Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 242130

Area Forecast Discussion
National Weather Service Salt Lake City UT
330 PM MDT Fri Mar 24 2017

.SYNOPSIS...A Pacific weather disturbance late tonight through
Saturday. Another storm system will impact the area the first half
of next week.


.SHORT TERM (Through 12z Monday)...The ridge axis of a mid level
ridge has continued to slowly shift downstream today allowing a
mild southwesterly flow to translate overhead in advance of the
next land falling upper trough along the Pacific coast. A
relatively deep moisture tap is coincident with this trough
extending southwest off the SoCal coast, and it will remain so as
it continues to translate east into the Great Basin states
for a period overnight.

Upper level cloud cover is expanding rapidly over Nevada
downstream of the Sierra, and will trend to increase locally
through this evening and overnight. Though not overly cold
(trough), airmass differences will set up a decent cold front
tonight over the NV/ID/UT triple point increasing precip over the
extreme northwest tonight. This front does trend somewhat
frontolytic tomorrow morning with southeastward translation
however, this as the upper trough dives southeast and
weakens/splits slightly. Surface reflection is evident, but
weakening in time looking at MSLP trends in guidance. This said,
solid CAA coupled with copious deep layer moisture (PWAT pushing
.6 inches) will drive a period of moderate to heavy precip across
much of western/northern UT from frontal passage (morning hours)
through the mid level axis passage (afternoon hours). Breadth of
cold advection and anticipated PI rates will have the potential to
lower snow levels to or just below 6kft during that time, but
bulk of accumulating snow looks to remain above 7500ft where in
general 2-6 inches of snow will fall in the mountains (locally up
to 8 inches in the Cottonwoods). Regarding valley rain, .25-.50
inches looks like a safe bet across the north with less in
central/southern Utah.

The progressive nature of this trough will carry the mid level
axis east tomorrow afternoon, and although a somewhat moist CAA
NW flow will remain in place going into the overnight hours, flow
orientation will be trending anticyclonic and stabilizing quite
rapidly. Expecting bulk if not all precip to wind down tomorrow
evening as such.

The wave train continues from there, with ridging/warming for
Sunday, a few showers across the north associated with warm
advection Sunday night, and increasingly SW flow aloft ahead of
the next trough approaching Monday morn.

.LONG TERM (After 12Z MONDAY)...The long term period begins with
a trough centered upstream of the area over the Pacific coast. Since
yesterday`s 12Z cycle, guidance has come into much better agreement
regarding the evolution of this feature... essentially keeping it as
an open wave while it moves over Utah late Mon/early Tues before
finally closing it off as it digs south over the four corners. Given
this projected track expect widespread precip to impact much of the
state Mon-Tues... with relatively high fcst. precip totals over
Northern/Central Utah Monday afternoon-evening in association with
an active (although not particularly tight) mid-level baroclinic
zone and adequate moisture (~90-95th percentile PWATS are forecasted
by the GFS ensembles). As this trough and associated front pass
through the state, average snow levels will fall from around 7k feet
Mon afternoon to around 5k feet Tuesday morning. Despite most
guidance keeping snow levels just above N.Utah valley floors for the
duration of precip can`t completely rule out some snow falling on
the northern valley floors Monday night/Tues morning. However, this
period of snow would likely be brief with limited impacts (as the
bulk of the precip should occur when snow levels are over 6k feet).
There is potential for some higher elevation impacts with this
system, however, fairly low snow ratios may limit accumulations even
in locations where most of the precipitation falls as snow.

As the low progresses eastward into New Mexico during the day on
Tuesday, precipitation should taper pretty quickly across the state
from west to east as heights rise and mid level temps warm. This
warming trend and tranquil weather should continue into Thursday,
before another colder Pacific trough is progged to dig/amplify over
the state of Utah. This scenario would result in a fairly potent
frontal system impacting the state and also a rather cold
postfrontal airmass settling over the state late Thurs/early Friday.
For example the GFS forecasts H5 temps around -29C and H7 temps
around -11C in Northern Utah early Friday, which if verified would
be the coldest temps we have seen in over 3 weeks. Such a system
would likely result in snow levels falling to valley floors and at
least a day or two of rather cold (by late March standards)
temperatures. However, being 6 days out (despite relatively good
agreement between guidance at the moment) did not make too many
fcst. edits besides lowering snow levels/decreasing temps slightly
and raising PoPs Thursday into Friday


.AVIATION...Increasing clouds will continue through the overnight
hours at the SLC terminal. Cigs approaching 6000 ft and lowering by
12z Saturday as rain showers develop area wide. Conditions should
transition to MVFR after 14z in the heaviest rain, and improve to
VFR after 20z. Winds should remain southerly through the overnight
hours and may be gusty at times.





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