Area Forecast Discussion
Issued by NWS Grand Junction, CO

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322
FXUS65 KGJT 211740
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1140 AM MDT Thu Jun 21 2018

.SHORT TERM...(Today through Friday)
Issued at 419 AM MDT Thu Jun 21 2018

A shortwave trough over the Pacific Northwest will shift eastward
over the northern Rockies today, driving a cold front to just
north of the Uinta Mountains and the northwest Colorado border by
days end. In response, the high pressure ridge to the west will
shift east of the Continental Divide and southwest winds will
increase a bit ahead of the front during the afternoon. A modest
influx of mid-level moisture over the eastern Uinta, Elkhead, Park
and Flat Top Mountains may be sufficient to fuel isolated
thunderstorms over these ranges, though model convective
parameterization schemes were mixed as to whether this will occur
or not. Meanwhile, warm air advection ahead of the front will
drive afternoon temperatures upward on the order of 5 to 10
degrees over Wednesday`s highs.

The shortwave trough over the northern Rockies makes a turn to
the southeast tonight driving the cold front across northeast
Utah and northwest Colorado by sunrise Friday. Mid-level upward
forcing and jet level divergence across the north will provide
dynamic lift across the north, but moisture levels likely to limit
shower/thunderstorm activity to isolated coverage. Again, model
QPF output varied greatly and the GFS in particular looked too wet
in the absence of a tap to Pacific moisture. The surface pressure
gradient should keep winds moving during the night which will
buoy overnight temperatures resulting in above normal lows.

The shortwave will continue to move to the southeast on Friday,
forcing the cold front to continue southward with the front
reaching the southern border of Colorado and Utah late in the
day. Deep mixing associated with the trough passage and an
enhanced surface gradient will lead to another breezy afternoon,
though temperatures will moderate a bit in response to cold air
advection associated with the front. Instability and forcing
associated with the trough passage will bring another round of
isolated to low end scattered showers and thunderstorms over
Colorado`s northern and central mountains, mainly during the
afternoon.

.LONG TERM...(Friday night through Wednesday)
Issued at 419 AM MDT Thu Jun 21 2018

A shortwave trough will dive southward through the northern
Rockies on Friday night into Saturday morning. Expect an increase
in high cirrus cloud cover throughout the morning and afternoon
hours. Despite rather potent dynamics with this system, a notable
lack of a connection to Pacific moisture exists. Any moist
convection and resultant precipitation will be largely driven by
increasing lapse rates as colder air aloft moves in with the
upper-level low on Saturday evening. Most guidance keeps all areas
dry except for the eastern Uinta range before sunset.

Overnight on Saturday, low pressure slides southward into far
northern Colorado resulting in a few showers which will affect
areas north of I-70, and in the central mountains. The 00Z GFS is
just a little bit farther south with the center of the low on
Saturday evening in comparison to prior runs. The farther south
this low tracks, the better the chances for precipitation will be
across the western slope over the weekend.

Low pressure slowly moves across northern Colorado on Sunday,
keeping conditions unsettled with mostly cloudy skies and
scattered showers in northeast Utah and northern and central
Colorado. It still appears as though the San Juan Mountains will
remain dry with this system. Sunday will also be the coolest day
in the forecast period as the core of colder air aloft passes
through. Temperatures will likely run around 5 degrees below
average. This slow-moving low pressure will finally head east of
the Divide by Monday, however clouds and showers will stick around
in the east for much of the day. Following this system, dry zonal
flow moves in for Tuesday and Wednesday of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1140 AM MDT Thu Jun 21 2018

VFR conditions will prevail over the next 24 hours with mostly to
partly sunny skies. Some clouds will increase across northeast
Utah and northwest Colorado as a disturbance crosses the northern
Rockies. Breezy afternoon winds will occur as well. Winds will
relax early evening after 02Z with light terrain-driven winds
overnight.

&&

.FIRE WEATHER...
Issued at 419 AM MDT Thu Jun 21 2018

An approaching trough and weak cold front will cause winds to
increase this afternoon where conditions remained dry. Winds will
be strongest over northwest Colorado with isolated, mostly dry
thunderstorms possible. A Red Flag Warning is in effect from 10 AM
to 8 PM for Colorado fire zones 200 and 202.

On Friday, the cold front will slowly move through the area and
strong and gusty surface winds will expand over across the region
in response to an increase in the surface pressure gradient.
Northwest Colorado, northeast Utah and much of southwest Colorado
will see the strongest winds while relative humidity is expected
to remain low. Meanwhile, thunderstorm chances will increase a
bit over northern and central Colorado. Due to the potential for
critical fire weather conditions, a Fire Weather Watch is in
effect for much of western Colorado and northeast Utah. A detailed
listing of the fire zones included in the watch can be found in
the Watch/Warning/Advisory section which follows this discussion.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Fire Weather Watch from Friday morning through Friday evening
     for COZ200-202-203-207-290>295.

     Red Flag Warning until 8 PM MDT this evening for COZ200-202.

UT...Fire Weather Watch from Friday morning through Friday evening
     for UTZ486-487-490.

&&

$$

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



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