Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 212230

Area Forecast Discussion
National Weather Service Salt Lake City UT
430 PM MDT Tue Mar 21 2017

.SYNOPSIS...A series of weak weather disturbances will impact
Utah through Wednesday. A stronger and significantly colder Pacific
storm system will move east across the Great Basin Wednesday
night through Thursday, followed by a return of high pressure for
the end of the week.


.SHORT TERM (Until 00z Saturday)...Water vapor imagery places the
cold core upper trough along 130W late this afternoon. This
trough will split as it reaches the west coast late tonight, with
the energetic southern portion of the split tracking east into the
western Great Basin Wednesday night, then continuing east through
Utah on Thursday.

Ahead of this trough weak shortwaves ejecting northeast through
the Great Basin have struggled to generate much more than clouds
and weak convection today. This trend will likely continue through
this evening before a near 700mb baroclinic zone settles into far
northwest Utah late tonight. expecting this boundary to turn
convectively active overnight as convergence into this boundary
increases with a few degrees of mid-level cooling and the left
exit region of the upper jet on top of the boundary working to
creating increasingly strong and deep lift. Moisture still remains
a limiting factor, so any convection that does develop is not
expected to generate much in the way of measurable precip.

Farther to the south still can not rule out isolated convection
from the favorably positioned jet and the potential weak dynamic
support from any shortwave ejecting out of the main trough. Again,
moisture will be quite limited, so any showers/storms that develop
likely to produce more winds than rain overnight.

Convection will likely stay focused along the baroclinic zone over
extreme northwest Utah through the day Wednesday. For the
remainder of the state convection will gradually build up across
the southwest as the southern portion of the split upper trough
advances into the Great Basin late in the day. Anticipating a
fairly rapid expansion of precip beginning Wednesday evening as
the core of the upper level trough closes in on southern Utah
Wednesday night. Strong dynamic lift and deep layer thermal
advection will develop fairly strong synoptic-scale lift across
southern/central Utah late Wednesday night/early Thursday. Add to
this the injection of fairly substantial moisture tapped by the
upper trough with the potential for heavy precip, especially
across the southern mountains. Snow levels will also be dropping
as the cold air at the core of the trough pours in across southern

Up north the onset of heavier precip will lag a bit, but should
start to ramp up by late Wednesday night or early Thursday. A
near 700mb circulation is expected to develop across northeast
Utah late Wednesday night, then drift into southwest Wyoming early
Thursday. The corresponding deepening of the surface low across
southwest Wyoming appears to be overdone in the GFS, though the
idea of some surface cyclogenesis appears reasonable. Increasing
synoptic-scale lift indicated by the deepening of the upper low
along with the arrival of low-level cold advection and a
significantly improved moisture profile all suggest widespread and
occasionally heavy precip across northern Utah/southwest Wyoming
into early Thursday evening. Heavy snow is a good possibility as
the mean layer flow will be favorable to engage orographic
influences, especially along the southern Wasatch range. Periods
of snow or rain/snow mixed could develop along the benches and
higher northern valleys in the more intense convective showers.

Showers will linger into early Thursday evening, then end quickly
overnight as dynamic subsidence takes hold with the arrival of the
upper ridge axis trailing the exiting trough. Temps will bounce
back a bit Friday from the cold (actually near seasonal) temps on
Thursday. A few showers could develop over the Uintas in the
lingering moisture early Friday, but should dissipate by afternoon
as the drier and stable air mass with the ridge prevails.

.LONG TERM (After 00Z Saturday)...The shortwave ridge over the CWA
on Friday quickly shifts eastward on Friday night, as the next
Pacific trough pushes across the western CONUS. Friday night should
be decently warm, as southerly flow and cloud cover increases ahead
of the storm.

The 12Z runs of the GFS/ECMWF/Canadian came into better agreement
with this trough and sped the system up a bit, with the best
precipitation across most of Utah during Saturday afternoon. As
such, have increased POPs and decreased temperatures for Saturday.
It looks like a few showers will likely linger into Saturday
evening, but should largely dissipate after midnight.

The progressive pattern continues past Saturday, as the storm exits
quickly and a ridge builds again for Sunday. The ridge currently
looks fairly flat, so warming for Sunday will likely be modest.
Forecast details, particularly pertaining to overall pattern
amplitude, begin to break down past Sunday, but in general models
indicate another storm system swinging through the Great Basin
Sunday night and Monday. Have kept POPs relatively high through this
period, with temperatures near seasonal normals, though colder
temperatures are possible Monday/Tuesday if the more amplified and
colder solutions verify.

Obviously confidence is even lower in the details beyond Monday, but
the pattern currently looks to remain fairly active through much of
next week, so expect this cooler and wetter weather to continue
through the rest of the month of March.


.AVIATION...Gusty southerly winds at KSLC will remain in place
through this evening, with periodic gusts in excess of 30mph
expected through 01-03z. Cloud cover will thicken and gradually
overnight bringing a 40 percent chance of showers to the terminal
after 06z. Cigs are expected to remain at or above 7kft however.





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