Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 271825

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1225 PM MDT Thu Apr 27 2017

Issued at 1224 PM MDT Thu Apr 27 2017

After looking at the latest guidance this morning, went ahead and
upgraded the Freeze Watch to a Freeze Warning for the Grand Valley
and Central Gunnison/Uncompahgre River Basin for tonight. While
clouds are expected to linger in these areas some clearing should
occur as drier air moves in from the northwest. The cold air mass
in place from last evening`s system and the (at least) partial
clearing should be enough to drop temperatures into the upper 20`s
to the low 30`s. Overnight lows look to again drop below freezing
for these same areas on Friday night but will take the next few
hours to examine this more closely before publishing the afternoon
forecast package.


.SHORT TERM...(Today through Friday)
Issued at 422 AM MDT Thu Apr 27 2017

Following a lull in activity during the evening, shower activity
became widespread over much of the forecast area overnight,
stretching from KDWX in the north to KDRO in the south. Snow
levels fell to near mountain bases early this morning with KPSO
reporting snow as of 09Z. Showers were supplied by deep moisture
ahead of a short-wave trough approaching from the west-northwest,
driven east on the nose of a 130 KT jet.

The short-wave will sweep across eastern Utah and northwest
Colorado during the morning with the lift provided enhancing
orographic forcing over the mountains, especially north. Models
were consistent in moving a cold front associated with the short-
wave trough from northeast Utah at 12Z to Colorado`s northern
ranges by midday. The influx of colder air behind the front will
keep snow levels relatively low in the Flat Tops and the Elkhead
and Park Ranges today as 7h temperatures near -6 deg C settle over
the area. The front then eases south during the afternoon as the
southeastward directed jet deepens the trough. Instability,
orographics, and mid and upper level upward forcing combine to
keep mountain snow widespread during the day. Significant
accumulations, especially in the northern mountains, remain
probable so will make no changes to the current winter weather
headlines with this package.

Models indicated snow will diminish across the southern mountains
and adjacent valleys this evening while lingering across the
northern and central mountains in response to a secondary wave
rotating over the area expanding the broad trough over the
Rockies. 7h temperatures beneath the core of the system are
expected to range from -10 deg C in the north to near -7 deg C
across the south. Consequently, surface lows are expected to be
unseasonably cold with freezing temperatures possible in the lower
valleys where budding vegetation is susceptible to freeze damage.
Therefore, have hoisted a Freeze Watch for the Grand and
Uncompahgre River Valleys. Uncertain if vegetation is at risk
outside of these areas and therefore urge the day crew to assess
whether other areas need to be added to the watch.

A southward flowing jet over the Great Basin will keep the now
closed upper level low pressure system anchored over the forecast
area on Friday. As a result, cooler than normal and unsettled
conditions will continue over the forecast area, especially across
the north.

.LONG TERM...(Friday night through Wednesday)
Issued at 422 AM MDT Thu Apr 27 2017

Another upper level trough will move across the forecast area
Friday night into Saturday with extended models even carving out
a cutoff low. Details still need to be worked out with this storm
and would definitely prefer to see some more model consistency.
Regardless, the cool, unsettled weather will continue through at
least the end of the work week. Temperatures will remain below
normal through the week and overnight lows will approach or reach
freezing in many valley locations.

Going even further into the extended, northwesterly flow appears
to set up over the region on Sunday. This will bring a shift
towards drier conditions although lingering moisture will more
than likely keep showers going over the northern mountains into
early next week.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1149 AM MDT Thu Apr 27 2017

Moisture and instability will continue to pop showers over most of
the region this afternoon. The instability will also allow
stronger winds to mix down creating gusts over 25 mph and may
impact landing and takeoff operations. VFR conditions are forecast
to prevail but the showers could quickly drop the mountain
forecast terminal conditions down to LIFR under brief moderate to
heavy precipitation snow rates. Northwest flow into the Rockies
will keep ILS conditions in place in many areas.


CO...Winter Weather Advisory until 6 PM MDT this evening for COZ009-

     Freeze Warning from midnight tonight to 9 AM MDT Friday for

     Winter Storm Warning until midnight MDT tonight for COZ004-013.

     Winter Weather Advisory until midnight MDT tonight for COZ010.



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