Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 090441

Area Forecast Discussion
National Weather Service Salt Lake City UT
941 PM MST Mon Mar 8 2021

.SYNOPSIS...A disturbance and associated cold front will bring
rain and snow showers to portions of northern Utah later tonight.
A cooler and more active weather pattern is expected Tuesday
through the end of the week as a mean longwave trough moves


.UPDATE...A mid level short wave currently lifting across
northeastern Nevada has driven the surge of a cold front across
northern Utah this evening, with current location of this front on
the doorstep of KSLC as of the time of this writing. Dry passage
thus far, as dewpoint spreads up to 45 degrees remain in place at
the surface, and deep layer mixing up to H5 (per 00z KSLC RAOB)
will take a bit to saturate. Said, likelihood of precip will
increase as the H7 baroclinic zone which is still lagging over
far northwestern Utah continues a southeastern trajectory during
the latter half of the night...modest frontogenesis expected
along the Wasatch Front developing by 03z. Bulk of Hi-Res guidance
continues to support showery precip namely Salt Lake Valley
north, but minimal amounts at best. About the only CAM not
supporting this remains the HRRR. Snow levels lowering to valley
floors by sunrise...bench dustings at best most areas, but
possibly up to an inch in the Cache due to closer proximity of
the wave passage.

Gusty pre-frontal southerly winds across the western valleys
continue to decrease as a combination of decreasing H7 speeds and
overnight decoupling continue this evening. The earlier Wind
Advisory was allowed to expire at 8pm this evening as such.

Updated earlier to boost PoPs many areas KSLC north, but still
treating this as a somewhat high side PoP/low (very low) QPF
frontal passage.


.PREVIOUS DISCUSSION...Yet another ejecting shortwave out of the
mean longwave trough, currently offshore of the PacNW, will move
across far northern Utah later this evening, bringing a round of
valley rain and mountain snow showers to areas from Salt Lake
County northward along the associated frontal boundary. Greatest
uncertainty with the current QPF forecast is whether or not the
valley areas will actually accumulate much precipitation given how
dry the near surface environment is, but generally thinking that
the more moderate showers will moisten the column enough to
squeeze out some measurable precipitation at the surface. Valley
precipitation will likely start as rain showers, quickly
transitioning to a rain-snow mix as the frontal boundary drapes
over the northern third of the forecast area. For the mountainous
terrain across this similar area, will see better chances of
accumulating snowfall with most focus from the far northern
Wasatch to Bear River Mountains. In large, expecting precipitation
to come to an end by the mid-morning hours as the boundary stalls
and the shortwave continues to move northeast of the CWA.

A brief lull in precipitation tomorrow will be accompanied by much
cooler temperatures across much of the forecast area, with the
exception of lower Washington County and the greater San Rafael
Swell area. By Tuesday afternoon, another ejecting shortwave will
begin to rotate through the area, bringing a round of more
widespread showers with the best chances along the higher terrain
of northern and central Utah. Valley areas are still expected to
see precipitation, likely mixed, as cooler temperatures and
broader forcing exists over the area as the wave moves through.
The H5 trough axis looks to clear the region by around 5AM
Wednesday, allowing for widespread precipitation to come to an
end. However, with lingering instability overhead, could still be
looking at light mountain snow showers continuing over the high

By Wednesday evening, the mean longwave will begin to slide along
the California coast with another wave pushing into the CWA,
increasing coverage of precipitation once again. Best chances of
precipitation are expected to be across the southern mountains
overnight Wednesday into Thursday morning as the wave pushes from
south to north. By this time, the colder airmass will have spread
across the entire forecast area, allowing for widespread valley
snow showers north of Washington County. With broader forcing and
much cooler temperatures, mountainous terrain of central and
southern Utah will likely fair the best during this event, but
will need to dial in the amounts closer to the event.

The closed low is expected to be parked along the California
coast Thursday morning. The various global models are in good
agreement that this system will slide slowly across the Desert
Southwest into Saturday, bringing a prolonged period of cool and
unsettled weather, primarily for central and southern Utah.
Agreement on the track of the system is quite good among the
ensembles, though there are some differences with regard to the
precise strength of the system. Timing of the best precip is a bit
difficult at this time because it will be determined by the
arrival of various embedded shortwaves which can be difficult to
time. Thus, have kept POPs primarily in the scattered range before
tapering off Saturday as the system exits.

High pressure is expected to allow for a temporary break in the
action for the latter part of the upcoming weekend before another
Pacific Northwest storm system begins impacting the area late
Sunday/early Monday. Ensemble spread is on the high side with regard
to the timing, track, and strength of this system so confidence is
low in any details for the end of the forecast period.


.AVIATION...Gusty northwest winds at the KSLC terminal will trend
to decrease rapidly between 05-06z, but will remain from the
northwest through the night tonight. A 40% chance of mixed rain
to snow showers is expected during the 08-12z window, but no
accumulations are expected.





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