Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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358
FXUS65 KSLC 232149
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
349 PM MDT Sat Sep 23 2017

.SYNOPSIS...An upper low over Nevada will slowly track across
Utah late tonight through Sunday, keeping cool and in some areas
wet weather in place. Another upper low will likely impact Utah
midweek.

&&

.SHORT TERM...Upper level closed low just west of the tri-state
area of Utah, Nevada and Arizona will drift east then northeast
across the state through Sunday. A dry slot over eastern Utah will
likely keep snowfall across the Uintas from reaching the full
potential with this system as the models do show it wrapping into
the 700mb low overnight. For now have 4-8 inches but this could be
overdone on the south face slopes. Farther south the -24C core of
the low moving into southwest Utah this evening will create good
instability and thus have thunderstorms in the forecast.

The 700mb temps lower to -4C at 700mb over the west central and
northwest Utah this evening, but as warm advection moves in from
the east across southwest Wyoming later tonight temperatures will
rise some. This warm advection of 0-2C air across SW Wyoming is
the reason why Uintah County was not included in the Winter
Weather Advisories. As the low moves into SW WYoming late Sunday,
the colder 700mb air over southwest and western Utah will filter
in across northern Utah and SW Wyoming bringing the snow levels
back down to near 6500 feet, which may require hoisting a WSW for
SW WYoming if the moisture hangs around through Sunday night.

The cold core of this system remains near enough to SW Wyoming and
northern Utah mountains Monday to keep a slight chance of showers
continuing. Temperatures during the next couple of days will
average 20-25 degrees below normal across northern Utah and
Southwest Wyoming while more like 10-20 degrees below normal
elsewhere.

.LONG TERM (After 12Z Tuesday)...A shortwave diving south on the
east side of the high amplitude eastern Pacific ridge will reach
the Great Basin early Tuesday. The various global model have been
locked in on a solution where this shortwave evolves into a closed
low over the southern Great Basin. The ECMWF remains the more
northern solution, with the center located over southwest Utah
Tuesday night. The GFS continues to dig this feature south,
forming the closed center across southwest Arizona by late Tuesday
night. Inclined at this point to favor the GFS solution as the
upper jet on the western side of this feature should be able to
carry the developing low at least into west- central Arizona.

The initial track of the still open shortwave could track slightly
further west in Nevada before closing off Tuesday night. Typically,
dynamic features tracking due south to the west of the Utah fail to
generate much in the way of precip outside of the Utah/Nevada border
region. This shortwave does not appear to be atypical as the best
vorticity and thermal advection look to remain near the border and
west across eastern Nevada.

Once the low closes off, moisture to the east of the low will be
drawn into the extreme southeast Utah Wednesday night/Thursday.
The initial round of precip will likely hit the four corners region,
with most of the precip activity remaining closer to the upper low
center over northern Arizona. By late in the week the low will begin
to drift northeast, drawing in more moisture across southern/eastern
Utah and increasing the chances for precip in the wrap around band
on the northern flank of the low.

For the remainder of Utah a generally dry, stable and warmer air
mass will be in place. Temperatures should climb back to readings
closer to the end of September norms, with little threat for any
precip.

&&

.FIRE WEATHER...Unseasonably cold and moist system moving across
the district tonight through Sunday will keep RH values very
elevated through Sunday with only the extreme southern zones of
Utah seeing Min RH values below 25%. By Wednesday this area of sub
25% RH will spread up into the central valleys of Utah. The
mountains from Central Utah northward will receive several inches
of snow above 7000ft through Sunday.

The cool temperatures will persist through midweek with Monday
and Tuesday averaging 20-25 degrees below normal across the north
while southern areas will be 10-20 degrees below normal.

Another upper low system will develop over the Great BAsin late in
the week which could bring more showers to the eastern zones for
Thursday, Friday and Saturday.

&&

.AVIATION...Cigs have fallen below 6000ft AGL at the KSLC terminal
this afternoon, and expected to fall further into the MVFR range
after 02z this evening. MVFR cigs are expected to remain in or near
the MVFR range through Sunday morning with increasing rainfall
potential developing overnight. Winds are expected to remain out of
the north through most all of the TAF period, but may briefly become
a light southerly in the 09-16z window Sunday morning.


&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until midnight MDT Sunday night for
     UTZ007>009.

     Winter Weather Advisory until 4 PM MDT Sunday for UTZ517.

WY...None.
&&

$$

SHORT TERM...Struthwolf
LONG TERM...Conger
AVIATION...Merrill

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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