Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 111121

Area Forecast Discussion
National Weather Service Grand Junction CO
421 AM MST Wed Dec 11 2019

.SHORT TERM...(Today through Thursday)
Issued at 347 AM MST Wed Dec 11 2019

A low amplitude ridge was over the area early this morning, being
driven eastward by a fast moving mid-level shortwave trough.
Except for some small pockets of low stratus in northeast Utah and
the Black Canyon of the Gunnison and thin cirrus moving through
the ridge, skies were clear or mostly clear this morning.

The shortwave trough clears the Great Basin this morning with the
trough axis moving overhead during the afternoon. There isn`t much
moisture associated with this disturbance, but most models
indicated some scattered light showers over the Elkhead Mountains
and the Park Range late this afternoon. Dry, mild conditions are
expected to continue elsewhere across the forecast area.

Some light orographic showers will linger over the northern
Colorado mountains tonight, though only light localized
accumulations are expected. Pacific moisture begins to trickle
into the north on Thursday with snow showers becoming likely
during the afternoon over the Elkhead and Park Mountains. As
discussed in the long term section, what begins as a trickle will
become a more substantial moisture surge, lasting well beyond
Thursday. Meanwhile, temperatures are expected to show little
change and will remain above normal for most locations.

.LONG TERM...(Thursday night through Tuesday)
Issued at 347 AM MST Wed Dec 11 2019

Mid to upper level winds will transition from the northwest to
zonal flow, which enhances Pacific moisture transported across
the Great Basin. An ample amount of moisture is projected to reach
the Colorado Rockies Thursday night and continue streaming in
through the weekend. This moisture with atmospheric river origins
supports a prolonged period of precipitation according to latest
models, bringing a mix of rain and snow showers across the CWA.
Due to the prolonged nature of this event, significant snowfall is
possible across the mountains this weekend.

Expect cloud cover to increase and spread across region Thursday
as a 150-160 kt jet approaches from the west. Light, orographic
precipitation will likely develop first across the central and
northern mountains along the Divide, as well as the Uintas in
northeast Utah. Precipitation will become more widespread late
Thursday into Friday, with higher moisture content moving across
the west and the jet maxima crossing overhead. As the system
deepens across the central CONUS, cooler temperatures will filter
in from north to south, likely lowering snow levels to the valley
floors. Meanwhile, the prolonged stream of moisture will continue
to provide showers through Friday and into the weekend. At this
time, forecast snowfall accumulations don`t meet warning
criteria for this prolonged event, though that could change if
later runs bring in higher levels of moisture. Winds in the
mountains are also a consideration and will let the day shift
weigh the merits of a Winter Storm Watch.

Another trough will swing south from the Pacific Northwest Saturday
night into Sunday, which will finally push the jet aloft east of the
Rockies. As this progresses, the best source for moisture will track
south and favor the southern mountain ranges. By this time, a cold
front crossing the entire region will bring much colder temperatures
regionwide for Sunday. In addition, precipitation along the
frontal boundary may bring brief periods heavy snowfall across the
San Juans, where slight instability combined with orographic lift
may result in convective snow showers. Northwest flow returns on
Monday, allowing showers to taper off by noon. Dry, colder
conditions prevail Tuesday with highs expected to be several
degrees below normal.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 421 AM MST Wed Dec 11 2019

High and mid level clouds will stream across the region during the
next 24 hours as an upper level disturbances tracks eastward
across the area. Any CIGS which form over TAF sites are expected
to exceed ILS breakpoints and VFR conditions are expected to
persist during the period. Winds will be light.






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