Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 070638
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1138 PM MST Tue Dec 6 2016

.UPDATE...
Issued at 1138 PM MST Tue Dec 6 2016

The mid-level short wave trough discussed in previous iterations
of this product was moving southeastward across eastern Utah and
western Colorado as models had predicted. However, snowfall
extended farther to the west than earlier indicated and therefore
boosted coverage of snowfall during the remainder of the night for
the central valleys. However, snow will be ending from northwest
to southeast as the trough and associated front continue to
propagate southeastward and south respectively.

&&

.SHORT TERM...(Tonight through Wednesday night)
Issued at 220 PM MST Tue Dec 6 2016

Some light snow is beginning to fall over the peaks with extensive
cloud cover after a decent lull occurred overnight through the
morning hours as expected. Latest model guidance indicates a later
push with the timing of the cold front arrival and trough passage
but overall message remains the same. Snowfall will begin to
increase in intensity over the northern and central Colorado
mountains late this afternoon into the evening with the cold front
beginning its push southward after 5 pm MST/00z. A low level
circulation center is evident in the 850-700 mb streamlines over
northwest Colorado late this afternoon and evening with the area
of convergence directed towards the central Colorado mountains.
This circulation center then shifts towards east-central Colorado
by midnight. The upper level jet will round the base of the trough
through southwest Colorado which puts the central Colorado
mountains in the left exit region and favored region for best
dynamics and lift. These ingredients all point towards the central
Colorado mountains (Grand Mesa, Elks and West Elks) seeing the
best potential for heavy snowfall. H5 winds of 50 to 60 kts still
indicate areas of blowing snow occurring over the higher
elevations through tonight.

A noticeable wind shift from west to northwest will occur with
the passage of the cold front which will pass through the central
mountains between 8 pm MST/03z and 11 pm MST/06z. Snowfall
intensity will be enhanced at this time in the vicinity of the
front with the focus once again over the central mountains and
along the I-70 corridor between Glenwood Springs and Vail Pass.
Snowfall will continue in a favored orographic northwest flow
favoring northwest facing slopes of the northern and central
Colorado mountains through early Wednesday morning before tapering
off to very light snow after 6 am. Some snow will also occur in
the San Juans and along Cerro Summit but accumulations appear to
be only a couple inches and not enough to warrant any highlites at
this time. It all depends on how far south this cold front
reaches, but with the upper level jet directly over the San Juans,
this does not favor the San Juans for significant accumulations.
However, winds will be strongest over the higher elevations of the
San Juans tonight as they remain directly under this upper level
jet where winds could gust in the 50s and 60s at the higher
peaks. Remaining Winter Weather Advisories remain on track so made
no changes there.

Much colder air will move into the region with this cold front and
trough passage tonight into Wednesday morning with H7 temps
dropping to -12C down south and -20C over northwest Colorado. This
translates to low temperatures tonight in the single digits and
below zero in the mountains and northern valleys with teens in the
lower central and southern valleys. Highs on Wednesday will
struggle to make it above freezing in this airmass of arctic
origins at most locations with the exception of southeast Utah
valleys as the cold air never quite reaches this area. Wednesday
night into Thursday morning will see the coldest temperatures so
far this season with the arctic air mass in place and good
radiational cooling due to clearing skies. Low temperatures
Wednesday night into Thursday morning will be some 15 to 20
degrees below zero in the higher valleys along the divide and
those areas prone to colder temps and stronger inversions (i.e.
Craig, Steamboat, Gunnison, Vail, etc.). The lower central and
southern valleys will see lows in the teens and single digits with
lows below zero in the mountains. Lowered temperatures well below
the blended guidance from tonight through Wednesday night as it
still was not cold enough, to better fit expectations and be
closer to the colder MAV guidance.

.LONG TERM...(Thursday through Tuesday)
Issued at 220 PM MST Tue Dec 6 2016

Temperatures remain cold on Thursday with the cold, arctic airmass
still in place but will be a few degrees warmer than Wednesday as
a flat ridge of high pressure works its way into the region from
the southwest. Temperatures will be closer to seasonal by Friday
as H7 temps increase to 0C near the Four Corners and -5C up north.
A southwest flow will shift to westerly and draw in some moisture
off the Pacific as specific humidities increase between 3 and 4
g/kg. Snow looks to begin again over the western Colorado
mountains by Thursday evening and continue on and off again
through Saturday night into Sunday morning as embedded
disturbances move through this moist, westerly flow. Some favored
upslope mountain areas could see a prolonged snow event but timing
these waves appears problematic given model differences unable to
resolve those details with great accuracy. Regardless, looks like
an unsettled period with snow in the mountains and milder
temperatures in the valleys closer to or a little above seasonal.
Colder air moves in again Saturday evening into Sunday morning as
a more distinct trough moves through the area. Drier conditions
expected after this trough passage with the moist, westerly flow
returning Monday afternoon into early next week. This moist,
westerly flow off the Pacific looks to have the makings of an
atmospheric river event. Keep an eye on the forecast for changes
in this active weather pattern as details come into better focus.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 946 PM MST Tue Dec 6 2016

As of 05z, a cold front was draped across western Colorado along
and just south of the I-70 corridor. A wind shift from the west-
southwest to west-northwest was been noted in GJT VAD wind
profile below 12kft over the past few hours as the front crossed
the Grand Mesa. The wind shift will slowly progress to the surface
at valley locations over the next 6 hours. IFR to LIFR ceilings
and visibility will continue at ASE through the overnight.
Elsewhere, MVFR to at times IFR conditions are possible in central
and northern terminals with a snow shower or two possible. From
MTJ southward, IFR conditions are unlikely. Gusty westerly winds
will affect TEX through 12z this morning.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 6 AM MST Wednesday for COZ004-
     008>010-012-013.

UT...None.
&&

$$

UPDATE...NL
SHORT TERM...MDA
LONG TERM...MDA
AVIATION...MAC



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