Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 222342

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
542 PM MDT Fri Sep 22 2017

.SHORT TERM...(Tonight through Saturday night)
Issued at 222 PM MDT Fri Sep 22 2017

Gusty winds remain the main story this afternoon across the
eastern Utah and western Colorado. The central Rockies are
positioned on the eastern fringes of a deep trough centered over
the northern Great Basin. The center of a 110 knot jet streak
remains over the western slope, and afternoon mixing has allowed
stronger wind gusts to descend once again today. Stronger wind
gusts have been curtailed by considerable cloud cover, which has
limited surface heating and resultant mixing to some extent. A
diffuse frontal boundary is evident with in thermal fields around
700mb, and stretches from the central Wasatch range in Utah
through the Uintas and into south-central Wyoming. Ahead of this
boundary, numerous showers have developed in an environment
characterized by 200-500 j/kg of SB CAPE and 30 knots of speed
shear. Some isolated thunder has been noted north of the Tavaputs
in northwest Colorado. A few weak thunderstorms will continue
through early evening along with numerous quick-moving showers,
before precipitation transitions to scattered showers overnight
in the higher terrain as instability wanes.

On Saturday, 700mb temperatures will drop by a few degrees as the
mid-level frontal boundary moves east and fizzles over the central
Rockies. Snow levels will descend to around 11-12 kft during the
afternoon hours, and guidance has consistently indicated the
potential for a few inches of snow above that elevation from
Saturday afternoon through the evening and overnight. Lower
elevations will experience another round of showers, which will be
most common from the San Juans northward along the Divide, as well
as in northwest Utah. Aloft, the western trough will cut off with
an upper-level low remaining nearly stationary in central Utah.
Southerly flow will favor the Uinta mountains for rain and higher
elevation snowfall, with several inches possible on Saturday night
through mid-day Sunday given proximity to the cold upper-level low
and ample lift/moisture.

.LONG TERM...(Sunday through Friday)
Issued at 222 PM MDT Fri Sep 22 2017

Afternoon guidance is in good agreement showing the upper-level
low shifting northeastward through the Uintas and into southern
Wyoming on Sunday afternoon. Dry air will filter in south of
Interstate 70, however one more day of unsettled weather is
expected to the north before the low exits the Rockies. Sunday
will feature the coolest weather during the forecast period as the
upper low and trough axis pass over. Temperatures will run around
15 degrees below average. A consensus between MET and MAV MOS
guidance was used for Sunday highs, and was adjusted downward
based on 12z ECMWF ensemble guidance. If the ECMWF were to pan
out, the Grand Valley may struggle to reach the 60 degree mark!
Have not gone that low in this forecast package, however these
notably low forecasts are worth noting for now.

Lingering moisture along the Continental Divide and low heights
will remain on Monday, enough that showers and an isolated rumble
of thunder can not be completely ruled out of the forecast in the
higher terrain. Elsewhere, a drying trend will set in. By Tuesday,
our next weather disturbance will dive southward through the Great
Basin. Forecast confidence decreases from Tuesday through the
remainder of the week as major global models indicate another
cut off upper-level low developing in the southwest. This low will
keep things unsettled in the San Juans and Four Corner region.
Overall, the trend of below average temperatures will continue
into next week, although the airmass will not be quite as chilly
as the fall preview in store for this weekend.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 541 PM MDT Fri Sep 22 2017

General winds and those produced by passing showers will continue
to be a threat to aviation operations this evening...with gusts of
25 to 40 mph possible. Overnight spotty showers will persist
though the threat of impacts to forecast terminals will be
reduced. Ridgetop turbulence will continue to be possible over the
next 24 hours as well. VFR conditions are most probable to
continue over the next 24 hours however some cigs are likely to
reach or surpass ILS break points. Some riming or icing is
possible as well with freezing temperatures lowering toward
mountain top level overnight.




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