Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 171020

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
320 AM MST Sun Dec 17 2017

.SHORT TERM...(Today through Monday)
Issued at 320 AM MST Sun Dec 17 2017

Eastern Utah and western Colorado lay between a closed low over
the state of Sonora in northern Mexico and the open wave trough
over the northern Rockies early this morning. The trough to the
north will drift eastward during the day while the low remains
relatively stationary. A lack of moisture in the col between these
two systems rules out precipitation across the forecast area. In
fact, dry weather will continue tonight and Monday as the low over
northern Mexico remains too far south to impact the region, and
moisture in the northern stream also remains well removed from
the region. Meanwhile, unseasonably mild temperatures will persist
with little change today or Monday.

.LONG TERM...(Monday night through Saturday)
Issued at 320 AM MST Sun Dec 17 2017

Tranquil weather is expected through the first half of the week
ahead of the next shortwave trough. This trough will dive through
the great basin on Wednesday afternoon, spreading cloud cover
over much of the Four Corners through the day. Guidance remains
in rather excellent agreement regarding precipitation onset,
frontal passage, and precipitation departure with this storm
system. High-altitude snow showers will develop in the prefrontal
environment after sundown on Wednesday in the eastern Uintas as
well as the northern ranges of Colorado. By sunrise Thursday, a
strong cold front will cross the Wyoming border and make its way
south through eastern Utah and western Colorado. A consensus of
guidance places the front roughly along the I-70 corridor around
noon on Thursday, and through the San Juans by sunset.
Temperatures will crash along and behind the boundary, with likely
the coldest airmass of the season thus far moving over the
western slope. What little precipitation does fall ahead of the
front will likely remain high enough in altitude that snow levels
will not be a concern. 700mb temperatures will drop into the teens
below zero celsius behind the front, sufficient for snowfall in
all locations. A combination of frontal forcing and dynamic
support for lift will result in the heaviest snowfall occurring
along and just behind this boundary from Thursday morning in the
north through Thursday evening in the south.

Regarding precipitation totals, guidance has generally centered on
around 0.5 inches of liquid in the mountains with a few hundredths
in the valleys. Specifics on how much snow actually accumulates in
places like Grand Junction down to Montrose are still a bit up in
the air, but confidence is increasing that many may see their
first snowfall of the season along the US-50 corridor. All
mountain zones look to see at least advisory level snowfall before
precipitation comes to an end from Thursday evening in the north
to around midnight Friday in the south.

Right on the heels of this system comes another shortwave/vort max
diving southward out of Canada. A vast range of possible scenarios
have been portrayed in recent deterministic guidance, depending
on exactly how deep and west the system is able to track.
Virtually any outcome from significant precipitation to light
mountain snowfall is on the table for the December 23-24th
timeframe. GEFS ensemble guidance does show h5 height anomalies
running around -1 sigma during the same time, adding some
confidence to at least cooler than normal temperatures leading up
to the Christmas holiday.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 320 AM MST Sun Dec 17 2017

Expect VFR conditions and CIGS above ILS breakpoints through the
next 24 hours. Winds will be light and terrain driven.






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