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Area forecast discussion 
National Weather Service Cheyenne Wyoming
1045 am MST Thursday Dec 18 2014

issued at 1045 am MST Thursday Dec 18 2014

Minor update to adjust forecast for current conditions. Lowered maximums in
the North Platte River Valley as cold air trapping seems to be
occurring there. Adjusted cloud cover otherwise.

Short term...(today through Saturday night)
issued at 319 am MST Thursday Dec 18 2014

The ongoing locally dense fog along the North Platte River Valley
will carry the greatest impact through the period. Light winds and
minimal disruptive middle-level clouds has yielded efficient
radiational cooling overnight. Visibilities have bounced between
1/4sm and 1/2sm from Torrington to Scottsbluff to Alliance during
the overnight hours. Latest satellite fog product does show the
low clouds/fog beginning to shift east out of the eastern County Warning Area...but
keeps it in the North Platte River Valley. Will likely be able to
cancel Box Butte from the dense fog advisory here shortly...but
will leave the rest going for now. West winds will finally kick in
this morning as easily discernible shortwave spins out of eastern
Montana into the western Dakotas. This will effectively scour out
morning valley fog. Should also see the wind prone areas become
breezy. Otherwise weak westerly flow will work into the area today
will keep dry conditions outside of perhaps some very light
mountain snow showers. Although some patchy fog will be a
possibility again along the North Platte River not
anticipate it to be the extent of what it was tonight. Did add
patchy mention along the North Platte however.

Subsidence associated with shortwave ridge axis will build in for
Friday. This will yield a dry...partly sunny day. Another quick
moving...but dry shortwave will clip east central Wyoming and the
northern Nebraska Panhandle for Saturday.

Temperatures will remain seasonal for both today and tomorrow with
highs reaching near 30 west of the Laramie range and near 40 out on the
eastern plains. The gradient will likely tick up again on
producing locally breezy conditions over the wind prone areas and Lee-
side adiabatic warming which will boost temperatures a few degrees out on
the eastern plains.

Long term...(sunday through wednesday)
issued at 319 am MST Thursday Dec 18 2014

Pretty active extended period with winds across southeast Wyoming and
mountain snows being the primary concerns. A strong upper level
jet will stretch from the Pacific northwest into Wyoming on Sunday. An
increasing low level gradient will likely result in high winds at least
across the wind prone areas on Sunday into Sunday night. The GFS
shows the 850mb cag-cpr gradient rising above 60 meters by 12z sun
and maxing out at 85 meters by 12z Monday. The models have been
consistent with this over the past few our stretch of
relatively weak winds over southeast Wyoming will be coming to an end.
Front will push across the area on Monday morning with the surface
pressure gradient beginning to relax. However...with good mixing
and low level lapse rates in the postfrontal airmass...windy conditions
will spread onto the plains.

Several things are also coming together for a good snow event
across the Sierra Madre and snowy ranges from Sunday into Monday.
The region will be in the left exit region of the upper jet with
excellent 700-500mb moisture streaming into Wyoming from the Pacific.
Strong northwesterly flow will provide good orographic lift.
The coarse and likely underdone GFS shows 1.25-1.75 inches of swe
from 12z sun through 12z Tuesday. Would not be surprised to see a few
locations in the Sierra Madre range pick up a few feet of snow in
this type of pattern. Outside of the mountains it appears there could
be a brief shot of snow right behind the front on Monday afternoon. It
will definitely be a chilly day on Monday due to the combination of
colder temperatures and wind. Snow will decrease in intensity on
Tuesday...though likely not end completely as moist orographics
continue. Attention then turns to the system that will impact the
region in the Wednesday night and Thursday timeframe. The GFS is more
progressive with the system compared to the European model (ecmwf) which amplifies
the upper trough across the southwestern United States. This is
still over a week will not discuss too much at this
time...however this system could definitely have adverse impacts
around Christmas.


Aviation...(for the 18z tafs through 18z Friday morning)
issued at 1045 am MST Thursday Dec 18 2014

VFR expected overall through the period with light winds. Some fog
may redevelop in the North Platte River Valley tonight but a more
downslope breeze should limit its extent or density.


Fire weather...
issued at 319 am MST Thursday Dec 18 2014

Seasonal temperatures and dry conditions will
continue through the upcoming weekend limiting any fire weather
concerns. The mountains will see periodic light snow but
accumulations will be relatively light. High temperatures will rise
into the 40s over the eastern plains for Sunday...but with sufficient
moisture in place minimum relative humidities will only fall into
the 40-50s. The windy pattern will return to the district for Sunday
through Tuesday with several more feet of snow expected over the
snowy and Sierra Madre ranges.


Cys watches/warnings/advisories...


short term...cah
long term...zf
fire weather...cah

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