Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 260404 AAA

Area Forecast Discussion...UPDATED
National Weather Service Grand Junction CO
904 PM MST Sat Feb 25 2017

Issued at 903 PM MST Sat Feb 25 2017

Main reason for the update this evening was to issue a Winter
Storm Watch for the central and southern mountains of eastern UT
and western CO for Monday and Tuesday. Models are trending higher
with precip ahead of strong upper level trough and cold front.
Moist and strong southwesterly flow intersecting higher terrain
over a prolonged period should produce copious amounts of snow.
Fringe areas like the Flat Top mountains and southern foothills of
CO south of the San Juans will have to be watched closely as
well and may need to be added to the watch later. Otherwise, the
mountains in the watch have the potential for 1 to 2 feet of snow
with considerable blowing and drifting snow adding to the hazard.

As for tonight, westerly flow with various weak hortwaves riding
in it will continue to over the southern two thirds of the area
producing light snow showers and light accumulations, mainly for
the mountains. Very dry low levels will evaporate most of the snow
for valleys, but cannot rule out a few flurries.


.SHORT TERM...(Tonight through Sunday night)
Issued at 215 PM MST Sat Feb 25 2017

A broad upper level trough remains positioned over the western
CONUS this Saturday afternoon. Several pieces of energy will
rotate around this main feature through the weekend and into early
next week, keeping the weather unsettled over the central Rockies.

Cloud cover and some very light mountain snow showers will remain
in the forecast this evening. Support for precipitation, coming
in the form of a broadly stretched vorticity maximum advecting
from the west (co-located with a saturated dendritic growth
layer), will continue through midnight. The axis of a shortwave
trough at 500mb will pass after midnight, and in addition to the
passage of a weak surface front, precipitation will gradually come
to an end by Sunday morning. Not expecting much in the way of
significant accumulations with this system. The central mountains
will do best through the evening and overnight, with as much as
2-4 inches in the Elk and western Sawatch mountains. An inch or so
is possible in the Parks, Flat Tops, Grand Mesa, and San Juans.

Sunday will turn out mostly dry across the region, except in the
highest terrain of the central and southern Colorado mountains
where a few snow showers will persist. Sunday highs have been
nudged upward a degree or two in this package to reflect a
slightly drier model consensus, with a bit little thinner overall cloud
cover in the valleys as well.

By Sunday evening, clouds will thicken ahead of our next storm
system. A more potent 500mb shortwave trough will dive southward
into the Pacific northwest, as southern-stream energy arrives
along the southern California coastline. Consensus between various
12z forecast models indicate that eastern Utah and far western
Colorado will remain dry until after midnight early on Monday

.LONG TERM...(Monday through Saturday)
Issued at 215 PM MST Sat Feb 25 2017

Precipitation will overspread the region after midnight on Monday
morning, with light snow beginning first in the eastern Uintas,
La Sals, and far western Colorado mountains. 700mb temperatures
will be cold with this event, ranging from -6C near the Four
Corners to near -10 along the Wyoming border. These temperatures
will support an all-snow event to start for the entire forecast
area. Valleys will warm up by mid- morning on Monday, with light
snow showers changing to rain showers in locations such as the
Grand Valley, Montrose, and valleys south of the San Juans. Most
of these locations will see liquid precipitation during the
afternoon hours Monday with temperatures topping out in the mid-
40s. Wind gusts will also be notable with this event as a strong
southern stream jet streak moves over the central Rockies by
Monday evening. 300mb winds are progged to be in the 140-160kt
range at this time. It is entirely possible that this could
translate to wind gusts of at least 50-60mph in the higher terrain
from Monday evening through early Tuesday morning.

By midnight Tuesday, a cold front will work south of the Wyoming
border and into northern Utah and Colorado. Precipitation will be
maximized along and just ahead of this front, and it is at this
time that travel conditions will be the worst during this entire
winter storm. Flow will turn northwesterly behind this front on
Tuesday morning, with the intensity of the snowfall slackening a
bit by Tuesday afternoon. Valley locations will cool on Tuesday
afternoon as the front passes, however precipitation will likely
remain liquid in the Grand Valley by the time it ends around
sunset. The axis of both the mid and upper level troughs will
pass by midnight Wednesday, with a few snow showers lingering in
the higher terrain through daybreak Wednesday. Regarding snow
totals, the central and southern mountains look like they will do
best with this storm, with at least a foot of snow possible in
the Elks, Sawatch, Grand Mesa, and San Juan Mountains. The
northern Parks, Flat Tops, La Sals, and Uncompahgre Plateau could
approach a foot as well. Gunnison, Durango, and the southern
valleys will also see snowfall, with a general 2-5 inches
falling...more the higher in elevation you go. Highlights for this
event will need to be considered in the next few forecast

A tranquil weather pattern returns to the central Rockies for mid
and late next week as a broad upper-level ridge moves in over the
western CONUS. A few snow showers may linger in the northern Park
range on Wednesday afternoon and evening. Overnight low temperatures
have been adjusted downward in this forecast package for each
morning late next week. Fresh snow and clearing skies should
allow for good radiational cooling conditions. Deterministic
forecast guidance as well as GEFS and ECMWF ensembles are in
excellent agreement showing dry weather over the entire forecast
area from Thursday through Day 7 (next Saturday).


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 431 PM MST Sat Feb 25 2017

Scattered snow showers will bring temporary IFR conditions to the
mountain areas including the KASE and KTEX terminals through at
least midnight. The western TAF sites should see little problems
with VFR dominating the forecast and only a flurry or very light
snow shower moving through. The system bringing this weather is a
quick mover so expect improving conditions by Sunday morning over
most areas. Winds will be lighter tomorrow behind this system. In
the Roaring Fork valley however northwest flow will dominate with
winds 33008kt possible by mid to late afternoon.


CO...Winter Storm Watch from Monday morning through Tuesday evening
     for COZ009-010-012-017>019.

UT...Winter Storm Watch from Monday morning through Tuesday evening
     for UTZ028.



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