Area Forecast Discussion
Issued by NWS Grand Junction, CO

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811
FXUS65 KGJT 052255
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
355 PM MST Mon Dec 5 2016

.SHORT TERM...(Tonight through Tuesday night)
Issued at 355 PM MST Mon Dec 5 2016

Current webcam and surface observations indicate snowy conditions
occurring over Rabbit Ears Pass and Vail Pass with a noticeable
wind shift from west to northwest across northwest and west-
central Colorado. This would be a good indicator that the initial
front and trough have passed through the region. All short term
models are in very good agreement that snowfall will continue over
the northwest Colorado mountains through this evening before
winding down by midnight. After midnight, a lull in snowfall will
occur as one trough exits and another approaches from the Pacific
Northwest. The main concern overnight through Tuesday morning is
going to be the winds as H5 winds are in the 50 to 60 kt range
with H7 winds of 15 to 25 kts. Blowing snow is likely to continue
over the mountain passes with gusts to 50 mph at times as well as
some light orographic snow showers continuing intermittently in a
strong northwest flow. Because of this reason, elected to let the
winter weather advisories for the northern Colorado mountains
continue through Wednesday morning despite the lull in significant
snowfall accumulation. Specific humidities drop to less than 1
g/kg after midnight tonight which should put an end to snowfall in
most places.

Snowfall will pick up again over the northern mountains by late
Tuesday morning and spread southward into the central mountains
and along the I-70 corridor by late Tuesday afternoon. This second
and more potent trough and associated cold front will quickly move
through the region from north to south Tuesday afternoon and
evening, bringing some bands of heavy snow in the vicinity of the
front. The upper level jet of 150 kts behind this trough will
extend from northern California to just south of the Four Corners
by Tuesday afternoon, translating to 130 kts at its core by the
time it reaches the Four Corners. The jet will lift into southwest
Colorado by Tuesday evening, which will limit how far south the
front extends. This will keep the best potential for heavy
snowfall in the left exit region of this upper level jet as far
south as the Grand Mesa and West Elk and Sawatch mountains in
west-central Colorado. Specific humidity values will climb to
around 2.5 g/kg Tuesday afternoon and evening, which is not
sufficiently moist but will be enough for advisory criteria
snowfall being reached in the northern and central mountains of
western Colorado. Expecting snowfall amounts in the range of 5 to
10 inches for the northern mountains with 4 to 8 inches in the
central mountains and 2 to 4 inches along the I-70 corridor from
Glenwood Springs towards Avon. Higher amts are expected towards
Vail and Vail Pass with totals of 5 to 10 inches expected.
Potential for significant accumulating snowfall should wind down
after midnight as the front exits the region, with snowfall
continuing through Wednesday morning before tapering off to light
orographic snow showers. Confidence was high enough to include the
Grand Mesa and West Elk and Sawatch mountains in the Winter
Weather Advisory for 4 to 8 inches, as well as the I-70 corridor
from Glenwood Springs to Avon for 2 to 4 inches, beginning Tuesday
afternoon and ending Wednesday morning.

The biggest message to convey with this storm aside from the
snowfall is the cold arctic airmass that will be entrenching the
entire region Tuesday evening into Wednesday, behind the strong
cold front and trough of low pressure. H7 temps drop to -14C near
the Four Corners and -20C across the far northern zones near the
Wyoming border by Tuesday night/Wednesday morning. This
translates to low temperatures Tuesday night/Wednesday morning in
the single digits and teens amongst most valley locations with the
mountain locations seeing lows in the single digits to below zero
readings. Lowered temperatures below guidance as they seemed a
bit too mild given the change in airmass.

.LONG TERM...(Wednesday through Monday)
Issued at 355 PM MST Mon Dec 5 2016

Some orographic light snow showers will continue through
Wednesday morning under northwest flow as the trough departs the
region, with a cold, arctic airmass in place. Daytime highs on
Wednesday will struggle to get above freezing in most locations
with highs on the order of teens in the northern valleys and 20s
in the central valleys. The mountain locations will see highs on
Wednesday in the single digits and teens. Wednesday night looks to
see some of the coldest temperatures so far this season with
most valley temperatures in the single digits with exception to
the northern Colorado valleys and Gunnison basin seeing below
zero readings. Lowered temperatures below guidance once again for
Wednesday and Wednesday night as they seemed a bit too mild given
the change in airmass.

Snow will return to the picture for mainly the western Colorado
mountain areas late Thursday through the weekend as a strong
moist, westerly flow advects some Pacific moisture into the
region with specific humidity values increasing up to 4 g/kg.
Some embedded disturbances will move through this flow from time
to time. This flow would favor west facing slopes and in
particular the northern mountains surrounding the Steamboat
Springs basin. Could be a prolonged snow event for some mountain
areas with not much break in the action. The upper level flow will
be strong enough to help mix the atmosphere so not anticipating
valley inversions to hold for very long with temperatures
returning to near normal values.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1039 AM MST Mon Dec 5 2016

VFR conditions expected at all terminals late this morning into
early afternoon. Forecast challenges Monday afternoon include
lowering ceilings with light snow in the northern mountains
through 06z. Snow showers unlikely to affect most terminals,
however may make it into ASE around or just after 00z, lowering
visibility for a time. A few vicinity snow showers elsewhere in
Colorado along and north of I-70. VFR conditions expected
elsewhere through the evening.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory from 3 PM Tuesday to 6 AM MST Wednesday
     for COZ008-009-012.

     Winter Weather Advisory until 6 AM MST Wednesday for COZ004-013.

     Winter Weather Advisory from 6 PM this evening to 6 AM MST
     Wednesday for COZ010.

UT...None.
&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...MAC



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