Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 152018

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
218 PM MDT Fri Sep 15 2017

.SHORT TERM...(Tonight through Saturday night)
Issued at 130 PM MDT Fri Sep 15 2017

GOES Water Vapor imagery reveals a distinct moisture axis
sweeping through central Utah as of 2pm Friday afternoon. This
axis is located along the leading edge of a trough which will
pivot through eastern Utah and western Colorado overnight into
Saturday. Precipitation along this trough has been limited to
mainly light showers from Vernal eastward through Hayden as cloud
cover has curtailed instability values over the north so far
today. A bit farther south along the Roan and Tavaputs up to the
Flat Top range, some sunshine has resulted in SB CAPE values in
the 250-750 range per SPC Mesoanalysis. This corridor would be
the most likely area to see thunder today. Areas south of I-70
will likely remain too dry in the mid levels to support any
substantial convection, and shower activity will remain confined
to the higher terrain of the San Juans and Continental Divide.

Westerly winds will increase, especially north of I-70, on
Saturday as the trough passes through around noon. Expect a quiet
afternoon as a dry continental airmass moves in behind the trough.
A chilly night is expected on Saturday evening into Sunday
morning, with mostly clear skies, dry air and diminishing winds
over much of the central Rockies.

.LONG TERM...(Sunday through Friday)
Issued at 130 PM MDT Fri Sep 15 2017

Not much change in the long term forecast in this afternoon`s
model cycles, and any changes from the previous forecast have been
minor. Warm air advection will resume over the Four Corners on
Sunday as west to southwesterly flow develops over the Four
Corners. Any isolated convection will be limited to the higher
terrain with plenty of dry air in place. As as been the case over
the past few days, the major story in the long range forecast
will center around our upcoming trough moving in to the west
coast. The strong jet that will move in along the PACNW coast has
been modeled slightly slower and a bit weaker today, however the
broad synoptic setup remains the same. A large longwave trough
will dive southward into the central Rockies on Monday evening,
which will remain over the western CONUS through the end of the
week and through the weekend. GEFS and EPS ensemble guidance
remain in good agreement with 500mb height anomalies in the
-2 sigma range during this time period. If anything, the GEFS has
 pushed those anamolous even farther southward into the central

The resultant weather for the central Rockies will be
temperatures running well below normal. 10 to 15 degrees below
normal seems to be a pretty confident forecast area-wide. ECMWF
and MEX MOS guidance has come into better agreement on the
temperature forecast today. Tuesday evening appears to be the
first shot at seeing some shower activity as a shortwave trough
passes to the north. The ECMWF is stronger with this trough and
indicates shower activity remaining locked in to the central and
northern zones through Thursday, while the GFS takes a more
piecemeal approach with one trough on Tuesday and then a second
and stronger system on Friday. Regardless of the outcome with the
mid and late-week systems, fall- like weather will be the
headline for next week.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1144 AM MDT Fri Sep 15 2017

Prefrontal windy conditions expected today with TAF sites all
transitioning to southwesterly winds shortly. Showers will be
mainly confined to NE Utah and NW Colorado, but along the cold
front passage around 00 to 05z... this threat is anticipated to
extend further south to KCNY and KGJT. Breezy conditions will be
the big aviation struggle of the day, with wind gusts to 20 mph
commonplace with potentially stronger gusts this evening with
showers and thunderstorms. ILS breakpoints may be reached at times
across NE Utah and NW Colorado.




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