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Area forecast discussion 
National Weather Service Cheyenne Wyoming
1152 am MST sun Dec 28 2014

issued at 1148 am MST sun Dec 28 2014

Inherited afternoon gridded forecasts in decent shape. Made some
minor modifications to
cover and snow coverage based on trends. One more relatively mild
afternoon before the Arctic cold front brings bitterly cold
temperatures Monday and Tuesday.

Update issued at 1044 am MST sun Dec 28 2014

Quick update to cancel our team High Wind Warning...set to expire
at 11 am...for Arlington...Elk Mountain...Bordeaux...the southern
Laramie range including the Interstate 80 Summit...vedauwoo and
Buford. Prognosticated low and middle level gradients suggest winds will
continue to slowly diminish through the afternoon hours.


Short term...(today through tuesday)
issued at 403 am MST sun Dec 28 2014

Current infrared satellite loop early this morning shows low to midlevel
clouds moving into the area ahead of the next winter weather
system. Ahead of the short wave trough currently over western
Montana...strong gusty winds continue this morning across the wind
prone areas of southeast Wyoming. Arlington just hit 58 miles per hour again
in the last 10 minutes as the pressure gradient is still pretty
intense ahead of the next cold front and associates shortwave
aloft. Southwest winds have not been as intense as previously
expected however...with only a few gusts hitting high wind
criteria. Models do show the 850-700mb gradient not quite as high
compared to yesterday...and basically 5 to 10 dm lower on average
compared to 24 hours ago. Expect the High Wind Warning to be
cancelled later this morning...especially further east near i25.
Attention will then shift to snowfall and the intense Arctic cold
front...which will rapidly push southward across the area late
tonight and into early Monday morning. High temperatures today
will likely be the warmest in a while...with readings in the middle
20s to around 30.

Snowfall will begin to increase in coverage this
afternoon...mainly west of the Laramie range and the northern High
Plains as the upper level shortwave quickly slides southeast.
Models show the Arctic cold front moving southward across Montana
by the afternoon and into northern Wyoming. This will likely be
the first part of the prolonged snow event for the forecast area.
Once this initial shortwave moves into the Great Plains...the
Arctic cold front will rapidly move southward into Colorado by
early Monday morning. Snow will then develop over the High Plains
late tonight and into Monday morning. Models are not indicating a
band of locally heavy snow along and just behind the Arctic front
any more as they were 24 to 48 hours decided to issue a
Winter Weather Advisory for the mountains excluding the southern
Laramie range for now...due to decent snowfall rates as upslope
flow and some frontogenesis will enhance lift over those areas.
There is still some concern that locally heavy snowfall
rates...for a brief period of time...may occur during frontal passage this
late this evening and tonight over the High Plains. This is
especially concerning given elevated Li/S between 1c to 3c and
Omega values between 2 to 4 mb/hour just ahead of the colder
Arctic airmass. Will let the day shift day another look at it as a
Winter Weather Advisory may also be needed for portions of the
High Plains. The Arctic airmass will push southward with
temperatures rapidly lowering into the single digits above and
below zero by sunrise Monday.

On Monday...the second portion of the winter weather event will
set up across the area as a strong 130+ knot jet strengthens and
lifts northward. This appears to be a low quantitative precipitation forecast/high pop event for
most of the area beginning Monday and continuing through Tuesday.
Expect 1 to 3 inches of snow across most of the forecast area as
the bitter cold temperatures spread across the area. Believe the
main impacts will be bitter cold temperatures and dangerous wind chills
instead of the snowfall...which appears to be pretty light due to
the very cold temperatures and lack of moisture. Highs on Monday
will likely occur in the morning with temperatures falling to
around zero by late in the afternoon. Periods of light snow will
continue through Monday night as temperatures continue to plummet
below zero with low temperatures generally between 10 below to 20
below zero. It will be breezy along and west of the Laramie
range...which will likely produce dangerous wind chills around 35
below zero. Will likely need wind chill headlines shortly for
Monday night/Tuesday morning.

Light snow will gradually taper to flurries on Tuesday as the
strong jet pushes eastward into the Great Plains. Extremely cold
temperatures are expected Tuesday afternoon as the bulk of the
Arctic air establishes itself over the High Plains. High
temperatures may not even break 0 degrees given current model
soundings. Will not be surprised if many locations across
southeast Wyoming remain in the single digits below zero through
the day...with wind chills between 15 below to 30 below zero.

Long term...(tuesday night through saturday)
issued at 403 am MST sun Dec 28 2014

The bitterly cold Arctic airmass will remain in place Tuesday
night as the 1050 mb surface high slides south across the area with
an elongated SW-NE oriented trough moving overhead. Light winds
are expected across the County Warning Area through the early overnight period and
skies will clear as the trough moves overhead. All signs point
toward overnight lows plummeting into the minus teens to minus
20s...with locations within the Laramie valley likely dipping
below the minus 30 mark. Winds will increase ever so slightly
through the early morning period as the SW-NE surface pressure
gradient strengthens in the wake of the departing surface high...but
temperatures will not increase much and wind chills will remain
dangerously cold through at least middle-Wednesday morning.

An upper low will become cut-off within the southwestern periphery
of the upper trough over socal and the Desert
Southwest...leaving the County Warning Area underneath dry split flow Wednesday
through Thursday night. The low will then eject to the east-
northeast and into the Central Plains on Friday. Temperatures will remain
cold on Wednesday and Wednesday night although moderated some due to
increasing downslope winds. Warm air advection will strengthen on Thursday and
Friday with temperatures returning into the 20s. The upper jet will move
overhead from the northwest on Friday night and Saturday with yet
another Arctic boundary moving south into the plains Friday night-
Saturday. The boundary looks to stall just to the east of the
Laramie range on Saturday...with north-northeast upslope winds developing
across the plains through the day. Should see snow increase over
the mountains as warm air advection increases and low level winds strengthen late
Thursday into Friday...and across the plains with and behind the
front late Friday into Saturday. On Saturday...the GFS moves a
stronger piece of upper level energy along the front than the
European model (ecmwf)...and therefore produces more snow across the County Warning Area. Kept
slight chance/chance probability of precipitation across the forecast area to account for
these differences.


Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1044 am MST sun Dec 28 2014

Wyoming tafs...mountains becoming totally obscured late this
afternoon and tonight. At Rawlins...ceilings and visibilities
deteriorating to IFR this afternoon and continuing through
Monday morning in snow...fog and low clouds. VFR prevails at
Laramie and Cheyenne through this evening...then an Arctic cold
front passes...with winds becoming north and ceilings and
visibilities lowering to IFR in snow...fog and low clouds.

Nebraska tafs...VFR prevails into the early to late afternoon...
with ceilings and visibilities lowering first at Chadron and
Alliance at early to middle afternoon...then at Scottsbluff and
Sidney early this evening...with an Arctic cold front passage
and becoming IFR at Chadron and Alliance by late afternoon or
early evening...and becoming IFR at Scottsbluff and Sidney
this evening in low clouds...fog and snow.


Fire weather...
issued at 403 am MST sun Dec 28 2014

No fire weather concerns this weekend and through next week with
very cold temperatures and a widespread surface snowpack
expected. Arctic air will push into southeast Wyoming and western
Nebraska starting Monday...with another good chance of snow
expected through late Tuesday night. Light to moderate snow
accumulations are expected.


Cys watches/warnings/advisories...
Wyoming...Winter Weather Advisory until 5 PM MST Monday for wyz103-112-114.



short term...tjt
long term...rjm
fire weather...tjt

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