Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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642
FXUS65 KSLC 151107
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
407 AM MST Wed Feb 15 2017

.SYNOPSIS...High pressure aloft will maintain dry and mild
conditions across the region today. Southwesterly flow will
develop Thursday, followed by an active pattern late in the week
into early next week.

&&

.SHORT TERM (Through 00Z Sunday)...Mid level ridging extends
across much of the Interior West this morning, downstream from a
predominantly zonal flow which extends across the eastern Pacific.
With the ridge axis shifting overhead today will continue to see
temperatures aloft climb, resulting in a warming trend area-
wide. Under sunny skies, temperatures should run a good 5 to 10
degrees above climo. The only exceptions would be the Cache and
Bear River Valleys where shallow cold air remains entrenched.

This ridge axis is forecast to shift east Thursday allowing a
southwesterly flow aloft to overspread the region, and with deeper
mixing max temps should continue to exhibit a warming trend.

The large scale pattern is expected to transition and become more
progressive heading into the weekend. The first in a series of
waves to cross the region is forecast to eject across the northern
Great Basin Thursday night primarily impacting areas north of
I-80 with valley rain and mountain snow. The models have been
trending weaker and further north with this wave, and as such
would suspect most valleys south of I-80 will remain dry, although
have retained slight chance PoPs for now.

In the wake of this wave, shortwave ridging is expected to
translate across the forecast area early Friday, as an upstream
wave approaches and then digs along the CA coast. As this wave
drops along the coast, backing flow will allow a plume of theta-e
advection to spread first into southwest Utah late Friday, then
northward into central and eventually northern Utah Friday night,
weakening with time. Have increased PoPs across the southwest
Friday afternoon/evening, while maintaining chance to slight
chance further north into Friday night given the weakening trend
of this feature. With the upper low expected to pass well south of
the forecast area, would only anticipate showers into the day
Saturday, and have kept PoPs in the chance category for this
timeframe. Snow levels Friday through Saturday are expected to
remain well above the valley floors given 700mb temps in the -2 to
-4C range and no surface cold air in place.

.LONG TERM (After 00z Sunday)...This extended period begins with
fairly good agreement between global models of an
elongated/splitting trough from central Cal to central Baja. Because
of this stretched out system the best dynamics will be either under
cutting the CWA or going by to the north. What is left of the trough
axis moves through the CWA Sunday afternoon. Although have continued
with likely PoPs over the higher terrain and northern valleys, the
amount of precip will be modest at best. Have been leaning towards
the EC solution past few runs and will continue to do so through the
rest of the extended as it has been the most consistent in its trend
of weakening the systems as they move through the Intermountain west
early next week.

In the wake of this trough axis on Sunday, a weak shortwave ridge is
expected to build over the region Monday, although some moisture
could creep into the northwest corner of Utah late in the day. Have
trimmed back PoPs during this period but may not have backed off
enough as the models show some blotchy QPF but don`t see the
mechanism creating the precip. The trend over the past several runs
of the global models has been to gradually back off and foresee this
to continue as the overall ridge position appears to remain over the
Great Basin. Have nudged temps up a couple of degrees for Monday and
Tuesday as the EC is now showing 0 to +2C degrees at 700mb across
most of the CWA. Depending on the amount of moisture/cloud cover
these temps could be underdone by several degrees.

By Wednesday, the EC shows a sharper and thus a slower trough moving
into the western Great Basin, while the GFS is more progressive and
has 700mb temps about 5-6 degrees colder than the EC. Confidence is
low at this point as there are multiple waves across the Pacific
shown by all the global models which often leads to timing and
amplitude problems. Have taken a broad brush approach for now with
above normal climo PoPs as patterns remains active.

&&

.AVIATION...No meteorological concerns at the SLC terminal through
the TAF period. Light and variable winds will become prevailing
northwesterly after about 17-18Z today and then switch to southeast
about 03-05Z tonight.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Seaman/Struthwolf

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