Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 171732

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1132 AM MDT Sun Sep 17 2017

.SHORT TERM...(Today through Monday)
Issued at 330 AM MDT Sun Sep 17 2017

Flow aloft shifted to the southwest in the wake of the storm which
passed through the area Friday into Saturday. Subsidence and
drying brought airmass moisture content down to 0.32 of an inch
according to the 00Z/KGJT sounding. However, moisture is expected
to rise to near one-half inch ahead of a shallow trough which was
pushing eastward across the Great Basin early this morning, and
near 0.6 of an inch over southwest Colorado during the afternoon.

The trough axis will move across western Utah during the afternoon
with mid-level vorticity advection providing synoptic scale lift
over northeast Utah. Meanwhile, jet-level divergence beneath the
right entrance region of a 75 KT jet is expected to provide lift
over southwest Colorado. In these areas dynamic lift, mid-level
moisture advection and diurnal instability will combine to
generate scattered showers and thunderstorms, mainly over the
mountains, from late morning into the early morning hours Monday.
Though forcing not as favorable over the northern mountains,
instability alone should be sufficient for isolated to widely
scattered moist convection.

As mentioned previously, showers and thunderstorms will linger
over the forecast area during the evening and into the early
morning hours as upward forcing persists as the shallow trough
passes overhead. Subsidence in the wake of the trough should bring
an end to showers before sunrise Monday. Flow again shifts to the
southwest as another Pacific trough deepens over the Pacific
Northwest. This time, an influx of drier air will inhibit showers
and thunderstorms over eastern Utah and western Colorado on

Warm air advection in the southwest flow both today and Monday
will allow temperatures to return to near normal levels.
Predictably, overnight lows will also moderate, especially across
the north where Craig reached the freezing mark as of 3 AM MDT.

.LONG TERM...(Monday night through Saturday)
Issued at 330 AM MDT Sun Sep 17 2017

The long term starts out with dry zonal flow over the area and a
tightening gradient aloft as a strong trough approaches from the
northwest. A 120 kt jet will begin to nose its way into northwest
UT Tuesday morning. The jet axis in the GFS rides along the
northern CO border Tuesday afternoon as the upper trough drags
across northern UT and CO. Not much moisture is projected to
accompany this trough so precipitation chances should be low.

Wednesday and Wednesday night will be dry, but then the strong
trough from the PACNW will move through the great basin. As this
trough moves east it begins to show evidence of being our first
split trough of the season, which should the turn to a more
Autumn-like pattern. Through Thursday the winds aloft will back
to the southwest and increase. As the surface from moves through
the Thursday evening the strongest south- southwest winds will
accompany it. Precipitation associated with the front should focus
over northern and central CO and UT with less chance of
precipitation over southern areas. Moisture will side east Friday
and Saturday though SW flow remains over the area and the main 500
mb trough will still be over the Great Basin.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1132 AM MDT Sun Sep 17 2017

Atmosphere is moderately unstable already late this morning with
scattered and storms popping over the southern mountains/valleys
with more isolated convection to the north. KTEX/KASE/KGUC appear
to have the highest probability of impacts from these storms
through 18/00Z and this reflected in the latest TAF forecasts.
Gusty winds will be the primary threat with some riming and small
hail possible aloft in the highest storms. Storm activity should
dissipate with sunset leaving VFR to dominate the forecast
overnight through mid morning.




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