Scientific Forecaster Discussion
fxus65 kslc 191257
Area forecast discussion
National Weather Service Salt Lake City Utah
557 am MST Fri Jan 19 2018
Synopsis...a cold and moist storm system will move off the eastern
Pacific and will track across the Great Basin through the weekend.
This storm will impact much of Utah and southwest Wyoming tonight
through early Sunday.
Short term (through 00z monday)...the cold upper trough upstream is
noted on water vapor satellite imagery extending from off the
northern and central California coast through the Pacific northwest
states this morning. Over Utah, southwesterly flow aloft has
increased to between 35-50kt at 700mb. A south/southeasterly mslp
gradient is in place across the area, and winds over the west-
central and southwest valleys of Utah as well as portions of the
Wasatch front have gradually increased overnight. Local gusts 30-40
mph are already noted on surface obs. Although winds will continue
to increase this morning and remain strong and gusty especially
across west-central and southwest Utah through much of the day,
models weaken the mslp gradient later this afternoon, and this is
expected to keep winds generally below advisory criteria.
The cold front associated with the approaching trough is already
approaching northwest Utah, and radar returns noted through much of
southern Idaho through north-central Nevada. This front will slowly
slide into northwest Utah later this morning. The surface boundary
is not expected to reach slc until around 21z, although the 700mb
baroclinc zone will arrive after 00z.
The front is not expected to be particularly strong initially, but
as the trough becomes more consolidated and less positively tilted,
frontogenesis increases and precipitation along the boundary should
intensify as it crosses the I-80 corridor through the west deserts
and approaching the Salt Lake Valley during the evening hours. The
front will then continue to slowly sag south, with further
intensification of precipitation expected heading into west-central
and southwest Utah overnight and into Saturday morning as it front
interacts with the right entrance region of the upper jet. The front
will then very slowly continue on into east-central and southeast
Utah through Saturday afternoon.
Because the colder air aloft lags the surface boundary, many valleys
are expected to see precipitation begin as rain. However, as
precipitation intensifies with the arrival of the baroclinic zone,
snow levels will quickly lower. Precipitation is expected to begin
mixing with or changing to snow across most northern Utah valleys
late this afternoon or during the evening hours. Bufr wet bulb zero
heights suggest that at slc, it will be towards the tail end of the
evening commute, although significant accumulations might not begin
until later in the evening. For central through southwest Utah,
expect significant accumulating snow to begin overnight.
Overall, the heaviest snow with this storm is still expected across
west-central and southwest Utah and adjacent mountains, given the
aforementioned intensification of the precipitation band through
this area, slow frontal progression, and northerly flow behind the
cold front which would be a favorable upslope flow. The I-15
corridor through this area would be particularly prone to seeing
heavy snow tonight through tomorrow. The western Uinta Mountains and
southwest Wyoming would also tend to see favorable upslope flow as
well as long duration snow. Winter storm warnings have been issued
for these areas through tomorrow night as a result.
Winter storm warnings were also issued for the Salt Lake and Tooele
valleys, southern Wasatch front, Wasatch Mountain valleys, southern
Wasatch mountains, and the Wasatch Plateau and Book Cliffs. Amounts
will be less in these areas, and the southern Wasatch front will
even see a period of north/northeast flow late tonight into tomorrow
morning where they will be less favored orographically. However,
given a period of intense snowfall along the strengthening
baroclinic zone and potential significant local impacts possible,
felt that a warning was warranted. Otherwise, farther north where
snow is expected to be lighter but accumulations are still expected
to impact travel this evening and overnight, winter weather
advisories are in place. Some of the higher valleys of eastern Utah
could potentially need advisories as well, but if so this would not
be until tomorrow, so there is still time to evaluate this further.
Gusty downslope winds will also be possible in the eastern valleys
tomorrow night behind the front.
Snow will gradually become more confined orographically along
north/northwest facing slopes tomorrow night before gradually
winding down Sunday morning. Obviously, colder air will spread into
the area behind the cold front for the weekend. Gone will be the
highs in the 40s and 50s, as these will be replaced by highs in the
30s for most valleys except in the far south and southeast. Welcome
Long term (after 00z monday)...models start off this extended
period in pretty good agreement but with a progressive pattern
prevailing through this time frame, the strength and timing of
shortwaves get out of sync pretty fast. The GFS has 40-50 meter
500mb height differences within its own members across the County Warning Area by
12z Wednesday but this value increases to near 80 meters by 12z
Thursday and to 100 meters by 00z Friday. The 12z ec also has
discrepancies of about 100 meters by 12z Friday between the
operational model and the ec ensemble mean. That being said, mostly
went with current forecast on pops except used a superblend/climo
mix for the last two periods.
Both the GFS and ec show a cold trough over the western conus by end
of the forecast period but the details are out of sync. Could be an
interesting period with colder temperatures than this weekend's
Aviation...south winds at the slc terminal will continue through 19-
20z then shift to northwest behind the cold front. Rain will develop
but will switch to snow by 01-02z with a 30% chance of holding off
until 03-04z. Periods of heavy snow are expected between 04z and 08z
with snow accumulation of 1-3 inches on untreated surfaces. Snow
showers will continue through the end of the taf period with
prevailing IFR conditions expected.
Utah...Winter Storm Warning from 6 PM this evening to 5 am MST Sunday
Winter Weather Advisory from noon today to 9 am MST Saturday for
Winter Storm Warning from 5 PM this afternoon to 11 PM MST
Saturday for utz003-004-006.
Winter Weather Advisory from 4 PM this afternoon to 9 am MST
Saturday for utz002-005.
Winter Storm Warning from 11 PM this evening to 5 am MST Sunday
Winter Storm Warning from 9 PM this evening to 5 am MST Sunday
Winter Weather Advisory from 9 PM this evening to 5 am MST
Sunday for utz014.
Wyoming...Winter Storm Warning from 6 PM this evening to 5 am MST Sunday
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