Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 141214

Area Forecast Discussion
National Weather Service Salt Lake City UT
514 AM MST Wed Feb 14 2018

.SYNOPSIS...The upper level low pressure system off the California
coast will turn back to the east and weaken as it moves through
Arizona and southern Utah this evening. The next trough will move
into Utah from the Pacific Northwest late tonight into Thursday.


.LONG TERM (THROUGH 12Z SATURDAY)...The large-scale pattern this
morning depicts the upper low which was responsible for our most
recent storm now residing off the southern California coast while
the next upper trough approaches the Pacific Northwest coast.
Downstream of these storms, Utah remains under a southwesterly flow
aloft. Low-level moisture remains in place with a stratus deck in
place across a large portion of the area. A few weak showers
continue this morning, mainly over and in the vicinity of the Pine
Valley Mountains.

As the PacNW trough makes its way inland, the upper low will
gradually weaken through the day and become absorbed into the
southwesterlies ahead of the digging trough this evening. This
weakening low will pull some of the moisture present across SoCal
and northern Arizona into Utah, and as a result, expect
precipitation to begin increasing this afternoon, especially near
southwest-facing slopes across southern and central Utah. However,
given the warming airmass and decent mixing owing to the increasing
southwesterly flow, snow levels are expected to remain above most
valley floors today.

The trough will push a cold front into Utah overnight and as a
result, precipitation will continue to increase. Snow levels will
fall, so expect some snow down to the valley floors. However, most
valley precipitation should remain brief and rather light, with the
best accumulations along the I-15 corridor. A moist and
destabilizing northwesterly flow aloft will then continue to provide
orographic enhancement to northwest-facing slopes tomorrow morning
through the early afternoon hours. The storm will then gradually
wind down during the late afternoon through early evening hours as
the airmass gradually dries out and large-scale subsidence develops.
With a colder airmass settling into the area behind the front (H7
temperatures -14C across northern Utah tomorrow), temperatures will
trend closer to or slightly below normal tomorrow.

Warm advection will develop late tomorrow night into Friday
morning. However, temperatures are not expected to warm much if at
all, especially given the cold start to the day. A weak, fast-moving
trough will graze northern Utah Friday afternoon/evening which could
bring some light snow to the far north.

.LONG TERM (After 12Z Saturday)...Some changes to the extended
models as of 00z runs, and in better agreement with each other (the
models) is a good sign. The latest (00z) GFS has trended toward the
ECMWF with a digging broad trough feature late in the weekend into
the Great Basin. As of the writing of this, the 06Z run of the GFS
continues to remain persistent in solution with respect to the EC,
so my confidence is growing with the late weekend storm system.

A tightening baroclinc zone is forecast to drop southeast into
northern Utah by late Sunday, with similar temperatures among models
cooling to near -14C by early Monday. Models suggest moisture
associated with this system being advected from the north, so not a
significant fetch but enough to keep things interesting with the
instability and northwest flow. Lapse rates from 700 to 500 of 15-
18C seem reasonable for instability development. Differences among
the models are still very present, so did not weight heavily on
either one but more of a blend since one model is more widespread
and earlier with the precipitation while the other is more reserved
and slower. As the trough swings east by Wednesday a few shortwave
troughs are forecast to move through the region, although timing and
details are now well out of sync among the models. Kept the going
forecast as a starter with some minor changes to the PoP forecast.


.AVIATION...Light and variable winds at the SLC terminal will
become predominantly southerly by 14z, and become gusty after 16z. A
northwest shift is anticipated between 23z to 01z with nearby
showers lowering cigs into MVFR category by 06z.




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