Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 152257
Area Forecast Discussion
National Weather Service Salt Lake City UT
357 PM MST Sun Jan 15 2017
.SYNOPSIS...High pressure aloft will settle over the Great Basin
for the first half of the week. A return to an active weather
pattern will occur late in the week.
.SHORT TERM (Through 00z Thursday)...Afternoon water vapor and H5
analysis indicate rapid downstream translation of the upper low
that has been residing over the desert southwest over the last
few days. A secondary low over far SoCal is currently following a
similar track and poised to translate across northern Mexico over
the next 24 hours. The deformation axis resultant of these
features continues to weaken over south central and southeastern
Utah this afternoon, and will continue to do so through this
evening, but still maintaining mention of scattered showers across
the far south this eve due to weak lift and plentiful low/mid
level moisture in place.
The primary forecast problem moving forward is the presence of and
eventual dissipation of valley/basin stratus, and fog during
primarily the overnight hours. Although a drier northeasterly flow
will spread over the area for the next few days, this drying will
be fighting against increased subsidence as mid level ridging
builds over the area coincident. Do feel stratus will remain in
place for the next 18-24 hours where it is, but continue to lean
towards dissipation within the drying airmass during peak solar
Expecting patchy fog to develop overnight across the lower
elevations of the south including the Powell/Swell areas due to
recent precip and a gradual decrease of mid/upper cloud cover as
the low continues to depart. Expecting formation in the prone
northern valleys such as the Cache and northern Wasatch front
where stratus isn`t present as well, and some of the Wasatch Back
including Heber city. The Bear River valley and southwest Wyoming
look primed again as well. Added to grids through mid morning
A modest N-S gradient and cold advection across the southwest will
provide gusty gap winds in the Dixie area tonight, but opted not
to issue an highlights due to lack of anticipated impact. Will
continue to monitor as gusts will likely be hovering around 45mph
in the prone localized spots.
The aforementioned high pressure will allow for a warming trend
at the higher elevations through midweek, but not expecting much
reflection down low until the stratus breaks up where it resides.
Decreased PoPs across the far north Wednesday as Globals continue
to slow regarding onset of the unsettled period for the latter
half of the week. More info in the long term below.
.LONG TERM (After 12Z Thursday)...Interesting changes in the models
past 24 hrs as the EC is now slightly faster than the GFS in
breaking down the ridge and spreading moisture into the CWA. In fact
the GFS barely has any QPF into the northwest corner of Utah by 12Z
Thursday while the EC now has moisture half way across the state
from Logan to Lake Powell westward. Backed off on the eastern
advancement compared to the various model guidances, but still may
not have downplayed enough.
Have increased the PoPs over most of the CWA for Thursday, even
though the timing of the GFS and EC are slightly different. The
reason being is that although this system is not as strong as
previously progged, there is still a healthy moisture tap so even
minor-moderate dynamics will be sufficient to produce precipitation.
The 12Z EC is showing a little more negative tilt than the 00Z run
and still holds a jet in place overhead Thursday afternoon along
with a 700mb trough axis moving across western Utah. These features
alone should combine for reasonable lift and at least widespread
light precipitation. The 700mb temps cool to just about what is
needed for snow in the western valleys and may actually come down to
the intensity as to whether the precip is rain or snow.
This system quickly washes out as it moves east due to the more
energetic jet and trough moving to the California coast by Friday
morning. This should produce a weak short wave ridge over the region
Thursday night into Friday or a little slower if the EC is correct.
However, with the fast and active jet timing the details this far
out are difficult at best so have just went with above climo PoPs
through the rest of the extended. Temperatures appear to be colder
by both models as we head into the weekend so precip type should be
all snow for western and northern valleys.
.AVIATION...MVFR cigs are expected to prevail at the SLC terminal
through midday Monday. VFR vsbys should continue into this evening
but lower to high end MVFR after 04Z with a 20 percent chance of
waiting until after 06Z. Winds will remain light and northerly
through the TAF period.
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