Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 151932

Area Forecast Discussion
National Weather Service Cheyenne WY
1232 PM MST Sun Jan 15 2017

Issued at 1232 PM MST Sun Jan 15 2017

In an earlier update, have cancelled the Dense Fog Advisory for
the Laramie Valley & the Upper North Platte River Valley as fog
had lifted. Dense fog continues to impact travel along the I-80
corridor from east of Walcott Junction through Rawlins. Have opted
to extend the Dense Fog Advisory for this area until 3 PM.


.SHORT TERM...(Today through Tuesday)
Issued at 330 AM MST Sun Jan 15 2017

Dense fog to the west of the Laramie Range will likely persist for
quite some time today. A stagnant weather pattern remains in place
early this morning, with light winds and abundant low-lvl moisture
beneath a strong surface high. Several WYDOT webcams show very low
visibility around one quarter of a mile along I80 between Rawlins/
Laramie, so the Dense Fog Advisory is in good shape. The HRRR does
show fog continuing well into the afternoon hours which definitely
makes sense in this environment, so we will consider extending the
advisory for longer than the current 9 AM MST expiration. A tricky
precipitation forecast with the upcoming storm system. Water vapor
satellite shows the low beginning to lift into southern New Mexico
at this hour. The models are in excellent agreement with the track
of the low, located southwest of Oklahoma City at 06z Mon at south
east of Topeka, Kansas at 00z Tue. The storm track is much too far
southeast to see significant snow in this CWA, and the GFS and NAM
both show very little if any QPF through Monday. In fact, even the
ECMWF has trended further east. The GEM sings a different tune and
has snow spreading along the I80 corridor early this evening, then
becoming widespread overnight through Monday morning. This results
in a swath of 0.25+ inch QPF roughly along and southeast of a line
from Laramie to Bridgeport which would definitely be Advisory snow
amounts if it comes to fruition. The combination of north to north
west surface flow and modest dynamic support makes this very tough
to believe with the other models showing a similar setup with very
little precipitation. Nonetheless, we have maintained low-end PoPs
farther north and west to account for this possibility. NAM BUFKIT
soundings show a modest warm layer aloft which may lead to FZRA in
the southern Panhandle tonight, so included a mix of -SN and -FZRA
along/southeast of a Pine Bluffs to Alliance line from 00z-06z.

Despite some lingering uncertainties, the bottom line is this will
likely be a fairly low-impact event for us. Since precipitation is
unlikely to extend far west of the I25 corridor, we can expect the
potential for fog yet again for areas west of the Laramie Range as
we head into tonight as light winds will likely be unable to scour
out the low-level moisture. The period from Monday night thru Tues
will be windy for the wind corridors w/the passage of a strong mid
level wave. H85 CAG-CPR gradients exceeding 50 meters suggests the
potential for a marginal warning event, but would like to see low-
level flow a bit stronger than the 35-40 knots currently suggested
by the GFS.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM MST Sun Jan 15 2017

Pretty quiet in the long term. Upper level ridge will be over the
area into Wednesday. ECMWF begins to show next shortwave energy
moving into the area Thursday night into Friday. This should be
our next chance for snow. In the meantime, looking at dry
conditions and seasonal temperatures.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1232 PM MST Sun Jan 15 2017

MVFR/IFR conditions persist at KRWL and KLAR this afternoon as
patchy fog/low stratus continues there. Suspect to see some
improvement but confidence remains low just how much and where.
Expect conditions to deteriorate at these sites later tonight as
light winds, solid snow-pack, and moisture increases. For
terminals east of the Laramie Range, conditions will progressively
deteriorate later this afternoon south to north. The two
exceptions will be at KAIA and KCDR, which should be far enough
north to escape impact. KSNY, KCYS, and KBFF will see lowering
CIGS and light precipitation develop by tonight. KSNY may even see
some brief FZRA through around 05z, before transitioning all to
snow. Any icing would be very light of less than .01. Snowfall
amounts of around 1 inch is expected nr KSNY by midday tomorrow.


Issued at 330 AM MST Sun Jan 15 2017

No fire weather concerns over the next several days w/ light winds
and high humidity values.


WY...Dense Fog Advisory until 3 PM MST this afternoon for WYZ109.



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