Area Forecast Discussion
Issued by NWS Grand Junction, CO

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139
FXUS65 KGJT 182304
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
404 PM MST Sun Feb 18 2018

.SHORT TERM...(Tonight through Tuesday night) Issued at 330 PM
MST Sun Feb 18 2018

An extensive storm system will make its way southward from the
Pacific Northwest and through the central Rockies from this
evening through Tuesday. Ahead of this system, a warm-air
advection regime and an approaching jet streak aloft have combined
for a unseasonably warm and breezy day across much of the western
slope. Temperatures have reached or exceeded 60 degrees at
several observing sites in the lower valleys of Colorado and Utah,
and winds have gusted in excess of 45 mph especially along and
north of the Tavaputs. Current wind advisories remain on track
through the remainder of the afternoon and temperatures will top
out at least 15 degrees above average across the board.

Some slight adjustments have been made to the start times for
precipitation in the central and northern mountains in this
forecast package. Guidance has come in slightly slower once again,
and it now appears as though the San Juans and even the central
mountains will remain mostly dry through the early evening hours.
PoPs ramp up by midnight however, and higher-elevation orographic
snows will begin by that time. It still appears likely that the
heaviest snowfall rates with this system will occur along and just
behind the mid-level frontal boundary, which will swing southward
from eastern Utah through Colorado during the day on Monday.
Enhanced lift in a saturated dendritic growth zone, combined with
cold air rushing in aloft over relatively warm surface
temperatures, could result in some thunder as well during the day.
In fact, various CAM guidance suggests some pockets of MUCAPE in
the 100-300 J/Kg range especially from I-70 southward on Monday.

While the snowfall forecast in the mountains has been pretty much
a lock over the past few days, some challenges and potential
pitfalls to the forecast remain. First and foremost is the high
temperature forecast on Monday. If the front progresses slower
than expected, warm air near the surface may be tough to root out
throughout the lower elevations and lower mountain slopes on
Monday. The snowfall forecast in the southern foothills of the San
Juans was heavily considered in this package, and it was
determined that warm air through much of the event will likely
limit accumulation potential along the US 160 corridor. That being
said, a few inches are still possible in places such as Hesperus,
Durango, and especially Pagosa Springs. As is often the case with
extremely convective, frontally-forced events...intense snowfall
rates can often overcome warm surface temperatures for a period of
time. This is the second potential pitfall in our forecast. FROPA
timed with sunset could result in roads quickly becoming
slick in a short amount of time. This would apply mostly south of
Grand Junction as the front moves through on Monday. Nowcasting
will be particularly important throughout the event to analyze
these trends and related impacts.

Guidance remains consistent showing a slow and steady end to this
system after the front reaches the Continental Divide. Lingering
saturation in the bottom half of forecast soundings combined with
cold temperatures aloft will support upslope snow showers and some
convective banding overnight on Monday and into the day on
Tuesday. As as been the message over the past few days, any
banding that does set up could give some mountain locations a
significant dose of post-frontal snow. While pinpointing these
features is quite difficult even a day out, short-term guidance
has consistently hinted at this potential in the northern San
Juans, the central ranges including the West Elks and Grand Mesa,
and the Tavaputs into the Flat Tops. Time will tell on the
accuracy of these forecasts.

Precipitation will finally wind down later on Tuesday evening as
low level moisture is eradicated and the trough axis moves east of
the Divide. Overall, all winter storm headlines remain on track
at this point and minimal changes have been made to snowfall
accumulations in this forecast package. See the latest WSWGJT
product for specific accumulation details.

.LONG TERM...(Wednesday through Sunday) Issued at 330 PM MST Sun
Feb 18 2018

Much cooler start to this extended period in the wake of the
exiting winter storm with temperatures generally running 5 to 10
degrees below seasonal normals. Dry conditions are expected on
Wednesday with the exception of a few light showers across the
northern mountains as a weak system drags across Wyoming and clips
this area.

Another deepening upper level low will be driving down the west
coast Thursday and Friday as broad cyclonic flow sets over the
western states. Models generating some light precipitation Late
Wednesday night and Thursday in weak warm air advection pattern
with southerly flow forcing the air mass up and over the San
Juans. This places the focus of shower activity over our southern
valleys and mountains including the Cortez to Pagosa Corridor up
to Silverton and Telluride. Better forcing and another shot of
colder air arrive Thursday night and Friday as the upper low kicks
out and progresses eastward. This is where the ECMWF and GFS start
to diverge with timing and strength issues creeping back into the
forecast. The GFS wants to roll the trough axis across us later
Friday night, while the EC is holding back a bit and waiting until
late Saturday from trough passage. In either case, the period from
Friday through Saturday should be interesting with potential for
more moderate to heavy mountain snowfall. Another wave is being
picked up by both models for later Sunday.

Temperatures will be starting the period off on the cool side with
some moderation expected Thursday and Friday before we cool right
back down over the weekend. Trend overall is for a cooler than
normal week for the bulk of the forecast area with this pattern
persisting for another week or so based on the latest CPC 14 day
outlooks.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 359 PM MST Sun Feb 18 2018

Surface winds will continue to be gusty across the area through the
night and into Sunday morning as the cold front associated with
the next system approaches. VFR conditions with CIGS above ILS
breakpoints will persist through 06Z this evening and thereafter
rain, snow and a mix of rain and snow showers will threaten TAF
sites across eastern Utah and western Colorado through 00Z Tuesday.
This activity will be light initially with MVFR visibilities the
main impact on aviation operations.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 8 AM MST Tuesday for COZ001>005-
     010-017.

     Wind Advisory until 5 PM MST this afternoon for COZ001>003.

     Wind Advisory from 11 AM to 5 PM MST Monday for COZ020-021.

     Winter Storm Warning until 8 AM MST Tuesday for COZ009-012-013-
     018-019.

UT...Winter Weather Advisory until 8 AM MST Tuesday for UTZ023-025-
     028.

     Wind Advisory until 5 PM MST this afternoon for UTZ027-029.

     Wind Advisory from 11 AM to 5 PM MST Monday for UTZ022-028.

&&

$$

SHORT TERM...MAC
LONG TERM...JDC
AVIATION...CC



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