Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 140950
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
250 AM MST Sat Jan 14 2017

.SYNOPSIS...An upper level low pressure system will track east
across northern Mexico this weekend. Moisture associated with this
low will reach into southern Utah, with some shower activity across
southern Utah Saturday. High pressure aloft will settle over the
Great Basin for the first half of the week, followed by a return
to an active weather pattern late in the week.

&&

.SHORT TERM (THROUGH 12Z TUESDAY)...
Water Vapor Satellite shows a deep closed low over northern Baja
Mexico, with a deformation area over Nevada. A wave is approaching
a ridge over the eastern Pacific. 400-200mb MDCARS wind
observations place a 100-125kt cyclonic jet from the west coast
into Mexico and up the Southern and Central Rockies. GOES/SLC 00Z
RAOB/HRRR indicate that precipitable water values vary from
0.10"-0.25" mountains to 0.35"-0.55" southern valleys.

Diffluent flow ahead of the closed low will continue to support
precipitation as it propagates eastward over southern Utah through
this evening. The deformation zone will delineate dry weather vs.
chance of precipitation to the south, and it is expected to
translate across central Utah. Precipitation may linger through
tomorrow near Lake Powell supported by the deformation zone which
will continue to drift toward the east.

PoPs and QPF are a challenge given limited low level radar. But
from what we have on radar, lack of cold cloudtops on satellite,
and no surface observations of precipitation, kept PoPs in the
chance range for much of southern Utah. QPF from the HRRR-3km and
lower resolution global models would paint a picture of definite
and in some areas moderate precipitation, while our WRF models are
much less impressive. We leaned on the current forecast and
aforementioned observations due to the lack of consistency in the
high resolution guidance.

With a deep cyclone to our south, there is a 12mb pressure
gradient across the state which may support pockets of gusty north
winds for canyon areas of southern Utah. Fast forward to tomorrow
afternoon and evening on the backside of the storm system, 700mb
flow strengthens up to 30kts with a period of cold advection. Due
to this, these winds will pick up Sunday night and Monday near
canyons and downslope areas, and a wind advisory may be required.

The other issue in the short term comes in the way of stratus,
fog and eventually urban haze with an inversion in place. For
stratus added in additional sky coverage based on 11-3.9u
satellite and surface observations of clouds. Kept mention of
higher cloud cover for some of the more socked in basins into
tomorrow afternoon when our WRF models scatter out the stratus.
Some areas on the periphery will begin improving as early as this
morning.

As for fog went with persistence into the morning hours. Will re-
evaluate potential for later tonight after assessing how well we
mix out today.

Finally, air quality will decline into next week with the
inversion. Generally like to wait 3 days after trapping
pollutants to mention urban haze, and DAQ graphs would indicate
that today would be the day. But given the gradual incline thus
far and the fact that less pollutants are added during the
weekend, waiting until Monday to begin mentioning urban haze. This
looks to last through mid week.

.LONG TERM (AFTER 12Z TUESDAY)...
The strong upper level ridge continues to push farther East in
the extended as the long range models pinch off the open trough
wave down over Mexico. The negatively tilted ridge spills
southward through the Great Basin and Central Rockies by Tuesday,
further stabilizing valley inversions. Gradual warming aloft
through mid week will also aide in enhancing the valley
inversions.

Not until Thursday will the ridge begin to break down with another
Pacific Northwest trough feature that stretches down the California
coast and swings onshore. A notable baroclinic zone passage early on
Thursday will begin to filter in colder air aloft associated with a
moist wave. Both the GFS and ECMWF hint at a somewhat splitting
nature of this wave, however both solutions do bring the wave
through Utah and cool temperatures aloft (700mb) to near -10C to -
12C, sufficient for snow to Valley floors. Again, with the onset of
precip and timing of the colder air aloft, a precipitation type
challenge may be in play. The potential exists for rain to freezing
rain at the onset before conditions cool enough to produce snow by
Thursday. At this time, confidence is a bit low to lean one way or
another, but a trend to watch as we progress into next week.

The EC has a stronger jet (180kt) as the trough moves onshore
Thursday into Friday and kicks the trough to the east faster, while
the GFS later develops a strong jet but has a longer duration
precipitation feed across the West Coast into the weekend. By the
end of next week, differences among the models are enough to lower
confidence leaning one way or another in fine details, but this can
be said. Another very active and unsettled week ahead looks like a
sure thing. An atmospheric river pattern set up with a prolonged
period of precipitation (though which type and changing to and from
types will be the challenge) is being advertised by forecast models.

&&

.AVIATION...
For the KSLC terminal, a period of MVFR conditions in fog from
the Lake will likely develop during the early hours. There is a 30
percent chance of IFR conditions from 13-17z. Ceilings should
erode away by early afternoon, with mostly clear skies through the
remainder of the TAF period. Expecting a southeast wind shift
after 03z.

&&

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

$$

SHORT TERM...Rogowski
LONG TERM/AVIATION...Dewey

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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