Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 261505

Area Forecast Discussion
National Weather Service Salt Lake City UT
905 AM MDT Wed Apr 26 2017

.SYNOPSIS...A series of progressively colder systems will cross
the region through Saturday, with high pressure returning late in
the weekend.


Water vapor loop shows a strong zonal flow across the Pacific with
atop a ridge. AMDAR 400mb-200mb wind observations reveal a
130kt-145kt anticyclonic jet from Oregon into Utah. GOES/HRRR/12Z
KSLC RAOB indicate precipitable water values range from
0.10"-0.20" mountains to 0.35"-0.50" most valleys. Blended
Precipitable Water Product shows an atmospheric river with origins
northwest of Hawaii penetrating the western Great Basin.

Warm advection pattern today with the nose of a strong jet moving
into the region. Strongest warm advection is expected this
afternoon, supporting an increase in precipitation coverage across
the north and some central Utah locations. Snow levels initially
near 6000ft this morning will climb to between 7500ft and 9000ft
by late afternoon. Kept the slight chance of thunder for the
western two thirds of the forecast area per SREF guidance as
stronger dynamics arrive.

Southwest winds become gusty across west central and southwest
Utah this afternoon due to the combination of 5mb-6mb/3hr
pressure falls and 700mb flow near 30kts.

Forecast in great shape. Did lower PoPs a bit across the Salt Lake
valley this afternoon given some shadowing with southwest 700mb
flow in place. Nevertheless the probability of measuring is fairly
high this afternoon. Some Utah county locations as well as Ogden
northward stand a higher chance of precipitation in this warm
advective regime.


The next shortwave trough to impact the forecast area is
currently moving onshore along the Pacific Northwest coast, and
will quickly translate southeast through the northern Great Basin
today, before crossing the forecast area tonight. This feature is
along the nose of a strong Pacific jet, which is associated with
an atmospheric river extending across much of the Pacific. Warm
advection ahead of this wave is currently overspreading northern
Utah resulting in a few showers along the Idaho border. As this
wave approaches today, the flow is expected to back which will
strengthen warm advection, and result in an increase in valley
rain/mountain snow across northern and eventually central Utah.

The cold front associated with this wave is expected to sweep from
west to east across the forecast area this evening, likely
enhancing precipitation for a time, as well as spreading this
precip south into southern Utah, with precipitation becoming more
orographic in nature from late evening into the overnight hours

A relative lull in precipitation is expected Thursday aside from
perhaps some light orographic snow along the higher terrain, with
a colder wave dropping in from the north Thursday night into
Friday. This wave looks to be associated with a continental
airmass and thus considerably drier, but will also drop snow
levels to most valley floors late Thursday night into Friday
morning, with some valley accumulation not out of the question on
elevated/grassy surfaces. A cold northerly flow will linger into
the day Saturday, and with the cold pool aloft still overhead
could certainly see snow showers continue into Saturday before the
trough begins to pull off to the east late in the day.

As the trough continues to exit to the east on Saturday night, a
weak ridge will build over the Great Basin, with clearing skies
and another cold morning Sunday. The ridge flattens and stronger
northwest flow returns through the day Sunday, as another trough
pushes across Idaho and Montana. For Sunday and Monday, at least,
Utah and southwest Wyoming will remain on the southern fringes of
this trough. This should leave most of the forecast area dry,
though some stray showers are possible across the north.

The trough intensifies and digs south late Monday into Tuesday,
though there is still some discrepancy about whether the brunt of
the storm will primarily impact Colorado, or whether it will make
a pit stop in Utah on the way there. For now the forecast is a
compromise between the colder/wetter solutions in the GFS and
Canadian, and the somewhat warmer/drier ECMWF solution.

Either way, showers look to decrease in coverage across the area
Wednesday as the trough moves toward the Four Corners. Looking
ahead to days 8-10 (Thursday-Saturday), models are hinting at what
has become a rare phenomenon: a high amplitude ridge over the
Intermountain West. Don`t get too excited yet, or you might just
scare it away.


VFR conditions are expected at the SLC terminal for the rest of
the morning. Cigs are expected to drop to or below 6000 feet by
18Z as showers develop in the vicinity of the terminal, with
conditions dropping to MVFR at times during the afternoon in
heavier rain showers. Winds are expected to remain southerly
during the afternoon hours, but erratic winds are possible in the
vicinity of showers.





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