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Area forecast discussion 
National Weather Service Grand Junction Colorado
1038 PM MST Thursday Dec 18 2014

Short term...(tonight through Friday night)
issued at 332 PM MST Thursday Dec 18 2014

The atmosphere seems to be very efficient in making snow today
especially with a bit of instability added to the mix. Impacts to
roadways seen on webcams around the region are minimal but there
could be some snow packed and icy spots on north facing slopes.
This is likely on The Mesa where the best snow has fallen since
middle morning. The wave adding ascent will be moving over western
Colorado this evening and should keep some terrain based showers
going through midnight...with north/west facing slopes favored
looking at the middle level flow. From sunrise into middle morning a few
showers may linger over the northern mountains...but as transitory
ridging brings in warm air advection aloft during the afternoon...the more stable
conditions should shut off most of the showers by early afternoon.
This ridging remains in place through early Saturday before
upstream energy pushes the ridge axis to the High Plains and the
flow turns more northwesterly and progressively more moist.
Temperatures under the ridge will struggle due to recent snowfall
and precipitation...so highs near or slightly below normal are in
place. The bigger challenge to the forecast will be lingering planetary boundary layer
moisture and drying aloft tonight. Fog was fairly common overnight
and this will be the case again overnight. There will be areas of
dense fog along some valley locations but confidence too low at this time
to hoist advisories.

Long term...(saturday through thursday)
issued at 332 PM MST Thursday Dec 18 2014

.Significant mountain snowfall possible late this weekend and
early next week. Another storm possible on Christmas day...

Models continue to point toward an extended period of unsettled
weather through next week. The persistent longwave ridge over the
western states flattens this weekend and then rebuilds farther to
the west over the northeastern Pacific Ocean. This will favor a more
consolidated northwest flow over our area for much of next week with
a generaly cooling in temperatures...versus the split flow pattern
of the recent past.

One shortwave disturbance crosses on Sat and helps to flatten the
ridge while bringing a chance of light snow to the northern and
perhaps central mountains

A strong and moist jet stream blasts into the Pacific northwest this
weekend and across Wyoming on Sunday. This 120-130kt jet then becomes
oriented northwest to southeast over Colorado Monday-Monday night as the
upper ridge begins to amplify over the northestern Pacific. Abundant
moisture combined with jet dynamics and orographic forcing should
result in an extended period of snow for the northern and central Colorado
mountains Sunday afternoon through Monday night. Scattered snow showers
will persist across the northern mountains to Vail Tuesday in northwest flow while snow
decreases elsewhere as the upper jet slips eastward. Windy
conditions are likely creating areas of blowing snow as well. Travel
over the Rabbit Ears and Vail passes late Sunday into Tuesday will
be negatively impacted and will continue with the Special Weather
Statement addressing this. But as stated in the earlier
discussion...warm advection Sunday afternoon/ evening may result in
lesser snow amts initially before colder air settles in Monday as
the jet moves overhead.

Models indicate a bit of a break late Tuesday into Wednesday as a
transitory ridge moves across ahead of the next trough expected to
enter the Pacific northwest. 12z ecwmf and 12z GFS now in better
agreement on bring this trough across our area on Christmas day for
a chance of snow. This is a rather significant change for the ecwmf
which had been showing the trough closing off a middle-level low over
the Great Basin Thursday. Will see if this model agreement continues.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1029 PM MST Thursday Dec 18 2014

Biggest challenge of the night will be visibilities. Already
seeing dense fog at kcag and ksbs with k5sm not far behind at a
half mile visible. Expecting dense fog to be intermittent throughout
the night and not widespread across western Colorado/eastern Utah.

Widespread VFR expected on Friday...with high clouds increasing
from west to east after 23z.



&&

Gjt watches/warnings/advisories...
Colorado...dense fog advisory until 7 am MST Friday for coz002-005.

Utah...none.
&&

$$

Short term...15
long term...jad
aviation...jam

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