Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 161554
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
854 AM MST Tue Jan 16 2018

.UPDATE...
Issued at 853 AM MST Tue Jan 16 2018

Wind Chill Advisory has been cancelled.

&&

.SHORT TERM...(Today through Thursday night)
Issued at 325 AM MST Tue Jan 16 2018

Overnight water vapor imagery placed the center of the central
CONUS upper trough over the western Great Lakes, with a upper ridge
axis from the Great Basin to the northern Rockies. Deep northerly
flow aloft on the backside of the trough was funneling very cold air
into the CWFA, with ambient temperatures in the single digits and
teens below zero. Despite the light surface winds, wind chill
temperatures were dangerously cold, ranging from 10 to 35 degrees
below zero. Wind Chill Advisories for much of the NE Panhandle
remain in effect until 9 AM this morning.

Models/ensembles are in good agreement with the dominance of the
a large scale ridge pattern through the short term. Rising heights
aloft and 700mb temperatures (-4 to -8C this afternoon and -2 to
-4C Wednesday afternoon) and increasing downslope westerly low
level flow will promote a warming trend. After a very cold morning,
highs this afternoon will be close to seasonal normals (25 to 35
degrees). Tonight will not be as cold with lows in the single
digits and teens above zero. A stretch of much above normal
temperatures begins Wednesday with highs in the 30s and 40s. Highs
Thursday will be in the 40s and 50s.

A period of stronger west-southwest winds begins Thursday as lee
surface trough develops over the high plains. 700mb winds increase
to 45 to 50 kt along and west of the Laramie Range. The Craig to
Casper 700/850mb gradients rise above 55m by Thursday morning. Wind
prone areas including Arlington, Bordeaux, and the I-80 Summit could
experience gusts approaching high wind criteria Thursday night.

.LONG TERM...(Friday through Monday)
Issued at 300 AM MST Tue Jan 16 2018

Main time frame of interest will continue to be the winter storm
system for this weekend.

Models continue to be in remarkable agreement with this trough for
the weekend despite the system still being five days out. The biggest
change has been to shift the track of the trough south, much further
south, than what was shown last night. Latest model runs have the
trough further south as it moves onshore, then diving south to the
Four Corners Region by Saturday evening. After that the trough becomes
a closed low over the KS/CO border and lifts east-northeast.

Even though the trough is deepening over the Four Corners Region, which
is normally a favorable track for producing significant snowfall amounts
for the forecast area, the trajectory and speed of the trough are still
not very favorable for significant snowfall. If the trough were coming
up from near Baja California, that would be more favorable. However
the current track has the trough starting to lift to the northeast
when the trough is too far east. The speed of the trough is also not
the best for producing significant snowfall. Despite the significant
shift to the south, the forecast area is still in a favorable position
to receive snowfall.

If the track of the trough continues to shift further south without
the trough turning to the northeast earlier than the KS/CO border,
the area of heavy snow will continue to shift further south of the
forecast area. This trough is still 5 days away, leaving plenty of
time for the models to change the track of the storm as well as
any potential impacts for the forecast area.

A secondary concern will be the likelihood of high winds developing
ahead of the winter storm system on Friday. Models have increased
the 700mb winds over the wind prone areas. With models showing this
wind signature for the last three nights, confidence continues to
increase that high winds may occur on Friday for the wind prone areas.
One caveat to this would be the location of the surface low, which
is currently forecast to be west of I-25. A much more favorable
location would be just east of I-25 where the isallobaric pressure
trend would help increase the gap winds. Even if the surface trough
does not shift slightly further east, the winds will be quite strong
if the current model data holds true.

On Monday models are showing favorable 700mb winds for producing
another high wind event.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 417 AM MST Tue Jan 16 2018

VFR conditions forecast for the TAFs. Light winds will vary in
direction some through the TAF period, but should remain around 10
kts or less.


&&

.FIRE WEATHER...
Issued at 300 AM MST Tue Jan 16 2018

Very cold temperatures will continue early this morning. Temperatures
will moderate beginning this afternoon and continue through Friday,
with minimum humidities from 25 to 30 percent. Winds will increase
later this week ahead of the next Pacific low pressure system, which
will bring more snow and colder temperatures to the districts this
weekend.

&&

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

$$

UPDATE...RUBIN
SHORT TERM...MAJ
LONG TERM...JTL
AVIATION...JTL
FIRE WEATHER...MAJ



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