Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 302213

Area Forecast Discussion
National Weather Service Salt Lake City UT
413 PM MDT Sun Apr 30 2017

.SYNOPSIS...A northwest flow aloft across the Great Basin will
usher in a series of weak weather disturbances tonight through
the early portion of the week. Strong high pressure aloft
will finally reach the area for the latter half of the week.


.SHORT TERM (THROUGH 00Z THURSDAY)...A northwesterly flow aloft
remains in place across the region this afternoon. Anticyclonic
curvature will turn more cyclonic as a weak upstream trough
approaches the area late this evening. In the meantime, clouds
continue to stream into northern Utah ahead of this trough with a
few weak showers across far northwest Utah. These showers will
increase in coverage across northern and central tonight and into
tomorrow as a shallow cold front crosses the area. However, these
showers will likely remain on the light side given weak mid and
upper level forcing.

The shortwave trough will exit the area late tomorrow afternoon
allowing for a brief break in the weather. However, a second trough
is then expected to cross the area late tomorrow night through
Tuesday. This trough looks to be slightly stronger than the first,
as the GFS shows some jet support across far northern Utah and
southwest Wyoming which would be conducive to producing some showers
across the north once again. However, the 700mb thermal gradient
looks to remain relatively weak, so these showers are not expected
to produce significant amounts of precipitation. It is also noted
that the jet in the EC is a bit weaker and remains a bit farther
north than the GFS, so the EC solution is less enthusiastic
regarding precipitation.

A high-amplitude ridge will build into the western CONUS beginning
Wednesday. This will usher in a period of drier and warmer
conditions to the area.

.LONG TERM (After 00Z Thursday)...The 700mb ridge axis is directly
overhead by 00Z Friday with the ridge axis at 500 and 300mb slightly
offset to the west, indicating warm advection continuing into the
CWA Thursday. Have boosted temps into the mid 70s Wednesday for the
Wasatch Front. Temperatures continue to warm Friday as the ridge
axis shifts eastward and the southwest flow aloft increases.
However, since the surface low is expected to be over northeast
Nevada the surface gradient is not as favorable to have strong winds
develop over the western valleys as when the surface low is forecast
to be over the GSL. Therefore, generally have 10-20 mph southerly
winds for Friday. Also, the GFS model is about 5-10kts stronger at
700mb by 00Z Saturday than the EC has, so felt comfortable in not
getting too wild with the southwest winds at this time.

The GFS shows some 600 mb moisture over the Uintas and far northwest
Utah by 00Z Saturday which was just enough of a concern to put some
minor PoPs in for the high Uintas and the NW corner of Utah. This
600 mb moisture continues to slightly increase Saturday. This
moisture increase along with rather warm 700 mb temps will increase
the threat of high elevation showers and thunderstorms. Have
increased the PoPs a little for the mountain spine from Nrn Utah to
Srn Utah and Uintas to cover this possibility.

Looking at the EC operational run compared to the EC ensemble from
00Z Sunday, the ensemble is about 80-120 meters higher at 500 mb
than the operational run for the lowest heights of the closed low by
00Z Saturday. Based on seeing just the last two runs of the Canadian
and the EC it appears that there is tendency for the closed low to
set up a little farther west. If this indeed does occur, which
climatologically is more favored, then the threat of showers will
remain below 10 percent in the valleys and isolated to widely
scattered in the mountains. Have leaned this way and gone with
slightly warmer temperatures Saturday and Sunday than previous
forecast and used an average of the QPF guidance. In addition, there
is a shortwave moving over the eastern Pacific Ridge into western
Canada which cuts off the northerly flow into the closing off low
resulting in a completely cut off low which will not have momentum
to carry it into the Great Basin. May even move westward with time.


.AVIATION...Operational weather concerns will remain minimal at the
KSLC terminal through the valid TAF period. Northwest winds
currently in place will switch to the southeast between 03-04z per
norm, and VFR conditions will be maintained. All cloud cover will
remain above 8000ft AGL.






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