Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 171742

Area Forecast Discussion
National Weather Service Grand Junction CO
1042 AM MST Fri Feb 17 2017

Updated Aviation section

.SHORT TERM...(Today through Saturday)
Issued at 404 AM MST Fri Feb 17 2017

Mid and high clouds spread across the region overnight in a broad
area of difluent flow aloft ahead of the deep trough of low
pressure over the eastern Pacific. Latest observations indicated
cloud bases in the 15 to 20 kft range while radar showed areas of
virga. Expect very little moisture is reaching the surface but
clouds have served to buoy temperatures during the night where
readings were running 5 to 10 degrees F warmer than 24 hours
earlier. Gunnison remained the exception where readings were 3
degrees F cooler 3 AM MST.

Moisture will deepen over the area today as the eastern Pacific
trough approaches the California coast. Lift provided by weak
overrunning beneath a broad area of difluence/divergence aloft
will begin to generate precipitation later this morning, but
primarily during the afternoon. Lapse rates around 4.5 to 5
deg/km which points to light stratiform precipitation. That said,
downplayed decidedly ambitious QPF totals coming from the
00Z/Friday GFS solution in favor of more measured ECMWF and NAM
output. Unseasonably mild temperatures will limit snowfall
accumulations to areas generally above the 10000 foot level with
rain below 9000 feet. Though temperatures will be mild, expect
clouds and diabatic cooling to bring a degree or two of cooling
relative to yesterday`s highs. However, expect the inverted
Gunnison valley to remain below freezing again today.

The weather regime described in the previous paragraph will persist
tonight as the Pacific low finally moves ashore late tonight, then
slowly eastward over southern California Saturday. Weak CAA will
begin as the colder air encircling the low eases into the area.
This combined with diurnal cooling will bring snow down to mountain
bases by Saturday morning. Overnight lows will be more moderate
than earlier in the week as the blanketing effect of cloud cover
inhibits radiational cooling. Since flow continues from the south,
don`t expect more than slightly cooler temperatures on Saturday.
This will cause snow to retreat back to the higher elevations
again during the afternoon with snow shifting back over the rain
on the lower slopes.

Snowfall accumulations during the next 36 hours will be fairly
light, generally ranging from 3 to 6 inches with some higher
amounts in the western San Juan Mountains and in the higher
reaches of the La Sal and Abajo Mountains. Consequently, have no
plans to hoist winter weather highlights with this package.

.LONG TERM...(Saturday night through Thursday)
Issued at 404 AM MST Fri Feb 17 2017

Forecast models indicated trough will hold together structure
wise but losing some of its oomph and moisture as it passes
through the mountains in Nevada and Utah Saturday night. Forecast
PoPs are on the low side as thinking between the southerly flow
ahead of this trough will keep activity orographic and showery,
but these may be on the low side. 700mb temps still remaining
between 0C and -3C over the weekend which will keep the heaviest
snow above 10000 ft with some snow as low as about 8000 ft. The
elongated trough eventually makes it to eastern Utah and western
Colorado with the best forcing mechanisms and moisture on Sunday
with its associated cold front. This should be the big day for
precipitation, and will be our biggest impact day for travel. Snow
level on Sunday will likely drop to about 6000 ft by end of day,
but by then the trough will have passed and conditions will begin
to stabilize. A break in the weather is expected Monday through
Tuesday before the next possible storm moving in next Wednesday.
Forecast models have flip flopped a bit from run to run but have
been indicating a strong cold front associated with a closed low
passing through the intermountain west sometime between Wednesday
and Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1042 AM MST Fri Feb 17 2017

Cloud ceilings are beginning to drop with the approach of a moist
Pacific multi-day weather system moving into the region. Virga
will be seen at many TAF sites, but precip really only expected
to fall over the southern and central CO mountains. Winds may pick
up this evening with virga creating some mixing, but overall winds
will remain light throughout the day today with the exception of
locations above 8000 ft ASL. Cigs will continue to lower
throughout the day and precip will begin this evening across the
higher terrain.




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