Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 190534
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1034 PM MST Sat Feb 18 2017

.UPDATE...
Issued at 841 PM MST Sat Feb 18 2017

Latest IR satellite imagery and automated surface observations show
smattering of mid and high level clouds moving north across the
region in the southerly flow ahead of upper level trough near
CA/AZ border. Radar returns must be showing clouds or very light
precipitation for area web cams and surface stations not
indicating much if any precipitation reaching the ground. Short
term and high resolution models also backed way off on expected
precipitation overnight. Looks like we will need to wait for
stronger forcing and increased available moisture associated with
upper level trough to approach from the southwest early Sunday
morning before precipitation returns in earnest. San Juan Mtn area
and eastern UT higher terrain to be impacted first Sunday
morning...then points farther north and east during the afternoon.
Evening update to forecast includes decreasing PoPs and precip
amounts overnight and blending in with expected increasing trends
Sunday.

&&

.SHORT TERM...(Tonight through Sunday night)
Issued at 329 PM MST Sat Feb 18 2017

Moist southerly flow is in place over the region upstream of the
split flow along the left coastline. The southern split has formed
a closed low which is dropping southeast into the northern Baja
this afternoon where the biggest height falls were on this
morning`s 12Z H5 hand analysis. The strong Pacific jet is now
reforming on the upstream side of the trough over central
Colorado. Upper level downglide associated with the jet is
producing somewhat of a dry slot in the dendritic layer and this
has shut off much of the precipitation over the southern
CWA. The erosion of moisture in the ice bearing layer looks to
limit snow production over much of Colorado this evening through
tomorrow morning. The interface of deeper moisture will be near
the eastern Utah border and showers will continue to run northward
through that region overnight. The NAM did have a more defined
area of mid level frontogenesis and a strip of heavier
precipitation from the western end of the Paradox Valley through
the northwest corner of the state. No other model suggested this
and the 18Z NAM has backed off this as well. The Baja low will
begin to lift northeast after midnight and track south of the 4
Corners into W.Kansas through Sunday night. It will be phasing
again with the northern stream tomorrow afternoon and this will
create decent large scale ascent over western Colorado and
precipitation should quickly expand through the late morning and
early afternoon hours. The southern Utah and Colorado mountains
will continue to be favored by southerly flow as these features
merge. As the afternoon rolls on however cold air advection behind
the trough and a well defined surface front with cyclogenesis on
the front range will provide good low level support and
instability for strong ascent. This will be the favored period for
this storm and snow rates should allow snow to quickly pile up
over the central high mountains by Monday morning. Temperature
profiles still indicate the best snow accumulations above 9500
feet even though snow will make it to the valley floors where
melting on contact will limit initial accumulations. The strong
ascent will have ended by sunrise Monday with orographics taking
over as dry air filter in.

.LONG TERM...(Monday through Saturday)
Issued at 329 PM MST Sat Feb 18 2017

Flat transitory ridging with warm advection aloft will be moving
through on Monday. The drier air and more stable atmosphere will
help end the snow with a decent day forming up. Inversion don`t
appear to have much time to set up as the flow aloft will be
backing to the southwest and increasing as the next batch of
Pacific moisture and energy move into the West. Some of this
mositure moves into our northwest CWA Monday night with some very
light snow shower possible over the high peaks of the northern
mountains.

High pressure over the Baja Peninsula region will build over the
Desert Southwest on Tuesday with a fast southwest flow over the
region. Mid and high cloud cover will pass through the region as
several disturbances ride up and over this ridge affecting the
Pacific Northwest and northern Rockies states. Sufficient mixing
looks to occur Tuesday afternoon for mild temperatures with above
normal highs. Some convective showers look to develop over the
favored higher terrain late Tuesday afternoon and again Wednesday
afternoon under the influence of this pre-frontal mild southwest
flow. Wednesday`s highs also look to reach above normal values
until the showers and cloud cover become more extensive late in
the day due to an upper level trough moving onto the West Coast.
The potential exists for some isolated thunderstorms Wednesday
afternoon as low CAPE values in the 200 to 300 J/kg range and
steep lapse rates around 8 degrees C/km combine with added
moisture and lift to generate that potential.

This activity on Wednesday is in the pre-frontal stages of an
upper level trough that will be moving across the region Wednesday
night through Thursday with the associated cold front passing some
time Wednesday night into Thursday morning. The GFS and EC differ
by about 6 hours in terms of this frontal passage and the arrival
of the coldest air, with the GFS being more progressive. Snow
levels will be fairly high above 10,000 feet Wednesday afternoon
with the southwest facing slopes of the southern and central
mountains favored for snowfall. Temperatures drop sufficiently
behind this cold front Wednesday night into Thursday morning with
H7 temps dropping into the -10C to -13C range by Thursday
morning and afternoon. This should act to lower snow levels down
to most valley floors with rain mixing in with or changing over to
snow. The upper level flow will shift to the northwest by Thursday
morning and afternoon with the northwest facing slopes of the
western Colorado mountains favored for the best snowfall. The
upper level trough departs the region Thursday evening with a
lingering northwest flow through Friday morning, providing some
continued snow for the northern mountains under favorable
orographics. Improving conditions should be seen throughout the
day on Friday into Saturday with cooler temperatures closer to
normal as a shortwave ridge slides into place. Another storm
system dives down the California coast on Saturday and is showing
signs of splitting as it approaches us, but confidence is low Friday
and beyond as models have been showing some inconsistencies.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1035 PM MST Sat Feb 18 2017

Aside from ocnl mtn obscurations overnight conditions should be
largely VFR. The exception being KVEL where low clouds and light
fog will likely form. Ceilings will lower across southern areas
around 12Z as precipitation also spreads in. Flight conditions
will deteriorate...with many TAF sites falling below ILS
breakpoints...southwest to northeast Sunday with mtns obscured
and periods of moderate to heavy precipitation over the higher
terrain. Thunder is also possible generally from KASE southward in
the mid to late aftn. Some improvement likely after 00Z across
eastern UT and toward KGJT.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory from 11 AM Sunday to 7 AM MST Monday for
     COZ009-010-012-013.

     Winter Weather Advisory until 6 PM MST Sunday for COZ018-019.

UT...Winter Weather Advisory until 6 PM MST Sunday for UTZ028.

&&

$$

UPDATE...BEN
SHORT TERM...15
LONG TERM...15/MDA
AVIATION...BEN



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