Scientific Forecaster Discussion
fxus65 kslc 131057
Area forecast discussion
National Weather Service Salt Lake City Utah
357 am MST Wed Dec 13 2017
Synopsis...the strong and persistent upper ridge continue
retreating off the West Coast over the next couple of days.
Weather disturbances cresting this retreating ridge will begin to
erode the strong valley inversions, and possibly bring some
precipitation to the area by the weekend.
Short term (through 00z sunday)...in the near term, strong valley
inversions continue to plague northern Utah with widespread
stratus/fog extending across the larger Great Salt Lake basin
including the Wasatch front, as well as the Cache Valley and Bear
Lake areas. Snow has fallen at times from this fog bank around the
slc Airport and likely other locations along the valley bottom,
and would expect this to continue through this morning.
The mid level ridge which had resided across the Great Basin over
the past couple of days has retrograded along the Pacific coast,
and will continue to shift westward today, allowing a
northwesterly flow aloft to overspread the forecast area. A
relatively weak shortwave trough embedded within this flow will
dig southward through the forecast area this afternoon through
tonight, bringing a small chance for snow to the higher terrain of
the uintas as well as perhaps The Bear river range. With a very
dry airmass above the surface cold pools in northern Utah, it
seems unlikely precipitation will reach the valley floors (outside
of that falling from the persistent fog/stratus layers), and have
removed pops from the far northern valleys this afternoon/evening.
Any snowfall across the higher terrain will likely remain light
with perhaps a couple of inches of accumulation. As this boundary
pushes south, a tight northerly gradient will likely produce gusty
winds across prone areas of southwest Utah tonight into Thursday
The impact this wave will have on the stagnant valley inversions
remains uncertain. With 700mb temperatures falling to around -5c
this evening coupled with 30 kts of flow at 700mb and a bit of a
northerly surface gradient, it seems likely the inversions across
northern Utah will at least be weakened and its possible the
benches may experience some relief or possibly even mix out this
evening. However seems likely the valley floors will remain
entrenched within an increasingly shallow but still stagnant cold
pool. Assuming the inversions remain intact for the most part,
fog/stratus along with poor air quality and cold temperatures will
remain in place through Friday, and have basically maintained a
persistence forecast for these areas from today through Friday
The medium range models have remained in remarkable agreement for
several days now digging a shortwave trough and associated cold
front through the forecast area Friday night into Saturday
morning. Confidence is high that valley inversions will be swept
out with a combination of frontal passage and 700mb temperatures
falling below -10c Saturday morning. What's more uncertain is the
potential for snowfall with this feature as it translates through
the region. It seems likely the higher terrain will pick up
accumulating snowfall with this system and have bumped pops
across the northern mountains into the likely category for
Saturday, while leaving the valleys of northern Utah in the low
to middle chance category.
Long term (after 00z sunday)...global models are in fairly good
agreement that a second shortwave trough will dig into the
forecast area Saturday night into Sunday, although the ec is the
weakest with this feature. Both the GFS and ec show -12 to -14c
700mb temps moving southward across the County Warning Area with the GFS taking it
all the way to Dixie while the ec keeps the coldest air over the
far eastern portions of central and southern Utah. Confidence is
high now that this air mass is cold enough to scour out the
northern Utah inversions but as to bringing much in the way of
precipitation its not looking too good. The air mass is
sufficiently cold enough for snow in the valley but the lack of
moisture will make this system pretty anemic.
Although the ec is farther east with the cold air, both the ec
and GFS show good cold advection and a reinforcing surface
gradient across southwest Utah Saturday night into Sunday. Have
increased the winds in this area for both periods.
In the wake of this system, a ridge will build over California
and Nevada into Tuesday which will place northern Utah and
southwest Wyoming under a west northwest warm advection zone.
Although the threat of any precip will be marginal, there will be
periods of clouds moving through which should keep any strong
inversion from developing. However, the ridge builds in over Utah
Tuesday night through Wednesday for a chance of an inversion
developing. But fortunately, there is another disturbance forecast
for Thursday so if an inversion develops, it will be short lived.
Aviation...fog and stratus will be the weather concern through
the taf period. Vlifr conditions will generally prevail until
13-15z when conditions are expected to trend more into the LIFR of
IFR category, although there is a 40% chance of vlifr prevailing
well into the afternoon. Light snow will at times precipitate out
of the fog/stratus possibly creating light accumulations on
untreated surfaces. Otherwise, winds will remain light.
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