Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 192149

Area Forecast Discussion
National Weather Service Salt Lake City UT
349 PM MDT Mon Mar 19 2018

.SYNOPSIS...After a quiet start to the week, a wet and warm air
mass will move in for the latter half of the work week bringing
windy conditions, high snow levels, and widespread rain.


.SHORT TERM...Infrared satellite imagery this afternoon shows
scattered high clouds drifting through westerly flow aloft over
the Great Basin. A broad trough and associated circulation center
is present well off the California coast over the eastern
Pacific, while the storm which impacted our area this weekend has
pulled east of the Rockies. Visible imagery shows a bit of cumulus
development over northern Utah and southwest Wyoming this
afternoon, while area radars show a few light returns from these
cumulus, mainly around the Cache Valley northward. These light
showers should diminish quickly after sunset.

A low-amplitude ridge will continue to build over the Great Basin
tomorrow. Dewpoints have trended a little lower this afternoon,
so fog potential should be reduced in fog-prone mountain valleys
such as the Ogden Valley (which experienced foggy conditions this
morning). Meanwhile, a warming trend begins tomorrow that should
extend through midweek as the air mass warms (700MB temperatures
are progged to climb above freezing by Wednesday afternoon).

A significant weather pattern change begins for the Western US by
the middle of this week as a longwave trough approaches the West
Coast and (more significantly) a precipitable water tap in the
1.5-1.75" range is pulled up around the southern flank of the
trough and into SoCal. Elevated PWAT will eventually spread
through much of the western US late in the work week as the
longwave trough axis approaches, while models suggest that the
core of the aforementioned moisture tap (easily meeting the
criteria of an "Atmospheric River") could pass around the southern
end of the Sierra and potentially reach southern Utah. Meanwhile,
current guidance has snow levels reaching above 10,000 feet for
southern Utah and 9,000 feet for northern Utah and southwest
Wyoming. This possibility of widespread moderate to potentially
heavy rain up to high elevations could present a variety of hydro
concerns for our area. Stay tuned over the next few days as we
work with the Colorado Basin River Forecast Center to ascertain
these potential impacts.

One other lesser impact will be gusty south winds on Thursday,
especially over southwest Utah.

.LONG TERM (After 00z Friday)...Global models continue to depict an
upper low over the British Columbia coast Thursday evening, with a
mild southwesterly flow aloft over Utah ahead of this feature. In
conjunction with this southwesterly flow, an atmospheric river
containing climatological anomalous moisture is still expected to
spread into the area from the southwest. Latest model runs
suggest that precipitation will peak during the Thursday night
into Friday timeframe, ahead of and with a shortwave disturbance
which is progged to cross the area. There is relatively good
agreement between models at this time; however, timing has been
gradually pushed back run to run. As such, there is still a fairly
high confidence in a widespread and potentially heavy
precipitation event, but have had to bump up PoPs for Friday to
account for the slower timing.

Given the mild airmass, snow levels will remain high for the
entirety of the atmospheric river event. Temperatures at 700mb do
not fall much lower than around -4 to -5C, and even those
temperatures will remain across the north and occur towards the
waning part of the event Friday afternoon. Model QPF suggests storm
totals over an inch over the northern Wasatch Mountains and adjacent
valleys favored in southwesterly flow, as well as in the Pine Valley
Mountains area in southwest Utah. While this is not outrageous given
the abundant moisture expected to spread into the area, will have a
bit better handle on amounts as the event gets closer.

With the shortwave disturbance progged to exit the area Friday
evening, precipitation is expected to wind down. After that, the
aforementioned upper low over British Columbia is then expected to
open up and eject across the northwestern CONUS in several pieces.
This would push a couple of cold fronts into the area over the
weekend, with a potential for snow across northern and perhaps
portions of central Utah.


.AVIATION...Scattered clouds across the north will not impact
the KSLC terminal this evening. Ceilings expected to remain above
7000 feet, with clouds tied to higher terrain north and east of the
terminal. Northwest winds expected to shift to the south after 03z
this evening.




Van Cleave/Cheng/Dewey

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