Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 280446

Area Forecast Discussion
National Weather Service Grand Junction CO
1046 PM MDT TUE SEP 27 2016

.SHORT TERM...(Tonight through Wednesday night)
Issued at 200 PM MDT Tue Sep 27 2016

Rex block remains firmly in place today with high pressure over the
Great Basin and a low pressure system spinning over the northern
Baja peninsula just south of the old Mexico-Arizona border. This
is providing mostly sunny skies and light terrain driven winds
today. Not expecting much in the way of precipitation today as
high pressure seems to have a firm hold over the region with the
high clouds still staying south of the Colorado-Arizona border
but slowly inching closer. Tropical Storm Roslyn, located over
the Pacific just southwest of the southern tip of the Baja coast,
is slowly working its way northward. This will help push the
southern low a little further north into southern Arizona this
evening, allowing some high clouds to seep into the southern
reaches of our CWA. Otherwise, a quiet night is in store but big
changes on the way beginning Wednesday.

The Rex Block that has been in place the last few days will break
down on Wednesday as a low pressure system, currently located
over the Gulf of Alaska, moves down the British Columbia
coastline. This will shift the flow over our CWA to south-
southwest as the southern low elongates and becomes entrained into
the main upper level trough developing over the Pacific Northwest,
with the high pressure over Texas amplifying a ridge over the
central Plains. This allows sub-tropical moisture to be drawn
northward in the form of mid and high level clouds during much of
the day on Wednesday. The NAM is quicker and further north with
this moisture surge with showers developing over the higher
terrain by Wednesday afternoon, but seems to be the outlier as the
GFS and ECMWF have been consistent with showers increasing towards
Wednesday evening over southeast Utah. Will lean towards the
slower GFS and ECMWF. So, expecting mid and high level clouds to
increase beginning early Wednesday morning with showers
developing over southeast Utah into the Four Corners region
Wednesday evening. Some brief heavy rain is possible in southeast
Utah as the low levels become saturated late in the day and the
nose of the 60kt upper level jet nudges into the Four Corners

Temperatures through Wednesday will continue to see the warming
trend with values a few degrees above normal in the afternoons
despite the increase in cloud cover and relatively cool, fall

.LONG TERM...(Thursday through Tuesday)
Issued at 200 PM MDT Tue Sep 27 2016

Thursday and Friday: As sub-tropical moisture continues to increase
on Thursday, remnants of Tropical Storm Roslyn off the Baja will get
drawn up into the southwest flow with the southern low tracking
across the area as an open wave. Specific humidity values increase
to the 6 to 8 g/kg range with Precipitable Water values also
increasing to 0.75 to an inch plus, with the best moisture across
southeast Utah and southwest Colorado. Expecting more active weather
on Thursday with increased chances and coverage of showers and
thunderstorms. Due to the high moisture content and light winds,
heavy rain looks likely with flash flood potential for southeast
Utah and southwest Colorado. One concern is that there would be
sufficient cloud cover to limit the thunderstorm potential, but that
doesn`t completely rule out stronger storms from forming. Showers
and storms stand a better chance to continue through the overnight
hours Thursday night into Friday morning as a few embedded
disturbances move through. The Pacific Northwest low strengthens and
deepens a trough over the western states on Friday, which will allow
moisture to increase even more compared to Thursday with showers and
storms continuing with heavy rain likely in any showers and storms
that form. Temperatures will be fairly mild yet cooler under clouds
and showers with highs near seasonal.  Snow levels remain high
as well with snow levels around 12,500 feet.

Saturday and Sunday: A bit of a downturn is expected on Saturday as
drier southwest flow pushes into the region as the trough of low
pressure over the Pacific Northwest pushes inland.  Still enough
moisture remains for scattered storms to develop Saturday afternoon,
mainly over the higher terrain with a few drifting into adjacent
valleys. The latest 12Z GFS pushes this trough across the area
quicker than earlier runs. This faster push is all driven by a 130kt
upper level jet upstream punching into the Pacific Northwest and
across the northern Rockies. The upper level trough becomes
negatively tilted by Sunday evening with a cold front moving through
the area Sunday evening through Monday morning. Showers and storms
will develop out ahead of this system in southwest flow Sunday
afternoon with snow levels starting off near 12kft. Snow levels will
lower throughout the evening and overnight, dropping to around 9000
ft Sunday evening and as low as 7500 ft across the north overnight
through Monday morning.  The heaviest precip is expected along the
cold front and could cause impacts for the northern and central
mountains and passes if this forecast holds true.  This will be
something for future shifts to consider.

Monday and Tuesday: Drier air moves in by Monday but continued west
to east flow will allow orographics to take hold over the northwest
Colorado mountains and possibly Steamboat Springs being affected as
this flow tends to favor them. This continues into Tuesday as this
100kt upper level jet nudges into the NW Colorado mountains. Enough
moisture and cold air exists for orographic snowfall mainly above
8000 ft. However, unsure of snow amts as moisture does appear
limited. The rest of the CWA looks to be fairly dry with
cooler, below seasonal temperatures.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1045 PM MDT Tue Sep 27 2016

High clouds will increase from south to north overnight. Clouds
will lower and thicken through the day on Wednesday with a chance
of showers and thunderstorms spreading in from the south during
the afternoon and evening. The showers that do develop will
initially be on the light side into the evening and are not
expected to impact aviation. After 03z Wed evening local
obscurations may at times affect the southern mountains. That
said, VFR is expected at all TAF sites through the 24 hour period
The possible exception will be at KTEX where MVFR conditions may
briefly develop late Wed evening.


.GJT Watches/Warnings/Advisories...


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