Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 151104
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
504 AM MDT Fri Sep 15 2017

.SHORT TERM...(Today through Saturday)
Issued at 358 AM MDT Fri Sep 15 2017

The system which originated over southern California which then
lifted northeastward across Utah on Thursday had moved to southern
Wyoming early this morning. Though no longer a closed low, the
circulation center appeared as a well defined mid-latitude cyclone
in IR imagery, centered over the CO-UT-WY border triple point as
of 08Z. In keeping with the classic Carlson model, air was
descending south of the low center, curving in a clockwise
direction toward the center of the low. In response, skies had
cleared over southeast Utah and much of west-central and north-
central Colorado. Some clouds lingered over the San Juans and
Colorado`s central mountains beneath the weak (50-60 KT) jet
moving through the base of the trough. A few mountain sites
reported fog while the automated sensor at Wolf Creek Pass was
also reporting light snow in addition to the fog. Meanwhile, light
rain was falling from the comma head portion of the storm over
portions of northeast Utah and northwest Colorado according to
radar and confirmed by the KVEL ASOS.

Models indicated the system described previously will continue to
lift to the northeast this morning. The moisture wrapped comma
head will drift across northeast Utah and northwest Colorado
during this time bringing additional showers, some falling in the
form of snow above 10000 feet. Accumulations, however, will be
light and largely confined to areas above the timberline. The
subsident, dry airmass across the south will limit shower activity
and mostly sunny to partly cloudy skies will be the rule during
the early part of the day. A more vigorous trough system sweeps
toward the area from the northwest driving a cold front across
northeast Utah and much of northwest Colorado during the
afternoon. Precipitation will largely fall along the frontal
boundary, with embedded thunderstorms firing as the airmass
destabilizes with the heating of the day. Areas to the south will
mostly remain dry, though isolated moist convection cannot be
ruled out over the mountains.

The front will continue to move southeastward during the night as
the trough axis sweeps eastward across western Utah, stalling
along the northwest San Juan Mountains and the Abajo Mountains of
southeast Utah. Expect good dynamic lift on the leading edge at
the base of the trough, and therefore precipitation becomes likely
over areas north of the I-70 corridor overnight, with scattered
activity over the central mountains. Additional mountain snow is
likely in the northern mountains as snow levels fall to between
9,000 and 10,000 feet. At these elevations, snow is likely to
impact the high passes, though accumulations are expected to be
light. The trough axis pushes across western Colorado Saturday
morning and subsidence in its wake will bring drying and an end to
shower activity by early afternoon.

Cooler air has moved over the area in the past 24 hours, so expect
highs this afternoon will be below normal for the first time in a
while. This will also be reflected in cooler overnight lows tonight
which will then carry over into Saturday.

.LONG TERM...(Saturday night through Thursday)
Issued at 358 AM MDT Fri Sep 15 2017

Warm air advection will resume over the Four Corners on Sunday as
the trough of low pressure departs to the east. Isolated convection
will be limited to the higher terrain as a drier airmass filters in
from the west. The major story in the long range forecast will
center around the first large-scale west coast synoptic system. All
major global models indicate a potent 150-knot jet crashing ashore
in the PACNW through the day on Monday. This jet will drive a large
longwave trough into the central Rockies on Monday evening, which
will remain over the western CONUS through the end of the week. Both
GEFS and EPS ensemble guidance is in good agreement showing 500mb
height anomalies in the -2 sigma range during this time period.
The resultant weather for the central Rockies will be temperatures
running well below normal by as much as 15 degrees or so. In
fact, MEX guidance indicates highs in the GJT area in the mid-60s
by Wednesday afternoon. ECMWF ensemble MOS guidance is slightly
less bullish with the cold air, keeping temps in the low to mid-
70s for most of desert valley locations. Scattered showers will
accompany the cooler air as spokes of energy pivot across Colorado
and Utah through the week. The first will come on Tuesday evening
in northeast Utah and far NW Colorado. Another more potent
embedded shortwave looks to bring wet weather to the western slope
by Thursday. While clear differences remain between the
operational GFS/ECMWF/GEM in terms of specific shortwaves and
precipitation chances...confidence is very high for a cool and
fall-like weather pattern next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 504 AM MDT Fri Sep 15 2017

Showers will continue to impact KVEL this morning, with a chance
for additional showers and thunderstorms this afternoon and
evening. Elsewhere, MVFR CIGS will persist through 15Z at KGUC and
KEGE with KEGE also experiencing IFR VSBY in fog during this time.
Conditions improve over these sites with warming during the latter
part of the morning. Remaining TAF sites are unlikely to see much
weather and VFR conditions are expected through the next 24 hours.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...NL
LONG TERM...MAC
AVIATION...NL



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