Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 171725 CCA
AFDCYS

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
1025 AM MST Sun Dec 17 2017

.UPDATE...
Issued at 1018 AM MST Sun Dec 17 2017

Light mixed wintry precipitation and areas of freezing fog observed
earlier this morning have dissipated. Areas of low stratus affecting
Alliance, Douglas, Scottsbluff, Sidney and Torrington persist late
this morning. Latest model trends show these low cloud ceilings
lifting through early afternoon. Updated grids and zones to account
for these trends.

&&

.SHORT TERM...(Today through Monday)
Issued at 305 AM MST Sun Dec 17 2017

Deep low-level saturation and moist upslope flow is supporting fog
and patchy freezing drizzle, along with areas of light snow along/
east of the Laramie Range this AM. Radar & GOES-16 IR imagery have
been indicative of a decrease in overall precipitation coverage w/
precipitation ending in northern areas. WYDOT web cams show little
or no snow cover on roads in Converse county, so we have cancelled
the inherited Winter Weather Advisory. Areas of dense fog continue
along I-80 between Laramie & Cheyenne. We opted to remove the city
of Cheyenne from the Dense Fog Advisory as local visibilities have
improved and will likely not fall again with precipitation falling
at this time. We will re-evaluate the I-80 summit prior to the 13z
expiration. Generally minimal impacts from this, but will continue
to strongly message potential for areas of icy road conditions and
low visibility this AM. Conditions improving through the day w/sfc
flow becoming westerly in response to a passing trough axis.

Some concern for high winds in the wind corridors late tonight and
early Monday. A strong short-wave will track across Saskatchewan &
Manitoba during this time, enhancing low-lvl height gradients over
southeast Wyoming. NAM/GFS CAG-CPR gradients climb to 50-60 meters
at both H7/H85, but a lack of stronger flow aloft could limit risk
for warning-criteria gusts. The GFS peaks H7 flow around 35 kts at
ARL, and H8 flow near 50 kts at BRX early Monday. Am not confident
enough to issue any high wind headlines at this time, but the best
chance for 50+ kt gusts appears to be at BRX between 12-18z Mon.

.LONG TERM...(Monday Night through Saturday)
Issued at 305 AM MST Sun Dec 17 2017

Dry and somewhat mild conditions expected Tuesday and Wednesday
before big changes sweep in Thursday through the next weekend when
more typical Wyoming and western Nebraska winter conditions
return home for a holiday visit. Models in fair agreement bringing
a significant chance of snow to the area Wednesday night into
Thursday morning behind a strong Arctic cold front that will move
through late Wednesday. Model blends continue to be catching up
with trends towards higher snow amounts Thursday, so we went a
little above the blends on snow totals during the day on Thursday
when colder air becomes more deeply entrenched along the deep
moisture and lift axis as the upper trough moves across. ECMWF and
to some degree the GFS bring a closed low across Colorado
Thursday, which is a little stronger than the previous 00Z runs.
The low level pressure gradient Thursday would suggest northerly
winds 15 to 25 mph with strong cold advection and potential for
some blowing and drifting snow. So, there is the real potential
for travel impacts across the region on Thursday, and folks need
to stay tuned in on that. Also, in addition to the snow
potential, wind chills and cold temperatures will be the main
message for the latter part of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday morning)
Issued at 1017 AM MST Sun Dec 17 2017

Patchy MVFR/IFR cigs affecting mainly the Neb Panhandle will break
up by 20Z with VFR conditions then expected areawide through tonight.
Some gusty winds possible around the mtns later tonight as well.

&&

.FIRE WEATHER...
Issued at 245 AM MST Sun Dec 17 2017

Cooler temperatures and abundant low-level moisture will limit the
potential for critical fire weather conditions. Minimum RHs should
remain well outside of critical thresholds through the week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...CLH
LONG TERM...JG
AVIATION...RE
FIRE WEATHER...CLH


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