Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 181004

Area Forecast Discussion
National Weather Service Salt Lake City UT
404 AM MDT Tue Oct 18 2016

.SYNOPSIS...One last storm system will cross northern Utah and
southwest Wyoming today before high pressure returns for the
latter half of the week.


.SHORT TERM (Through 12z Thursday)...The last in a recent series
of shortwave troughs to traverse the Pacific Northwest/northern
Great Basin is currently moving inland across Oregon, and is
forecast to quickly translate east-southeast across the northern
Great Basin this morning, before crossing northern Utah this
afternoon and evening. As the 500mb cold pool crosses northern
Utah steepening lapse rates coupled with a low level WNW flow
should provide a window of favorable orographic for areas from
roughly Salt Lake County northward. Additionally with cooling
aloft and daytime heating instability should be sufficient for
isolated thunderstorms. 700mb temps fall to around -5C this
afternoon which will drop snow levels as low as 6500 feet along
the ID border. All in all it looks like a high POP/low QPF event
for northern Utah as the upper trough quickly moves off to the
east this evening.

A low amplitude mid level ridge is forecast to build across the
eastern Great Basin late tonight into the day Wednesday. With the
associated surface high in place Wednesday night will likely see
frost develop along outlying areas of northern Utah.

.LONG TERM (After 12z Thursday)...Ridge of high pressure will be in
control through the weekend with dry conditions prevailing.
Temperatures will be moderating during this period with the
mountains warming a little more rapidly as an inversion across the
valleys will likely inhibit them from warming to their full
potential through Friday. By Saturday and Sunday the northerly
surface gradient will vanish as the high moves to the east
and a southwest flow aloft develops.

The EC and GFS show moisture getting tapped for early next week but
details of timing and strength of system moving through the region
is questionable. Reason being is once again we have two Typhoons in
the western Pacific which tend to cause havoc on the extended
forecast over the eastern Pacific and western North America. Case in
point, the GFS ensemble members have a standard deviation
variability of 60-90 meters at 40N and 140W by midday Sunday which
increases to 100-120 meters between 40N and 50N along 140W by
Tuesday afternoon. This variation will surely have an impact on the
strength of any systems trying to penetrate western North America.
Consequently, have only gone slightly higher than climo PoPs for
Monday and Tuesday.


.FIRE WEATHER...More valley rain showers and mountain snow showers
later today and this evening across northern Utah while the south
will remain dry. The cold front which will be bringing the unsettled
weather to northern Utah will bring nothing more than a wind shift
and slightly cooler temperatures across southern Utah this
afternoon and evening.

Ridge of high pressure will build over the region Wednesday through
the weekend with moderating temperatures. Due to a valley inversion
temperatures will moderate faster at the higher elevations than in
the valleys. This inversion will also cause min relative humidity
values over the upper elevations of the northern mountains to lower
into the teens and lower 20s Thursday into Friday. The southern half
of Utahs valleys will have min RH values in the teens Thursday and
Friday before gradually increasing through the weekend.

An uptick in the winds across southwest valleys will occur this
weekend but nothing like we had this past weekend. Moisture will
likely increase early next week across the entire area but low
confidence in timing and amounts at this time.


.AVIATION...South winds will prevail at the SLC terminal through
about 18Z with a 40 percent chance of holding off until 19Z when a
cold front arrives. Showers with cigs below BKN070 should occur
between about 20 and 23Z although there is a 20 percent chance
that cigs remain above BKN070. Cigs are expected to increase above
BKN070 between 03 and 05Z this evening.





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