Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 211805
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1205 PM MDT WED SEP 21 2016

.SHORT TERM...(Today through Thursday)
Issued at 255 AM MDT Wed Sep 21 2016

Some light rain has been falling for much of the night for our
southern valleys including Durango and Pagosa Springs. Models have
initialized well allowing confidence to increase in their
solutions. The remainder of today looks to bring plenty of broken
to overcast skies to the region with thicker, denser clouds for
our southern zones. Flow remains from the southwest which will
continue to pump in moisture allowing showers to continue through
the day. Like Tuesday, favored areas look to be from the
Bookcliffs southward with chances increasing the further south one
goes. Instability is rather modest today and with cloud cover,
convection may be inhibited this afternoon. However, a relatively
brief break in the clouds may cause enough instability for some
storms to fire. With that in mind, the chance for some afternoon
convection does exist and included in forecast. Later shifts can
remove if it looks too stable.

Things start to get interesting tonight and Thursday ahead of a
deep closed low and strong cold front out to our west. Starting
tonight, eastern Utah will start to see more pronounced showers as
a strong 105kt jet starts moving overhead. H7 winds will also
start picking up in response to a tightening gradient with
sustained winds of 30kts expected. As Thursday progresses, the
front will continue to slowly move east with models still favoring
eastern Utah with heavier rainfall and some convection. As the day
progresses, lapse rates look to steepen significantly as a break
in the clouds will further destabilize the atmosphere. As
mentioned, upper level winds will continue to increase and any
convection that forms could tap into these winds causing some
strong downdraft winds. Some downdraft winds of 50 MPH or higher
certainly look possible at this point with the stronger
convection. The heaviest precipitation will fall just ahead of and
along the cold front which looks to stay over central Utah for
much of Thursday until around midnight when it finally starts
entering our CWA. It should be fairly obvious when the front moves
through as winds will increase noticeably for a brief period of
time.

Suffice to say, Thursday looks to be very active mainly for
eastern Utah and then western Colorado towards the evening hours.
Gusty surface winds will be common for much of the CWA.
Temperatures will remain warm though this will change quickly
after the front moves through. Snow also looks possible as the
front moves through as described below.

.LONG TERM...(Thursday night through Tuesday)
Issued at 409 PM MDT Tue Sep 20 2016

Thursday marks the beginning of a transitional phase into some
wintry weather that will impact the area Friday and Saturday.
Winds will pick up ahead of an approaching cold front driven by a
deep upper-level trough moving eastward across the Great Basin. At
this point, it does not appear winds will reach advisory criteria,
though, when combined with convective cells, could generate very
strong outflow winds. Precipitation coverage increases as the
front pushes eastward across eastern Utah during the night and
through western Colorado on Friday.

As the cold air sweeps in behind the front Friday night into
Saturday, snow levels will lower to near 7500 feet over northwest
Colorado and to around 8500 elsewhere. However, both GFS and ECMWF
indicated a lull in activity from late Friday afternoon into
Friday night which will limit mountain accumulations. Better
chances for accumulating snow arrive Saturday into Saturday
evening as the cold pool aloft sinks southward over the area.
Given snow levels around the 9000 foot level, it appears likely
there will be impacts due to accumulating snow across the
northern and central mountain passes and, to a lesser extent, the
southern passes in the San Juan Mountains.

Conditions improve Sunday and Monday, though moisture may return
Tuesday as the low begins to move northward after stalling and
possibly retrograding. Models diverge on the placement of the low
so confidence is low early next week

Temperatures will drop markedly Friday, possibly by as much as 15
degrees relative to Thursday. Freeze warnings are likely later
this week and may bring an end to the growing season for a number
of the higher valleys. Cool conditions will linger into the weekend
with a slow warmup beginning Sunday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1205 PM MDT Wed Sep 21 2016

Moist south-southwesterly flow will continue to pump moisture into
the region. Light to moderate rainfall will favor the I-70
corridor south with the best chances looking to be from the San
Juans south. VFR conditions will persist at all TAF sites but
brief periods of MVFR will be possible at times, especially in
stronger storms. North of I-70 will have more scattered coverage
with just VCTS favored. More widespread coverage is expected
beginning tomorrow after 12Z as a strong cold front takes aim at
eastern Utah and western Colorado.

&&

.GJT Watches/Warnings/Advisories...
CO...None.
UT...None.
&&

$$

SHORT TERM...TGR
LONG TERM...NL
AVIATION...TGJT



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