Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 242102

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
302 PM MDT Mon Apr 24 2017

.SHORT TERM...(Tonight through Tuesday night)
Issued at 302 PM MDT Mon Apr 24 2017

Clouds and moisture are increasing across the forecast area under
a fast west to east flow driven by a 140 kt jet streak over the
Pacific Northwest that is translating to 100 kts over our area.
Mixing is occurring despite extensive cloud cover with breezy
conditions at many locations, yet due to increased moisture, the
relative humidity values are staying above critical thresholds so
fire weather concerns are minimized today. Expect showers to favor
the northeast Utah and northwest Colorado this afternoon with
isolated thunderstorms possible but CAPE appears fairly low around
200 J/kg. Precipitable Water (PW) values increase towards 0.4 to
0.6 inches across the area with specific humidity values in the 4
to 5 g/kg range, so plenty of moisture to work with.

A weak low circulation develops over Wyoming this evening keeping
the northern and central mountains favored for snowfall, while
the main low over Oregon opens up and slides across the Great
Basin and our area by Tuesday afternoon. This will shift the focus
towards the central and southern mountains for snowfall as the
flow shifts to northwest behind the cold front and trough passage.
Rain will primarily occur at the lower elevations Tuesday
afternoon with better potential for thunderstorms as CAPE
increases towards 400 to 800 J/kg and lapse rates steepen towards
8 degrees C/km on average. Showers decrease in coverage by late
Tuesday evening into Tuesday night as the system exits and drier
air moves in on the backside with upper level forcing waning.
Expect the best periods of moderate to heavy snowfall to occur
with each frontal passage with the two aforementioned lows: the
first tonight into Tuesday morning over the northern and central
mountains, and the second Tuesday afternoon/evening over the
central and southern mountains. H7 temps only cool down to as low
as -4C to -6C with this system, with a slight moderation Tuesday
afternoon before cooling again, so expect impacts and best
snowfall accumulations associated with this system to remain
mainly above 9000 feet. Winter Weather Advisories remain on track
for 4 to 8 inches of snow mainly above 9000 feet for the western
Colorado mountains through Tuesday evening.

.LONG TERM...(Wednesday through Monday)
Issued at 302 PM MDT Mon Apr 24 2017

After a break Wednesday morning while in between weather systems
in this unsettled pattern, moisture and showers increase once
again by late Wednesday afternoon into Wednesday evening as the
next system approaches. The upper level jet stays oriented from NW
to SE across the Pacific Northwest towards the Four Corners during
this period, keeping our CWA in the left exit region of the upper
level jet. The strongest jet streak is over the Pacific Northwest
of 170 kts which translates down towards 110 kts over the Four
Corners. The cold front associated with this system moves across
early Thursday morning with northwest flow lingering through
Thursday afternoon and evening, favoring the northern and central
Colorado mountains. Another round of decent snowfall is possible
Wednesday evening through Thursday afternoon, so will need to keep
an eye on the details as we near closer to the event. One concern
is that a ridge of high pressure will amplify over the Pacific
Ocean, pushing the upper level jet further northeastward, allowing
drier air to traverse from the northwest and effectively cut off
the moisture supply and put an end to most precipitation by
Thursday afternoon.

As this Pacific ridge amplifies and pushes further east, this
also amplifies the troughiness over the Intermountain West.
Previous forecasts were indicating the potential for a significant
storm system to develop by late in the week and impact the area
through this weekend. The latest 12Z guidance is now showing this
low developing further east than previous runs with the low
developing over the northern Utah- Colorado border Friday morning
and tracking southward towards the Four Corners by Friday evening.
This brings increasing chances of widespread precipitation across
the area and colder air advecting in with H7 temps lowering
towards -10C Friday morning through Saturday morning. There is
potential for below freezing temperatures in the lower valleys so
something to keep an eye on. The amplified Pacific ridge moves
further eastward into the western states and draws concerns for
the going forecast in terms of significant snow potential for the
mountains with this system as this also draws much drier air into
the region during this period. PW values remain less than 0.25
inches from Thursday afternoon through at least Sunday, so this
doesn`t bode well for significant precipitation. The ridge helps
to quickly push this system out of the region towards the Plains
by Saturday evening with an unsettled but drier northwest flow
over the area Sunday and Monday, keeping the northern Colorado
mountains favored for continued orographic snowfall. Still kept
the PoPs on the conservative side for late in the week as even
though the 12Z GFS and EC are in good agreement with this further
eastward and drier solution, run to run consistency is lagging.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1144 AM MDT Mon Apr 24 2017

An approaching cold front will cause some gusty winds today at
most TAF sites with southwest winds gusting from 20 to 30kts this
afternoon. Clouds will continue to thicken and drop as the day
goes on. VCSH looks possible for valley TAF sites while mountain
TAF sites have the best chance for any showers this evening and
overnight. Some MVFR is possible for the mountain sites as these
showers move through. KASE and KEGE will likely see ILS
breakpoints being met while other TAF sites will get close.
Conditions will start to improve Tuesday morning onwards.


CO...Winter Weather Advisory from 9 PM this evening to 9 PM MDT
     Tuesday for COZ004-010-012-013.

     Winter Weather Advisory from midnight tonight to 9 PM MDT
     Tuesday for COZ018-019.



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