Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 220501
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1101 PM MDT Thu Sep 21 2017

.UPDATE...
Issued at 907 PM MDT Thu Sep 21 2017

Allowed both the Wind Advisory and the Red Flag Warnings to
expire. Though winds continued to be strong and gusty at some
locations, they had fallen below advisory criteria and should
continue to decrease as the atmosphere decouples. Meanwhile,
falling temperatures have allowed relative humidity to recover
to above 15 percent for most locations.

UPDATE Issued at 651 PM MDT Thu Sep 21 2017

Relative humidity recovery slower than anticipated and winds
continued to gust well over 25 MPH across the area within the Red
Flag Warning (RFW) early this evening. Anticipate winds will
remain strong and gusty and RH low for the next couple of hours.
Therefore, have extended the RFW through 9 PM for fire zones 207
and 290.

&&

.SHORT TERM...(Tonight through Friday night)
Issued at 359 PM MDT Thu Sep 21 2017

Windy conditions are occurring in most areas today where gusts 35
to 45 mph are common. Some areas are even gusting to 50 mph or
above, mainly over northwest Colorado valleys. Dry conditions are
in place as well with mostly sunny skies, leading to critical fire
weather conditions in some areas. See the Fire Weather section
below for more details. The storm system developing to our west
responsible for sufficient mixing of these upper level winds with
a 130kt jet parked over Utah. Our CWA remains in the left exit
region of this jet, which is allowing for windier conditions
rather than any precipitation as the atmosphere remains dry. Wind
Advisories remain in effect through 9 pm this evening for much of
southeast Utah, northwest Colorado and west-central Colorado
valleys. This also includes the Paradox valley in southwest
Colorado as well as the Tavaputs Plateau in both east-central Utah
and west-central Colorado.

Breezy conditions will remain overnight as the 130kt upper level
jet shifts eastward over the Four Corners region and across
western Colorado by Friday afternoon as the upper level low drops
into the northern Great Basin from the Pacific Northwest. Plenty
of cloud cover and moisture is associated with this upper level
jet as seen on GOES-16 satellite data today. This mid and high
level moisture is expected to move over the area heading into
Friday with increasing clouds as a result. Daytime temperatures
will lower by some 10 degrees from today. Do not anticipate
needing any additional wind or fire weather highlites for Friday
as relative humidity levels are expected to increase with the
increasing cloud cover and moisture. This cloud cover should also
prevent the stronger upper level winds from reaching the surface
with just breezy conditions expected. Some showers will develop
mainly over the higher terrain of the eastern Uintas and San Juans
overnight, with some development over the rest of the higher
terrain by Friday afternoon. Not expecting much in the way of
precip accumulation as low levels still remain dry, but the best
chances appear to be over the San Juans, with still high snow
levels around 12kft. Not enough to warrant any winter highlites as
light accumulations are expected at the higher peaks. H7 temps
still remain fairly mild as our CWA remains in the warm sector.

.LONG TERM...(Saturday through Thursday)
Issued at 359 PM MDT Thu Sep 21 2017

The upper level trough will continue to amplify on Saturday as the
upper level low shifts southward towards southern Nevada and
southwest Utah and deepens. Even though models are not in
agreement on how far south to take the upper low and deepen the
trough, the models are in good agreement on the overall forecast
trends. Expect better chances of shower and thunderstorm
development on Saturday as better instability and shear exists
with CAPE in the 400 to 600 J/kg range and lapse rates approaching
8 degrees C/km. The better shear over southwest Colorado and the
divide mountains south of I-70 may result in a little better
organization for some stronger storms to develop. Snow levels
still remain high with the highest peaks above 12kft seeing some
snow accumulation, but still remains minimal at this time due to
mild H7 temps around 6 degrees C.

Drier air punches in across the south on Sunday as the upper low
tracks across Utah from southwest to northeast, keeping scattered
convective showers across the north. Colder air finally begins to
move into the region as this upper low tracks across the area. The
coldest air arrives by Sunday night into Monday as H7 temps dip
down to -3C across the north and 0C across the south with some
moderation by Monday afternoon up to 0C across the north and 3C
across the south. Snow levels look to drop down to 7000 feet by
this time. The northern mountains stand the best chance to see
snowfall accumulations but details are not clear at this time.
There is also good potential that some other lower elevations will
see season ending freeze as lows dip to the upper 20s to low 30s
in some areas, mainly the northern and southern valleys, as well
as the valleys along the divide (eastern most portions of the CWA).
Something for future shifts to consider as the time gets closer.

The trough is expected to remain over the region through Wednesday
as another piece of energy dives down the backside of the trough
towards the Four Corners region for an uptick in shower activity.
A ridge of high pressure moves back into the region by Thursday
for a slow warming trend Tuesday through the end of the week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1101 PM MDT Thu Sep 21 2017

A deep trough of lower pressure will shift slowly eastward across
the Great Basin over the next 24 hours. A cold front associated
with this system is expected to ease over the eastern Uinta Basin
in the evening. Ahead of the front, breezy to windy southwest
winds will be the big story, especially Friday afternoon and early
in the evening, though early indications suggest a bit less windy
than on Thursday. Showers and afternoon thunderstorms are possible
over the higher terrain across eastern Utah and western Colorado,
however this activity is unlikely to impact TAF sites with rain.
However, outflow winds from thunderstorms added to prevailing
flow could result in gusts to 50 MPH or more. Aside from wind, VFR
conditions will prevail over the next 24 hours and CIGS will
remain above ILS breakpoints for all sites.

&&

.FIRE WEATHER...
Issued at 907 PM MDT Thu Sep 21 2017

Winds look to remain breezy through the weekend before this
system moves eastward, with moisture levels coming up above
critical thresholds. Southeast UT will avoid critical conditions
due to non-critical fuel conditions.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

UPDATE...NL
SHORT TERM...MDA
LONG TERM...MDA
AVIATION...NL
FIRE WEATHER...TGJT


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