Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 130421
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
921 PM MST Mon Feb 12 2018

.UPDATE...
Issued at 920 PM MST Mon Feb 12 2018

Latest Nested Nam showing bands of snow lifting across the I-70
corridor between now and 06Z this evening. Expect an increase in
snowfall across this area with another 1 to 3 inches possible from
DeBeque east to Eagle. Some accumulating snow also possible across
the Grand Valley, although amounts will be lighter, generally an
inch or less. Activity is expected to wind down after midnight
with showers ending in this corridor by 4 am or so. The entire
complex will shift north after midnight with snowfall increasing
across the Bookcliffs and areas north through the early morning
hours. All current highlights remain on track for now and will
continue to monitor latest radar, satellite and sfc reports for
future updates.

Side note: Freezing rain has been reported at the Rifle ASOS.
Calls to law enforcement indicate this is not the case with light
snow falling in town with amounts around an inch. Webcams support
the snow reports. Will continue to monitor this as well.

UPDATE Issued at 631 PM MST Mon Feb 12 2018

Convective showers really wreaking havoc with I-70 this evening
with bands passing over the higher dropping moderate to heavy
snowfall. Latest HiRes models continue to sweep new bands
northeast across the I-70 corridor through late this evening.
Snow accumulations already adding up to another 2 to 3 inches on
top of the snow that fell earlier today. With the sun now down,
highways are expected to remain treacherous for much of the night.
So went ahead and issued an advisory for this corridor from the
DeBeque Canyon east to Vail with another 2 to 5 inches of snow
possible through late tonight. Also lowered the elevation
restrictions for the central mountains and included zone 10
(Aspen/Vail areas) as snowboards are approaching low end advisory
criteria.

Now down south, several reports of snowfall ranging from 2 to 5
inches in and around the Durango area. At this point, the majority
of the snow looks to have fallen with main thoroughfares wet. Low
level flow will be shifting to the southwest over the next few
hours, which usually shuts down the orographic component and the
convective element will also be waning with the loss of solar
heating. Additional snowfall will only total an inch or two with
significant impacts to travel not anticipated in this area as
temps hover in the lower 30s.

&&

.SHORT TERM...(Tonight through Tuesday night)
Issued at 351 PM MST Mon Feb 12 2018

A closed mid level low over the Great Basin and associated trough
over the Dakotas is stretching energy across the region and has
allowed for significant southwest to northeast bands of snow to
develop across the region today. The energy shifts slightly
northward this evening and eventually leaving precipitation only
across the northern part of Utah and Colorado Tuesday morning.
Since this morning, a 90 kt upper level jet has entered the
region from the southwest and sheared the snow bands out a bit,
however spots of moderate to heavy snow will likely remain set up
through the evening hours from southwest to northeast.

There remains to be huge discrepancies in model solutions in the
short term forecast models. The models are having a difficult
time resolving the location of snow bands today. P-type has also
been difficult, with forecast models greatly waffling between rain
and snow below 7000 feet. In localized spots such as Redstone on
Hwy 133 have been receiving moderate snow for much of the day
despite being at 7200 ft. Other spots across the state remain
shadowed from orographics, or switching back and forth between
rain and snow. The feature we are most confident on tonight and is
the reason for issuance of additional winter wx advisories is the
shift in precipitation northward this evening and ending in the
Four Corners valleys tonight. Showers may continue over the high
terrain through Tuesday afternoon before a short break Tuesday
night. A rumble of thunder is not out of the realm of
possibilities through the evening.

.LONG TERM...(Wednesday through Monday)
Issued at 351 PM MST Mon Feb 12 2018

Southwest flow will remain over the area Wednesday as the remnants
of the current low pressure system affecting our area, spins just
off the southern California coast.  Breezy conditions will occur,
especially at mountain top level, as a 165 kt upper level jet
remains overhead. This SoCal low will pull moisture into Arizona and
up through the Four Corners region Wednesday evening into Thursday
morning, as it merges with a colder system diving down from the
Pacific Northwest. Specific humidities will be in the 3 to 4 g/kg
range under the milder pre-frontal environment with high snow levels
to start with. H7 temps will be in the -3 to -5C range Wednesday
evening, before lowering with the colder air post-frontal late
Thursday morning into the afternoon, as H7 temps drop into the -7 to
-12C range. This system, like the previous, also looks convective,
so anticipate heavier bands of precipitation setting up throughout
the event. These bands are hard to predict, but have a tendency to
cool temps rapidly and drop snow levels, changing any precipitation
over to snow and can overcome any orographic influence. The best
chance for precipitation will be Wednesday evening into Thursday
afternoon, with much drier air moving in behind the system by late
Thursday afternoon into the evening. At this time, it looks like an
advisory event for the mountains with roughly 5 to 10 inches
possible. Valley snowfall may all depend on where the bands set up.
Something we will have to look at more closely in the next few
model runs.

Dry northwest flow will set up on Friday with the flow shifting to
the west by Saturday ahead of yet another storm system.  This next
storm looks to occur Sunday into the coming work week, potentially
lasting through Tuesday. This system looks particularly interesting
as it is much colder than previous systems. While there is some mild
air ahead of it Sunday afternoon, H7 temps lower to below -8C up
north late Sunday afternoon, and -10 to -15C across the rest of the
forecast area by Sunday evening as the initial cold front drops
north to south through the day on Sunday. The trough then digs
through the Great Basin on Monday, maintaining a southwest flow
across the area until trough passage. The models are in good
agreement overall through Sunday, but differ with regards to how to
handle this trough. The GFS is further east, bringing the trough
through Utah and Colorado before exiting late Monday night into
Tuesday morning.  The ECMWF on the other hand is deeper and further
west, bringing the trough base through Nevada and keeping snow and
cold air in place through Tuesday evening. This means all areas
would have potential to see some snowfall as the cold air is timed
well with the precipitation and moisture as specific humidities
remain in the 2.5 to 3.5 g/kg range until trough passage, which
could be Monday evening/Tuesday morning or Tuesday evening/Wednesday
morning depending on which model is the winner. This storm is still
a ways out and lingers beyond this forecast package, but bottom line
is that these series of storms are good news for our snowpack, so
that is something to be hopeful for.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 920 PM MST Mon Feb 12 2018

Widespread rain and snow will continue across much of eastern Utah
and western Colorado through early Tuesday morning. Expect
widespread CIGS below ILS breakpoints with areas of IFR and MVFR
Conditions near showers. Mountains will be frequently obscured in
showers. CIGS start to break up by midday Tuesday with showers
becoming more scattered in nature. Expect VFR conditions to return
by mid-afternoon with occasional dips into the MVFR category at
the higher valley sites as showers pass.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 6 AM MST Tuesday for COZ002>005-
     007>010-012-013-017>019-023.

     Winter Storm Warning until 6 AM MST Tuesday for COZ019.

UT...None.
&&

$$

UPDATE...JDC
SHORT TERM...JAM
LONG TERM...MDA
AVIATION...JDC



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