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Area forecast discussion 
National Weather Service Cheyenne Wyoming
527 PM MST Monday Nov 24 2014

Short term...(this evening through tuesday)
issued at 526 PM MST Monday Nov 24 2014

Rather vigorous snow showers continue over the a good portion of
the County Warning Area early this evening as a shortwave moves overhead in the
lfq of a strong jet. Near whiteout conditions occurred at times
this afternoon in the heavy showers and gusty convective
winds...with this activity expected to continue to diminish this
evening. Slick and snow covered roads are anticipated as temperatures
drop tonight and freeze snow/ice to roads...especially I-25 and
I-80 near extreme caution is advised if traveling
through these areas tonight.

The main story for Tuesday will be strong winds and continued
chances for snow. The 130 knots jet will move over Wyoming tonight
and will shift east into the Panhandle through the morning...all
the while intensifying another 20 to 30 kts. Strong midlevel
energy collocated with the jet will slide southeast across Montana
in the morning...and into South Dakota/Nebraska late afternoon and
evening. Ahead of the wave and strengthening jet...the leeside
trough will rapidly deepen late tonight through tomorrow
morning...with 800 - 700 mb winds increasing to 50 to 70 kts
respectively by sunrise...or shortly thereafter. The inversion
looks to set up near mountain top level in the morning...with
increasing subsidence aloft as the rfq of the jet then moves
overhead. Therefore expect strong gradient winds and active
mountain wave gusts to occur for many areas along and west of I-25
and for eastern Laramie County in the morning and afternoon...and
possibly through the evening as well. Upgraded to a High Wind
Warning for these areas and timed it such that the more wind prone
areas from Bordeaux...southern Laramie range and
foothills...Arlington and northern Albany/Carbon counties start
earlier in the morning and end later to capture mountain wave and
gap influences...with the remaining zones expected to see the
strong winds tied more to diurnal mixing tendencies.

A rather strong cold front will drop south across the plains late
Tuesday afternoon and evening. Models show some differences with
timing of this feature with the GFS perhaps a bit faster with the
front moving through and stalling along the Laramie range shortlyafter
00z. This will have implications for when the winds across the
plains will shut off...and when snow will develop across this
area. For now...have winds dying off in the evening while
frontogenesis strengthens northwest-southeast across the plains after 00z
Wednesday and on through the night as the surface front stalls across
this area. So do not feel there will be a long period overlap of
winds and snow across the plains...and feel the wind highlight vs
winter highlight is appropriate for now. Have 1 to 3 inches of
accumulation for much of the plains along the axis of
frontogenesis...with the jet subsidence possibly acting as a
limiting factor for additional snow. Will have to see whether
heavy snow bands do develop though...and will then need to entertain
higher amounts and perhaps winter headlines for Tuesday night
across the plains. Further west...not expecting as much widespread
snowfall in the high valleys where winds will be strongest as most
of the forcing will be tied to orographics. Again felt wind vs
winter highlights best for these areas as well...but mentioned
blowing snow in the High Wind Warning statement for now.

Regarding the Winter Storm Warning for the mountains...decided to
just extend this warning through Tuesday night as the lull in
snow/winds tonight will be relatively shortlived with more heavy
snow for tomorrow. Likely to see a 12hr period of minimal snow in
the snowy and Sierra madres...but with winds rapidly increasing
around sunrise near the peaks and restarting the snow
machine...felt it best to just keep the warning going. Orographic
snow showers will occur through the day and much of the night as
strong west-southwest mountain top winds and high low level moisture
persist...with another 4 to 12 inches of snow accumulation
expected or perhaps higher near the peaks.

Long term...(wednesday through monday)
issued at 526 PM MST Monday Nov 24 2014

The main concern with this portion of the forecast package is the
potential for the lingering morning snowfall in areas east of the
Laramie range Wednesday morning followed by a warm up towards
Thanksgiving weekend.

models are advertising the band of 800-700mb frontogenesis
persisting through middle morning in response to a baroclinic zone
setting up between Cheyenne and Douglas with good isentropic lift
between 295-300k. Beyond that time frame...snowfall should come to
and end by midday with conditions improving as the Lee side trough
develops. This Lee side trough will also allow for some subsidence
to kick in during the afternoon. As a result...we should see wind
speeds increase to 40 to 50 miles per hour mainly along and west of the
Laramie range with higher gusts possible near The Gap regions of
Arlington and Bordeaux. In fact...the GFS/NAM are showing 800mb
gradients around 60 to 65 meters.

Wednesday night-thursday:
wind speeds will be reluctant to diminish during the Wednesday
night into Thursday timeframe. However...the subsidence will be
quite a bit weaker which may limit mixing somewhat Thursday. We
may still have to deal with some gusty winds in The Gap
areas...but overall there should be a trend to see lighter wind
speeds. Temperatures on the other hand will climb to the 40s and
50s with good downslope flow.

the upper level pattern will continue to favor mild conditions
into the weekend. However...the European model (ecmwf) is showing a fairly strong
cold front moving through the region late Saturday night into
Sunday morning. This boundary may bring a band of snow along with
it. The GFS is not advertising as cold of air as the European model (ecmwf) at this
point. For now...we will have to watch later runs to see how this
pattern evolves.


Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 526 PM MST Monday Nov 24 2014

Main concern with this taf issuance is when the snow showers will
diminish and when wind speeds will pickup tomorrow.

Latest radar loop was showing the snow showers gradually
dissipating. However...there are still a few rogue snow showers
that are reducing the visibility to less than one half mile at
times. These snow showers should continue to diminish this evening
before wind speeds start cranking up overnight starting at rwl and
lar taf sites after 09z or so. These strong winds will then begin
to push east towards areas east of the Laramie range after
daybreak. Next upper level wave is prognosticated to move into the rwl
taf site during the afternoon and possibly bring a chance of snow.


Fire weather...
issued at 526 PM MST Monday Nov 24 2014

Though winds will be strong at times...humidities will remain
below critical no fire weather concerns are


Cys watches/warnings/advisories...
Wyoming...Winter Storm Warning until 7 am MST Wednesday for wyz112-114.

High Wind Warning from 3 am to 11 PM MST Tuesday for wyz104>106-

High Wind Warning from 8 am to 7 PM MST Tuesday for wyz107-115-



Short term...rjm
long term...rec
fire weather...rjm

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