Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 191659

Area Forecast Discussion
National Weather Service Salt Lake City UT
1059 AM 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
high snow levels and widespread rain.


.Update...Mostly quiet weather across Utah and southwest Wyoming
this morning as low-amplitude ridging builds in over the Great
Basin. Patchy valley fog in areas such as the Cache Valley has
all but dissipated as of this writing.

The focus today turns to the wet and warm pattern setting up
later this week. The NAEFS and ECMWF ensemble both show an
impressive moisture transport scenario extending SW to NE around
the southern end of the Sierra and into our area by
late Wednesday and Thursday. Widespread rain of 0.5-1.5" is
suggested by models, along with snow levels rising to 9-10
thousand feet. Will need to consider various hydro concerns of
rain on melting snow at mid slopes and lower, as well as the
typical trouble spots in southern Utah. Expect to hear more about
that soon as we discuss the possibilities with the River Forecast
Center. For now, the message is wet weather on the way.

Updated sky cover today to better match satellite trends and
latest guidance. Also updated QPF/Snow forecast mid week to pull
in more recent guidance.

.Previous Discussion...issued 418 AM MDT...

.SHORT TERM (Until 00Z Friday)...Heights are beginning to build
across the Great Basin in the wake of the exiting trough pressing
east into the central/southern plains. The broad low amplitude
upper ridge over Utah during the first half of the week will
maintain dry conditions across the forecast area with a warming
trend developing the next couple of days. Mid and high level
moisture moving through the ridge will not offer up any real
chance for precip, but will serve to inhibit warming a bit today
and Tuesday.

The next series of weather disturbances will develop out of the
strengthening upper trough digging south along the BritCol coast
Wednesday. This trough will eventually settle in along the BritCol
through Pacific Northwest coasts and will be centered around a
closed low near 50/130W.

Initially, the atmospheric river (AR) to the south of the
advancing trough will focus across southern/central California,
with some of this moisture breaking off and extending into the
Great Basin Wednesday. Only looking at limited showery precip with
this early moisture as dynamic/thermal support for convection will
be weak at best. The areal coverage/intensity for precip should
increase Thursday as shortwaves ejecting out of the coastal trough
will draw in additional moisture and generate somewhat stronger
dynamic/thermally-driven lift during the day.

Snow levels look to be fairly high on Thursday, so will keep any
accumulating snows generally above 8000 feet. Will also keep an
eye on far southwest Utah where the persistent southwest flow into
the upslope areas could lead to significant rains extending into
the long term period beyond Thursday afternoon.

.LONG TERM (After 00Z Friday)...A shortwave breaking off from the
main Pacific Northwest storm system is expected to slide across the
forecast area Thursday night into Friday. Associated precip in the
very most airmass is initially expected to cover the majority of the
forecast area, but the focus is expected to shift into southern Utah
Friday afternoon as the associated frontal boundary shifts the flow
over northern Utah to a more W/NW direction as the wave exits,
resulting in a drier and more stable airmass there.

The main storm system is then expected to pass across Utah and
southwest Wyoming Saturday into Sunday. The system is now stronger
in the GFS compared to the EC, which shows it grazing northern Utah
as it splits. Despite this, global models all indicate the best
moisture will be confined to northern Utah, though moisture is
somewhat limited with this system, especially compared to the
exceptional moisture with the previous system.

The system and its associated front should, however, cause
noticeable cooling with respect to temperatures with 700mb temps
currently progged to drop into the -12C to -13C range by Sunday.
Have kept POPs primarily in the isolated to widely scattered range
over the weekend as guidance has been overall inconsistent in the
evolution of the Pacific Northwest system and its associated waves.
The GFS indicates high pressure returning by day seven while the EC
keeps things unsettled with some shortwave energy still impacting
the forecast area. Have included some isolated POPs through the end
of the forecast period to account for this.


.AVIATION...Scattered clouds nearby will quickly clear through
this afternoon at the SLC terminal. VFR conditions expected as south
winds shift to the northwest between 18z and 19z, and again back to
the south after 03z this evening.




Van Cleave/Dewey

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