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Area forecast discussion 
National Weather Service Salt Lake City Utah
328 am MST Wednesday Nov 25 2015

Synopsis...a cold upper low will settle into the Great Basin
today. This low will continue to impact the area through the
Thanksgiving weekend.


Short term (through 12z friday)...
water vapor loop shows a closed low over the western Great Basin.
Amdar 400-250mb wind observations places a 100kt-125kt cyclonic
jet extending into the northern rockies from central California.
GOES/hrrr/GPS/00z slc radiosonde observation indicate the precipitable water value
ranges between 0.10"-0.20" 0.20"-0.33" most

Southerly winds will continue gusting ahead of the storm across
southwest Utah...expanding eastward this afternoon supported by
5mb/6hr pressure falls...45kts 700mb flow and an 6mb southeasterly
pressure gradient across eastern Utah. Advisory level winds are
possible though once again believe winds may fall just shy of the 3-
hour duration requirement for an advisory. Locally higher gusts
should once again fall short of warning criteria.

Based on radar/satellite trends and rap guidance have trended probability of precipitation
lower especially along and east of the Wasatch front through sunset.
While light activity should get going in the higher terrain...will
have to wait until 700mb cold front arrives this evening for the
event to really unfold locally. Meanwhile...expect snowrates to
begin to present Road hazards this afternoon across the west desert.
Have upgraded this area to a Winter Storm Warning due to expected
travel impacts along Interstate 84 and State Route 30 near the Idaho
border...with more intermittent impacts forecast along Interstate
80. These conditions will likely persist through tomorrow morning
with the arrival of the 700mb cyclone. This feature arrives across
northwest Utah this afternoon and sits into tomorrow morning when
it weakens.

This gives the northern zones the best opportunity for travel
impacts...with much more limited impacts near Salt Lake City and
especially southward. Soutwhesterly flow favors Davis/Weber
counties...while easterly flow will support the Wasatch back during
this period. Will allow day shift one more opportunity to
collaborate with partners before deciding on whether an upgrade is
prudent for the Wasatch back and soutwest Wyoming. The rap
solution and later arrival time both had US lean against upgrading
these zones on the overnight shift.

Another player will be the arrival of the Arctic boundary from
Wyoming for temperature forecast and potential for greater travel
impacts and snow rates. Current indications are this boundary
makes it into southwest Wyoming tonight...seeping into northern
Utah tomorrow. Best pressure gradient and cold advection appear
Thursday night into Friday morning. Will have to monitor for
potential downslope windstorm in addition to enhanced canyon

Long term (after 12z friday)...
a large upper level low will be in the process of cutting off
across the southwest United States at the start of the long term
forecast period. Several shortwaves are expected to rotate around
the upper level low through Saturday. Meanwhile...a fairly potent
vorticity maximum will approach the California coast Saturday. This vorticity
maximum will combine with the intensification of a Lee cyclone across
Wyoming to finally eject the upper level low out of the western
United States.

At 12z low/mid-level gradient combined with easterly
cold air advection should support gusty winds near canyons along the
northern Wasatch/Cache Valley. A few of the model solutions hint at
stronger winds or a possible downslope wind event. At this
seems unlikely all the ingredients for a downslope wind event will
come expect wind gusts to 45 miles per hour Friday morning in
favored locations.

The other impact this slow moving upper level low will have to Utah
will be a period of unsettled conditions through at least Sunday.
With a deformation axis across Utah...expect occasional snow showers
across at least central portions of the state. Added low end probability of precipitation to
match the location of the best lift.

The model guidance varies quite a bit with several of the models
completely out of phase by Monday. Given this trend...kept climatology
probability of precipitation in the extended portion of the forecast with temperature
averaging near to below normal levels state-wide.


Aviation...expect southerly winds to become gusty in the morning
and remain elevated through much of the day at the slc terminal.
Winds are expected to shift to the southwest or even west between 20-
22z...though approaching showers will likely cause erratic/gusty
winds at times.


Slc watches/warnings/advisories...
Utah...Winter Weather Advisory from noon today to noon MST Thursday for

Winter Storm Warning from noon today to noon MST Thursday for

Wyoming...Winter Weather Advisory from noon today to noon MST Thursday for



Short term...rogowski
long term/aviation...Kruse

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http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)

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