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FXUS65 KSLC 212143

Area Forecast Discussion
National Weather Service Salt Lake City UT
343 PM MDT Wed Mar 21 2018

.SYNOPSIS...A moist southwest flow aloft focused on California
will spill into the Great Basin late in the week. A weak
disturbance embedded in this moist southwest flow will cross Utah
Thursday night through early Friday. A second cold upper level
trough off the Pacific Northwest coast will move east across the
region this weekend. 


.SHORT TERM (Through 00Z Sunday)...Southwesterly flow extends from
the eastern Pacific inland across the Interior West this
afternoon. A subtropical moisture plume/atmospheric river (AR) 
embedded within this flow continues to make landfall and impact 
portions of southern California, with associated moisture aloft 
extending further inland across the Great Basin. This continues to
result in showers across northeast Nevada and northern Utah, and 
this will likely continue into the evening hours. 

The flow aloft is forecast to back and strengthen overnight into
Thursday morning, as an upper low currently off the British
Columbia coastline sags southward along the Pacific Northwest
coast. As this happens, a plume of moisture advection is forecast
to lift northward across the forecast area, which may bring a more
organized band of showers to southern Utah this evening, then 
northern Utah late tonight into Thursday morning.

The flow aloft is expected to strengthen overnight through the day
Thursday, and coupled with a tightening pressure gradient will
result in strong gusty winds across western Utah. Have upgraded to
a High Wind Warning across west central and southwest Utah, and
expanded north with the primary concern being the I-80 corridor. 

The remnants of the landfalling AR is forecast to spread across
the area Thursday afternoon through Thursday evening, and in
conjunction with an ejecting shortwave lifting through the region
will bring the heaviest precipitation to areas favored in
southwesterly flow, with snow levels remaining at or above 9000
feet. Favored locations may see rainfall totals in excess of 2
inches through late Thursday night. With widespread stratiform 
rainfall across southern Utah will likely see most normally dry 
washes, small streams, and slot canyons running, however mainstem 
rivers should remain well within their banks. The same is expected
across the north where a couple of rivers may push bankfull but 
are not expected to flood. 

The moisture plume is expected to shift eastward as a weak cold
front pushes into northern Utah late Thursday night into Friday
morning, dropping the snow levels below 7 kft before precipitation
quickly ends late Friday morning. This should bring the best
chance for accumulating snowfall across most of the higher terrain
of northern and central Utah. 

Southwesterly flow redevelops late Friday into the upcoming 
weekend, with a weak ejecting shortwave bringing another chance 
for showers to northern Utah Saturday. Colder air with this wave
may allow snow to mix in with any rain along the valley floors. 

.LONG TERM (After 00z Sunday)...Weather in the extended will be 
dominated by an expansive closed-low which will very slowly 
progress through the Great Basin early next week. An ejecting wave
ahead of this feature will move over northern Utah Saturday 
evening bringing scattered precipitation to areas favored in 
southwesterly flow. The main event for most of the state currently
appears to be on Sunday, when guidance progs the mid-level low to
begin moving eastward into the Great Basin. The associated 
baroclinic zone pushes through most of the area Sunday 
afternoon/evening with widespread precipitation forming along and 
behind the boundary and snow levels dropping down to valley floors
everywhere outside of Dixie. Frontogenetic forcing with this 
feature currently looks to be maximized along a line extending 
southwest from SLC to just north of Cedar City. However, given 
that this is a closed system the exact orientation of the 
kinematic fields and thus the frontal orientation is more 
uncertain than in a more progressive/open system. Regardless given
a moist postfrontal airmass, continued height falls, and 
generally north/ northwest low-lvl flow expect snow to continue 
into Monday morning over northern Utah. Monday looks to be a cool 
day across most of the area with maxes running 10-15 degrees below

Uncertainty increases considerably into early next week as guidance 
depicts the closed low drifting south and cutting off in the 
vicinity of the forecast area. These systems are typically not 
handled well by guidance but feel confidence is high enough to broad-
brush southern/central Utah with higher than climo PoPs and also 
keep the entire state (but particularly the south) cooler than 


.AVIATION...Southerly winds are expected to continue through the
remainder of the day at the SLC terminal, though winds may become
light and variable for a time between 22-01Z. An area of showers 
is expected to move across the terminal between 22-03Z, likely
bringing lower ceilings at or near 4000-5000ft possibly a period 
of erratic and gusty winds. Much stronger southerly winds are 
expected Thursday morning into Thursday evening.


UT...High Wind Warning from 10 AM to 9 PM MDT Thursday for UTZ003-005-




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