Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 211910
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1210 PM MST Wed Feb 21 2018

.UPDATE...
Issued at 1210 PM MST Wed Feb 21 2018

After looking in detail at the southern valleys with this next
disturbance set to move through this evening into Thursday,
decided to issue a Winter Weather Advisory for the Animas Valley
including Durango and Bayfield (COZ022). Also, decided to
upgrade the San Juan River Basin, including Pagosa Springs
(COZ023), to a Winter Storm Warning. Time heights over Pagosa
Springs indicate very strong lift and deep low to mid level
moisture, extending through the dendritic layer. The southwest
flow is the perfect setup for this area, including Pagosa Springs
up to Wolf Creek Pass, to get sufficient snowfall, as it is
orographically favored. Emphasized in the WSW statement as well,
that even though the SW San Juans look to see advisory amounts in
the 5 to 10 inch range, the area east of Pagosa Springs in the San
Juans stand the best chance to see higher amounts above a foot,
up to 15 inches.

In regards to the Durango to Bayfield area, looks like solid
advisory with 2 to 4 inches for Durango and higher amounts east
of Durango towards Bayfield and Ignacio. The southwest flow would
favor the Bayfield area for up to 6 inches of snow expected. In
order for Durango to see better potential for snow, they would
need a more southerly component. This happens only in the lower
levels but for a brief period of time tonight around midnight
until the drier air comes in from the southwest Thursday morning,
keeping snow going in the eastern portion of the zone. Still think
advisory is warranted for this area as lift is still present,
although not nearly as strong as Pagosa Springs, and temps remain
cold enough with H7 temps cooling from -8C this evening to -12C
by Thursday morning. The best moisture also remains across the
southern valleys and SW San Juans with a respectable 0.3 inches of
precipitable water sneaking in this evening through Thursday
morning.

&&

.SHORT TERM...(Today through Thursday)
Issued at 400 AM MST Wed Feb 21 2018

Following a brief break in the action overnight, snow is expected
to develop over the San Juan Mountains from late this morning into
the afternoon as a trough along the West Coast begins to push
inland. Modest moisture will move into the Four Corners region
ahead of the trough where the right entrance region of the jet
will add dynamic lift to a weak overrunning regime. GFS appears to
go a bit overboard with the QPF given the modest moisture levels
in a weak warm advective environment. Consequently will continue
with scattered coverage over the southern and central mountains of
Colorado. Left isolated coverage in the northern mountains since
flurries can`t be ruled out there.

Snow becomes widespread across the San Juans tonight and spreads
into the central and northern Colorado mountains as the trough
rotates eastward across the Desert Southwest in concert with a
cyclonically curved jet aloft. Snow also becomes likely at KDRO
with better chances to the east at KPSO. The negatively tilted
trough sweeps northeastward across eastern Utah and western
Colorado Thursday morning with moisture and energy continuing to
favor southwest Colorado. The trough axis pushes east of the
Continental Divide Thursday afternoon resulting in a significant
reduction in dynamic lift. However, expect snow showers will
remain fairly numerous as the -30 deg C 5H cold pool moves
overhead yielding steep lapse rates. Model snowfall estimates
still in line with Winter Weather Advisory (WWA) for the southwest
San Juan Mountains and adjacent lower elevations in and around
Pagosa Springs. Though adjacent areas are likely to experience
accumulating snow, believe amounts will fall short of advisory
levels so made no additions to the current WWA`s already in place.

Temperatures will continue to be unseasonably cool today, though a
bit warmer than yesterday for most locations. Tonight, clouds and
moisture will result in more moderate lows. Modest warming will
continue for all but southwest Colorado on Thursday, though
afternoon highs are unlikely to recover to normal levels.

.LONG TERM...(Thursday night through Tuesday)
Issued at 400 AM MST Wed Feb 21 2018

Fairly active weather is expected through the extended period with
multiple weather systems projected to move through the area.
Thursday night eastern UT and western CO will be under the warm
advection ahead of strengthening trough over the Great Basin. This
trough will be a large one and cover much of the Rockies and the
Intermountain region into Friday. By sunrise Friday a strong cold
front should be moving through eastern UT and push through western
CO by sunset. Precipitation should be widespread, in the valleys
as well as the mountains, with -12 to -14 C at 700 mb over all of
western CO and eastern UT. The 700 mb trough should be east of the
Continental Divide by 03z, according to the GFS, so NW flow ought
to produce enhanced orographic snow on favored slopes from
Steamboat Spring to Durango and all parts in between.

Most of the area should clear out Saturday, except for a good
chance of precipitation continuing over in the NW flow over the
northern CO mountains. Then late Saturday night through Sunday
another trough drops through the forecast area from the northwest.
Monday and Tuesday look like the "in between days" as moisture
erodes over the area under strong zonal flow as the next strong
storm develops over the Pacific Northwest.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1028 AM MST Wed Feb 21 2018

Clouds and the potential for snow showers will be on the increase
across western Colorado and eastern Utah through midday Thursday.
Expect high CIGS to gradually drop to near ILS breakpoints at most
sites by 09Z tonight as a weak Pacific storm moves northeast
across the region. Mountains will become obscured across southwest
Colorado tonight with occasional obscuration into the central
mountains later tonight and Thursday. Areas of MVFR/IFR conds are
expected late tonight and Thursday morning at KDRO, KPSO and
KTEX.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory from 8 PM this evening to 6 PM MST
     Thursday for COZ019-022.

     Winter Storm Warning from 8 PM this evening to 6 PM MST Thursday
     for COZ023.

UT...None.
&&

$$

UPDATE...MDA
SHORT TERM...NL
LONG TERM...CC
AVIATION...JDC


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