Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Cheyenne Wyoming
303 PM MST sun Dec 21 2014
Short term...(this evening through tuesday)
issued at 300 PM MST sun Dec 21 2014
Powerful Pacific low pressure system continues to move into the
area from the west this afternoon...bringing widespread strong
winds and near blizzard conditions along portions of Interstate 80
out near Arlington. Have seen some reports of wind gusts
approaching 70mph and had an 80mph gust reported at a upper site up
on the south Laramie range. Forecast challenges continue to be
mainly winds...but later this evening...winter weather conditions
will take shape.
Currently...1015mb surface low currently over central Utah early
this afternoon. Perhaps our strongest gradient is yet to come as
surface analysis shows the strong pressure gradient still over
northwestern Colorado and northeastern Utah. Craig to Casper
surface gradient right around 12mbs at 1 PM this afternoon. Latest
wrkhgt products has 850/700mb Craig to Casper height gradients of
75 and 73mtrs respectively. Mosaic radar showing most of the snow
over northwestern Colorado and up over northwestern Wyoming.
Height gradients remain strong over southeast Wyoming through the
evening hours. Will keep all wind headlines going as they are for
now as the GFS shows 700mb winds still around 60-65kts at 06z
tonight. European model (ecmwf) shows the 500mb shortwave moving through central
Wyoming into western Nebraska after 06z with winds shifting
northwest behind the front tonight. Areas west of the Laramie
range stay in the favorable left front quadrant of a 170+ knots
jet...so heavy snow looks to continue for the snowy and Sierra
Madre ranges. Lower elevations will continue to see strong
winds...though not as strong as today...but enough to keep winter
headlines going out there through the overnight hours.
Attention turns to the Panhandle for Monday. GFS 850mb winds
behind the front out over the Panhandle increase significantly
overnight. Looking at 850mb wind speeds of 40-45kts for much of
the Panhandle. Went ahead with a Wind Advisory for now covering
much of the day. Later shifts may need to upgrade to high wind
warnings if later guidance shows anything stronger than what I am
seeing now. Heavy snow continues for the snowy and Sierra Madre
ranges all day Monday.
Finally begin to see an easing on the snow out west Monday night
into Tuesday as the upper low moves east. Current Winter Storm
Warning for the mountains looks good through 12z Tuesday...no
changes made in headlines.
Long term...(tuesday night through sunday)
issued at 300 PM MST sun Dec 21 2014
Main focus in the long term will be Tuesday evening through
Friday due to Holiday travel...and the potential for winter
weather on Christmas day. Models are maintaining run to run
consistency for the potential winter weather on Christmas...but
with such a large discrepancy between individual model runs...the
picture is not looking any clearer today in terms of the weather.
Earlier in the week...the beginning of the peak Holiday travel
period appears to be quiet and low impact...with all models
showing a ridge of high pressure quickly moving across the
northern Rocky Mountains and into the High Plains by Wednesday.
Temperatures will rebound slightly on Wednesday as downslope flow
develops...with highs in the middle 30s to middle 40s...warmest along
the i25 corridor due to downslope flow near the surface. This
slight warming trend will be short lived however.
By early Christmas morning...models indicate a rapidly moving
Pacific upper level trough pushing into the area from the
northwest United States. A strong Pacific cold front is forecast
to push through southeast Wyoming sometime Christmas morning with
snow developing behind the front. There is the potential for a
high wind event across the wind prone areas of southeast Wyoming
ahead of this front...so continued to increase winds over the
Arlington and Bordeaux areas. Wind impacts should be minimal
however for Holiday travelers...due to a relatively brief period
of strong winds...and the winds mainly affecting high profile
Forecast becomes tricky on Christmas day in terms of snow
potential as there is only slightly more confidence compared to
yesterday that a decent portion of the area will receive
accumulating snow. All models now show the system moving rapidly
across the area into Thursday night...which is important because
it greatly lowers the chance of winter storm conditions. The 12z
European model (ecmwf) is slightly faster and showing widespread light to moderate
snowfall nearly everywhere across southeast Wyoming and western
Nebraska. The 12z GFS has been consistently further north with the
trough axis and port maximum...and thus much less snow due to
downslope winds. The primary difference between these two
solutions...other than the quantitative precipitation forecast...is the storm track and jet stream
location. The European model (ecmwf) has the jet digging further south with a
surface low developing near the Wyoming/Colorado border and moving
southeast. The GFS on the other hand shows the jet stream over the
state of Wyoming with a surface low much further north across
Montana and the Dakotas. The net result is broad dry downslope
flow and low level subsidence compared to the European model (ecmwf) which shows plenty
of northeast upslope flow. The Gem is somewhere in between these
two extremes but a little closer to the GFS. European model (ecmwf) quantitative precipitation forecast values
range from one quarter of an inch to about a third of an inch.
Which means there is potential for light to moderate snow amounts.
Increased pop slightly across the i80 corridor to around 30 to 40
percent for now...with values near 80 percent for the mountains
which have the best chance of seeing 6+ inches of snow.
Hopefully...models trend toward a similar solution over the next
24 hours...but have a feeling that may not happen until Tuesday
when the system begins to move onshore. There is high confidence
that much colder temperatures will enter the region for the
remainder of the week and into next weekend with highs in the 20s
to middle 30s...and lows in the single digits (above and below zero)
Aviation...(for the 00z tafs through Monday afternoon)
Potent storm system will bring a combination of snow and high winds
to areas along and west of the Laramie range...producing prevailing
MVFR conditions with periods of LIFR conditions in moderate
snowfall. Light rain and snow is possible across the eastern High
Plains this evening...but the timing of this precipitation will be
difficult to predict...so kept vcsh in the forecast for now. MVFR
conditions are possible with this activity as it moves southeast
across the area. Strong winds will slowly diminish late this
issued at 110 PM MST sun Dec 21 2014
No fire weather concerns through the middle of the week. Very windy
conditions are expected through this evening along and west of
the Laramie range with gusts between 60 and 70 miles per hour likely.
Increasing moisture will keep the risk for rapid fire growth low.
Significant mountain snows will persist over the next 24 to 48
hours with total accumulations in excess of 2 feet possible in the
snowy and Sierra madres before all is said and done.
Wyoming...High Wind Warning until midnight MST tonight for wyz104>107-109-
Winter Storm Warning until 5 am MST Tuesday for wyz112-114.
Winter Weather Advisory until 8 am MST Monday for wyz113-115.
Winter Storm Warning until 11 am MST Monday for wyz110-111.
NE...Wind Advisory from 8 am to 5 PM MST Monday for nez002-003-