-70%
chance of continued below normal temperatures.


.AVIATION...KSLC...Gusty southerly winds up to around 30-35kts will
continue through the afternoon, likely decreasing with a westerly
wind shift around 03-04z with a frontal passage. Around this time,
expect rain showers and MVFR VIS for 2-3 hours along with a 20%
chance of thunder. Another round of showers is expected around 15-
19z, with some uncertainty in exact timing. This round is likely to
be stronger and colder, bringing snow showers or wintry mix, reduced
VIS, and a brief northwesterly wind shift. There is a 60% likelihood
of IFR conditions and 20% of LIFR conditions with the second round.

.REST OF UTAH AND SOUTHWEST WYOMING...Gusty southerly winds will
continue through the afternoon, with the highest gusts across
western valleys (including Tooele Valley, KCDC, KMLF). Winds will
likely decrease and shift more westerly/northwesterly behind a
frontal passage late this evening. This front will bring a band of
moderate-heavy precipitation, mainly rain in lower valleys. The
front will stretch roughly along the UT-NV border around 23-00z, and
will reach the Wasatch Front around 03-05z, KBCE around 05-06z, and
KEVW around 07-08z. Expect MVFR to IFR conditions with this feature
and a 15-20% chance of thunder. Another band will move across the
area mid-day Thursday, which will be stronger and colder than the
first, with higher chances for IFR to LIFR conditions and valley
snow showers.


UT...Wind Advisory until 9 PM MDT this evening for UTZ101-102-115-122.

     Winter Weather Advisory from 9 PM this evening to 9 PM MDT
     Friday for UTZ110>112.

     Winter Weather Advisory from 9 PM this evening to noon MDT
     Friday for UTZ117.

     Winter Weather Advisory from 9 PM this evening to 9 PM MDT
     Thursday for UTZ125.



Webber / Warthen / Cunningham

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