Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 212235
AFDGJT

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

.SHORT TERM...(Tonight through Thursday night)
Issued at 335 PM MST Wed Feb 21 2018

Southwest flow will increase this evening ahead of a long wave
trough developing over the western states. A disturbance will move
through the Four Corners region in southwest flow ahead of this
long wave trough. Snowfall will begin this evening, with the
focus for heaviest precipitation over southwest Colorado, in
particular the SW San Juans and the southern valleys from Durango
to Pagosa Springs, where precipitable water values increase to
around 0.3 inches. This region remains in the favored right
entrance region of an upper level jet moving through New Mexico
and Texas. Looking at time heights over various points in
southwest Colorado, the most impressive is over Pagosa Springs.
The 12Z guidance was coming in with very strong lift this evening
through Thursday morning over Pagosa Springs, with omega (vertical
motion) values of -130 mb/hr through a deep dendritic layer in a
very favorable southwest flow. In fact, the 18Z guidance has come
in stronger with omega peaking at -160 mb/hr! Thinking that the
San Juan River Basin, including Pagosa Springs, up towards Wolf
Creek Pass will do very well with snowfall, which is the reason
for the upgrade to a Winter Storm Warning for the Pagosa Springs
area (COZ023) where 4 to 8 inches of snow is expected.

The lift is still present, although much weaker over Durango, so
thinking advisory amounts look reasonable with 2 to 4 inches
expected. Bayfield on the eastern end of the Animas River Basin
tends to do better in southwest flow, so thinking higher amounts
up to 6 inches are likely near Ignacio and Bayfield. As far as the
SW San Juans, advisories still look on track there for 5 to 10
inches with locally higher amounts. The eastern portion of the San
Juans, areas east of Pagosa Springs up to Wolf Creek Pass, look
to do better with amounts approaching 15 inches in spots. This all
due to favorable southwest orographics and impressive lift, with
the deepest moisture focused here. So, indicated in the grids and
WSW statement of higher amts east of Pagosa Springs. Other
mountain areas will see snowfall with this passing wave, but look
to stay below advisory amts at this time. Even some other valleys
will see some light snowfall through the night with minimal
accumulations expected. H7 temperatures remain in the -8C to -12C
range throughout this event, so plenty of cold air for all snow.

Snow will become more scattered in nature Thursday afternoon in
this unsettled southwest flow with periods of light snow here and
there. Breezy conditions look to occur Thursday afternoon with
winds picking up a bit more Thursday evening ahead of a stronger
trough dropping into the Great Basin from the Pacific Northwest.
More on this in the Long Term section.

.LONG TERM...(Friday through Wednesday)
Issued at 335 PM MST Wed Feb 21 2018

Snowfall looks to begin Friday morning as a deeper long wave
trough digs into the Great Basin, with a 110 kt jet rounding the
base of the trough through the Desert Southwest up through the
Four Corners region. Winds will be breezy on Friday with H7 winds
of 20 to 30 kts ahead of the cold front set to move through the
region Friday evening. H7 temps will remain cold still, with a
little warming ahead of this system but not by much. Temps get as
warm as -8C over southwest Colorado, with -10C to -12C elsewhere.
This means that precip type should remain all snow with this event
with not much changeover to rain throughout. Specific humidity
values increase to a modest 2 to 2.5 g/kg with a drop to 1.5 g/kg
or less by Saturday morning.

This system for Friday morning into Saturday morning looks very
convective, with banding likely setting up throughout, which plays
havoc on forecasting snowfall amounts for valleys especially.
Overall, looks like a solid advisory level storm for all mountain
areas with 5 to 10 inches on average although some spots could see
over a foot of snow depending on where the banding sets up. The
valleys look to get snowfall as well but how much still remains
the question. This will again all depend on the banding which is
something that will evolve over time and need to be looked at
closely throughout this event. H7 temps drop behind the cold front
Friday night, with temperatures well within the efficient
dendritic layer in the realm of -12C to -18C and the flow shifting
around to the northwest. So expecting a very cold night Friday
night into Saturday morning.

As this system shifts eastward over the Front Range by Saturday
morning, our snowfall looks to quickly come to an end as a surface
low deepens over SE Colorado and tracks northeast into the Plains,
with the H7 low also deepening over the Front Range and moving
into NW Kansas and SW Nebraska. This is also known as lee side
cyclogenesis and when this happens, this tend to take all the
energy and snow with it. Therefore, after the snow falls Friday
morning through Saturday morning, expecting a break in the action
and relatively drier day on Saturday. But wait, that`s not all...the
fun isn`t over yet as yet another storm system moves in on the
heels of the previous on Sunday. This one looks to favor the
mountains of western Colorado.

A ridge of high pressure looks to build in across the Plains, with
southwest flow returning by Monday into Tuesday. H7 temps also
take a climb towards -3C to -8C with winds increasing into the 30
to 40 kt range by Monday afternoon. This will result in a dry,
milder and breezy day across the area, with temperatures taking a
return back to near normal values. The warming however may be
tempered a bit, depending on how much snow falls, but should be a
drier day. The pattern does remain progressive as yet another
system dives down the west coast and moves into the Four Corners
region as an open wave trough by late Tuesday into Wednesday,
bringing more much needed precipitation to the region. Models
overall are in fairly good agreement through Monday.

&&

.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.
&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JDC


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