Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 151744
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1044 AM MST Mon Jan 15 2018

.UPDATE...
Issued at 1016 AM MST Mon Jan 15 2018

Latest water vapor loop showing the jet energy continuing to build
south which is taking the bulk of the synoptic scale forcing south
into northern Colorado. Along with this forcing, moisture is
continuing to push south as well, due to the arctic high building
into the forecast area. As a result, we are cancelling the Winter
Weather Advisory. Temperatures have been fairly steady over much
of the area except for the Nebraska panhandle where they are
showing temperatures climbing during the last couple of hours.
That tells us that the cold air advection may be ceasing a bit
over that region. We had considered on issuing a Wind Chill
Advisory for that region this afternoon. However, with full sun
and lack of cold air advection we decided to hold off for now.
Although, it will still be breezy in areas east of the Laramie
Range today which will keep wind chills below zero in most areas.
The main concern still remains tonight as temperatures really
radiate out tonight and cause wind chills to be well below zero
(15 to 25 below) especially in parts of the Nebraska panhandle.

&&

.SHORT TERM...(Today through Wednesday night)
Issued at 330 AM MST Mon Jan 15 2018

Overnight water vapor loop showed a polar vortex centered over the
Upper Midwest, with a strong upper level jet diving south through
MT and WY. Arctic cold front extended from southwest WY to a low
near Laramie then southeast through northeast CO. Band of post-frontal
snow has been spreading to the southwest, extending from Casper to
Cheyenne to northeast CO. Accumulations thus far have been light,
less than a half inch. Temperatures have fallen through the teens
and 20s since the frontal passage. Winds were northerly 15-25 kt
with wind chills in the single digits and teens.

Models have been consistent with southward motion of the Arctic
cold front this morning. Main concerns will be snow accumulations
this morning and very cold temperatures tonight and early Tuesday
morning.

Expect snow to increase in intensity this morning as the boundary
briefly hangs up along the southern Laramie Range, with QG lift
from left-front quadrant of upper level jet and moist low level
upslope. Mid-level frontogenetical banding maximizes between 12Z
and 18Z this morning. Looking at 2-3 inches of snow from southern
Laramie Range eastward to near Cheyenne. Added the southern Laramie
Range foothills to the Winter Weather Advisory valid through noon
today. Up to four inches may fall over the Snowy and north
Laramie Ranges.

The snow will taper off over the high terrain this afternoon.
Temperatures today will remain in the teens and single digits east
of the Laramie Range owing to strong cold air advection with 700mb
temperatures -10 to -15C. It wil be brisk with wind chill temperatures
between 0 and -15 degrees. Clear skies and diminishing winds tonight
will set stage for excellent radiational cooling. Temperatures
will plummet into the single digits above to lower teens below
zero late tonight. It wont take much wind to drive wind chills
as low as -25 degrees across much of western NE Panhandle. Added
Sioux, Morrill and Cheyenne counties to the Wind Chill Advisory
from midnight tonight through 9 AM Tuesday. Next shift may need to
adjust this advisory based on the 12Z MOS temperatures and winds.

The upper level trough translates east across the Plains and Midwest
Tuesday with an upper ridge building west of the Divide.
Temperatures begin moderating Tuesday, closer to seasonal normals
from 20s to lower 30s. Even warmer Wednesday with upper ridge
overhead and downslope westerly flow resulting in upper 30s to
upper 40s. It will be breezy along and west of the Laramie Range.

.LONG TERM...(Thursday through Sunday)
Issued at 245 AM MST Mon Jan 15 2018

Focus for this period will be on the winter storm system for this
coming weekend.

Models continue to be in good agreement with the track and speed of
this trough, which is rather surprising considering it is four to
five days away. The track of the trough has shifted slightly further
south, putting the forecast area in a better position for receiving
precipitation. Both the ECMWF and the GFS have the trough beginning
to deepen further to the west, which has caused the track to shift
further south. If the trough continues to shift further south, the
precipitation will begin to be focused more to the south of the
forecast area. Even though the track of the trough is further south,
it is still coming from the west. This is not the favorable track
for a trough to take that would produce heavy snow. The more favorable
direction is northeast from the Four Corners Region. However, based
on the latest track the forecast area will be on the northwest side
of the trough where the higher snowfall amounts are generally found.

The latest run of the GFS has this trough deepening into a closed
low over CO, which is stronger than the previous run. If the trough
does indeed become closed off the trough will slow down, giving more
time for the forecast area to receive precipitation.

700mb winds continue to show a potential for high winds for the wind
prone areas of the forecast area Thursday and Friday.

Strong winds will likely occur on the backside of the trough beginning
as early as Saturday. Snow should still be occurring as the winds
strengthen. If so, this would lead to blowing snow with low visibilities
creating very poor driving conditions.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1041 AM MST Mon Jan 15 2018

Wyoming TAFS...VFR, except occasional MVFR at Laramie through 19Z.
Wind gusts to 25 knots through 01Z.

Nebraska TAFS...VFR, except occasional MVFR at Chadron through
20Z. Wind gusts to 30 knots through 00Z.

&&

.FIRE WEATHER...
Issued at 245 AM MST Mon Jan 15 2018

An Arctic cold front will push south across the districts this morning,
bringing occasional light snow and much colder temperatures. Very cold
temperatures are forecast tonight, ranging from 5 to -12 degrees. Wind
chills will range from -15 to -25 degrees across much of the western
Nebraska Panhandle late tonight and early Tuesday morning. Temperatures
will begin moderating Tuesday, with above normal values expected
Wednesday. Fire weather concerns will be low through the week. It will
be breezy to windy Wednesday through Friday along and west of the Laramie
Range.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...Wind Chill Advisory from midnight tonight to 9 AM MST Tuesday
     for NEZ002-003-021-055-095-096.

&&

$$

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


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