Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 230340

Area Forecast Discussion
National Weather Service Salt Lake City UT
940 PM MDT Sat Apr 22 2017

.SYNOPSIS...A series of progressively colder and energetic storm
systems will cross the region beginning Sunday, and continuing
through the upcoming work week.


.SHORT TERM (Through 00Z Monday)...The mostly opaque high clouds
across the forecast area overnight will serve to keep lows
elevated heading into Sunday morning.

Increasing southwest flow aloft developing ahead of the first in a
series of weather disturbances will eventually translate into south
winds at the surface for late tonight through Sunday. The
deepening surface trough through central Nevada this evening will
pivot into northwest Utah late tonight, then stretch out
southwest-northeast over northern Utah Sunday. The increasingly
favorable surface gradient created by this surface boundary along
with fairly strong southwest flow aloft will create strong south winds
across west-central and southwest Utah during the day. Could see
the need for wind highlights Sunday, though if extensive opaque
cloud cover hangs on, mixing will be reduced and will keep winds
in check.

The first best threat for precip will arrive by Sunday afternoon
with the arrival of the first in the long series of shortwaves.
This dynamic feature will ripple along the weak low-level
baroclinic zone north of the surface trough across far northern
Utah. Sufficient low-level forcing combined with dynamic forcing
and cooling aloft should bring a gradual increase in precip during
the afternoon hours.

.PREVIOUS SHORT TERM (00z Monday Through 00z Wednesday)...A short
break in the weather will occur late Sunday night into early Monday
as a flat ridge moves overhead. However, the active weather pattern
will bring another short wave across the area with warm advection
precipitation breaking out along the Idaho-Utah border and northern
Mountains mid morning. The shortwave itself along with the nose
of the jet moves into northwest Utah late Monday so have increased
PoPs for northern Utah from about Provo northward. The jet digs
southeast across Utah Monday night which will increase lift and
also spread moisture farther south.

This jet is quite impressive as the associated moisture plume can
be seen on satellite at this time from near the Date Line
stretching back to the northern Philippines. With such a zonal
jet there are always perturbations embedded within which will be
handled differently between models. This makes timing and strength
of systems hard to pinpoint even within 72 hours. Therefore have
basically broad brushed Tuesday with above climo PoPs for the
northern two-thirds of the CWA.

.PREVIOUS LONG TERM (After 00z Wednesday)...Longwave zonal flow
draped across the Western CONUS in the extended period sets the
stage for an unsettled period ahead. The jet noses down from the
Pacific Northwest into Nevada and Utah bringing with it cold
advection and diffluence aloft. A long fetch of moisture remains
upstream and while this setup has some atmospheric river like
features, the placement is a bit off and forecasts the core of the
moisture plume to remain off the Pacific Coast. Plenty of
available moisture will still move inland for the interest of our
forecast area however. The overall pattern through the week
remains unsettled with several waves sweeping through the Great
Basin and Intermountain West. 700mb temperatures on Wednesday
begin to decline from around -4C to -8C by Thursday under a
northwest flow pattern. The jet placement will aid in instability
and almost daily opportunities for showers and even thunderstorms
across Utah. As temperatures aloft cool rapidly and shower
possibilities remain, an opportunity for valley snow exists on
Friday into Saturday, however differences begin to come to light
as the global forecast models diverge.

The latest ECMWF run has trended towards the latest GFS, while the
previous versions of each model had done the opposite. For the week
ahead, both global models are on the same track and differ only
slightly until late Thursday, so this increases confidence a bit.
By Friday, the GFS wants to close off an upper Low nearing the
four corners area while the EC remains under a northwest flow
pattern aloft.







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