Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 180407

Area Forecast Discussion
National Weather Service Cheyenne WY
907 PM MST Sun Dec 17 2017

Issued at 854 PM MST Sun Dec 17 2017

We are sending out a quick update to upgrade the high wind watch
to a high wind warning for Bordeaux tonight into tomorrow, and
issue a high wind warning for the I-80 summit and foothills.
Latest LAPS data was continuing to show a tight surface gradient
(10mb) between the foothills of the Laramie Range and Craig
Colorado. This should keep the strong wind speeds going around the
Arlington zone overnight. Meanwhile, strong jet energy appears to
moving through Montana which will induce more height
falls/pressure falls across eastern Montana and northeast Wyoming
by tomorrow morning. As a result, we may see some strong winds
(65-75 mph) in Bordeaux from daybreak onward. We are also
concerned that the lee trof will push east in response to the weak
shortwave moving through northern Montana and allow for strong
mixing to take place on the I-80 summit and the foothills where
50kt wind speeds will be in place. As a result, we are planning
on upgrading to a high wind warning for those areas as well with
strongest wind speeds in that area during the afternoon.


.SHORT TERM...(This evening through Tuesday night)
Issued at 215 PM MST Sun Dec 17 2017

Early afternoon water vapor loop showed a split flow aloft with
a southern stream low spinning along the AZ/northwest MX border,
and progressive northern stream trough bisecting the Dakotas into
northeast UT/northwest CO. A surface trough extended from east
central MT south along the laramie Range and northern CO Front
Range. Low clouds across western NE have cleared out nicely, with
some wave clouds west of the Laramie Range. Low level pressure
gradient was sufficient for occasional strong west winds between
Laramie and Rawlins, with gusts to around 55 mph. The gradient
should relax a bit later this afternoon, with winds decreasing
somewhat. Temperatures ranged from the mid 20s to mid 40s.

The near short term (tonight through Monday) will focus on high
wind potential for the WY 106 and 110 zones including Arlington
and Bordeaux from late tonight through Monday afternoon. The GFS
and NAM CAG-CPR 700/850mb height gradient progs are between 50 and
60m between 09Z and 00Z Monday. Prior runs of the 700mb winds were
west 35-40kt, have now increased to 45-50kt. Our Bordeaux local wind
probabilities range from 25 to 65 percent for the same time period.
Therefore, confidence is growing for a high wind episode from late
tonight through late Monday afternoon, and a High wind Watch was
issued. Aside from the winds, Monday will be dry with milder high
temperatures in the mid 40s to mid 50s east of the Laramie Range,
with 20s and 30s to the west.

Winds will diminish Monday night and Tuesday, but increase somewhat
across the southeast WY wind corridors Tuesday night. It will
continue dry during this period, with intervals of mid and high
clouds. A weak front tries to slip south into the northern NE
panhandle Tuesday where high temperatures will remain in the 40s.

.LONG TERM...(Wednesday through Sunday)
Issued at 215 PM MST Sun Dec 17 2017

One more mild day across the CWA Wednesday then winter weather
arrives Thursday and remains through the weekend. Snow will be
the main concern Thursday then very cold temperatures expected
after that.

An upper trough will dig southeast over the western CONUS Thur
with southwesterly flow aloft over this area giving us our last
mild day. The arctic front will then drop south across the CWA
Thursday evening with snow developing and spreading across the
CWA behind it. Much colder Thursday with snow continuing Thurs
morning then tapering off Thursday afternoon and evening as the
upper trough axis passes by. Models in decent agreement keeping
the system more an open and progressive trough so this should
keep snowfall amounts down a bit. Still looking like around a
foot or so of snow over the higher mtns with a general 2 to 5
inches over the plains by Thursday evening.

Friday looking to be cold with the next upper trough dropping
south out of Canada, bringing an even colder push of arctic air
with it. Should see another round of snow across much of the CWA
late Fri into Saturday as the trough digs well into the southwest
part of the country. GFS not as amplified with it and as a result
keeps QPF going over the CWA through Sunday while the EC is dry.
Either way very cold air will settle over the CWA Saturday through
Monday with fairly widespread mins temperatures below zero likely
particularly Sunday and Monday.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 434 PM MST Sun Dec 17 2017

Dry and milder conditions are anticipated during the next 24hrs
which will allow for VFR conditions to prevail. It will be breezy
again tomorrow afternoon across most of the TAF sites due to
downslope flow.


Issued at 215 PM MST Sun Dec 17 2017

Cool to mild temperatures and low-level moisture will preclude fire
weather issues through mid-week. Lowest humidities will remain above
critical thresholds through the week. Much colder temperatures and
accumulating snow are expected later this week.


WY...High Wind Watch from 2 AM MST Monday through Monday afternoon
     for WYZ106.

     High Wind Warning until 5 PM MST Monday for WYZ110.



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