FXUS65 KSLC 172252
Area Forecast Discussion
National Weather Service Salt Lake City UT
352 PM MST Sat Nov 17 2018
.SYNOPSIS...A weakening cold front will sweep south across Utah
overnight. High pressure aloft will return on Sunday, then remain
over the Great Basin during the first half of the upcoming week.
An upper level trough will move east across the region on
.SHORT TERM (Until 00z Wednesday)...The axis of the upper trough
working south from the northern Rockies has reached far northern
Utah late this afternoon. The associated surface cold front and
trailing near 700mb baroclinic zone over central Utah will
continue to work south through the remainder of the state tonight.
Precip has become less organized and more showery near the 700mb
baroclinic zone this afternoon. This trend of decreasing precip
will continue into this evening as the upper jet shears east and
weakens the mid-level trough, while the low-level baroclinic zone
continues south and becomes dissociated from the mid-level
support. Rapid drying behind the upper trough will lead to
clearing skies north-to-south with some fog developing up north
where precip was observed today.
The upper ridge currently over the eastern Pacific will gain
amplitude as it shifts east over the western CONUS beginning
Sunday. This ridge will remain the dominant feature over the
forecast area early in the upcoming week.
A shortwave embedded in the southern branch of the jet will
undercut the ridge on Tuesday. Models guidance takes this feature
east across Arizona with little in the way of dynamic/thermal
support to generate enough lift for any significant precip, even
across northern Arizona. Suspect that mid/high-level moisture will
advect north ahead of this feature and bring some cloudiness and
slightly cooler temps to southern Utah on Tuesday.
.LONG TERM (After 00Z Wednesday)...Upstream ridge amplification
(between 170-180W) during the midweek period will set the stage for
a more progressive northern branch pattern across the western CONUS
thereafter...at least briefly. Two known troughs to focus on, the
latter potentially more notable for post-Thanksgiving travel early
next weekend. It's too early for to much detail attm however.
To begin the period an elongated trough off the west coast will
eject inland by a deepening upstream short wave over the Gulf of AK,
splitting on shore as it does so. The southern split will be
weakening while trending positively tilted as it translates across
the eastern Great Basin region on Thanksgiving. This is the first.
Do note fairly deep moisture with this, and activation of a southern
branch jet streak oriented over the 4-corners region...so even with
a sub-par baroclinic structure, dynamic lift looks sufficient for
fairly widespread (but generally light) precip focused with trough
axis passage for the holiday. Modest CAA through the mid levels will
support lowering snow levels, though most likely remaining above
valley levels (especially along the Wasatch Front).
The aforementioned Gulf of Alaska trough will then have the means
to translate SE into the region late Fri into Sat, this as the
pacific ridge axis shifts east towards 140W. Globals continue to
differ in detail regarding trough evolution/amplitude, with the
operational GFS more progressive/less amplified. This said,
significant spread remains owing to the GEFs ensemble mean
pointing more closely towards the slower/more amplified EC.
Trended towards the latter this forecast. With the given track and
a more pronounced tap to a colder northern latitude airmass, do
see potential for lower valley elevation snowfall event (but
magnitude?). Something to watch, time will tell.
.AVIATION...Ceilings will continue to lift/scatter out as the
afternoon progresses in the wake of earlier showers. Expect sub-
7k.ft ceilings to erode by about 23-01Z. Northerly winds will
prevail before shifting to the southeast around 09-11Z. Although
there is some uncertainty regarding overnight wind direction, speeds
should be light enough to limit operational impacts.
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