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Area forecast discussion 
National Weather Service North Platte NE
1110 PM CST sun Dec 21 2014

issued at 348 PM CST sun Dec 21 2014

Water vapor imagery at 20z showed strong ridging over the eastern
Pacific with a trough centered through the plains. The upper jet
could be picked out from off the British Columbia/Pacific northwest
coasts which dug into the trough over the Continental U.S. Into the central
rockies towards the Southern Plains. Could pick out multiple
shortwave troughs moving over the ridge...the primary of concern was
one currently over western Montana...with the second just off the
British Columbia coast. Another very weak impulse of energy could be
picked out over eastern Wyoming as well. At the surface...a weak
Pacific warm front moved north through Nebraska early this morning
which brought warmer air into the region today as temperatures rose
into the upper 30s and 40s. Weak radar echoes have been observed
through a good portion of the day over western Nebraska and into
Wyoming. A substantial dry layer in the low to middle levels /below
650mb/ has prohibited any precipitation from reaching the ground as
ceilings remain at or above 7k feet.


Short term...(this evening through monday)
issued at 348 PM CST sun Dec 21 2014

As the shortwave over Montana will strengthen slightly and dig into
South Dakota/Nebraska tonight...then slowly progress the system east
through the day Monday. Models are in decent agreement with the
evolution of the middle level disturbance but do have some issues with
the degree of lift and the saturation which leads to some
discontinuity on where and when precipitation will be. The trend was
a bit slower with the progression of the system eastward from prior
model runs which is likely due to the middle level low strengthening
and digging a bit further south than previous model runs. The
surface low moves east across southern South Dakota tonight...which
keeps the majority of the moisture over South Dakota...on the north
and western sides of the low. Then as it gets over southeast South
Dakota later tonight and slows in movement...the moisture is able to
be pulled further south and into northern Nebraska. It appears that
the full column won't be saturated for much of the local area until
at least 03z tonight...and perhaps could be as late as 06z. That
being said...upslope flow over the Pine Ridge would provide some
extra lift and a bit more favorable conditions for precipitation so
chances remain higher in those areas through the overnight hours.
Chances for precipitation ramp up Monday morning across northern
Nebraska as moisture stays in place although the lift isn/T real
strong. Soundings and cross sections show enough Omega to keep light
snow in the forecast through the day. That being said...the best
Omega is below the dendritic growth zone so not expecting real high
snow to liquid ratios and used a range from 10:1 to 13:1 for the
forecast. This puts a couple inches of snow across the northeast
Panhandle and into the western sandhills through the day...with
lesser amounts to the south and east. With precipitation
type...temperatures will stay above freezing through much of the
night for most areas keeping the precipitation type as rain.
Temperatures will be cooling from west to east overnight as cold air
advection occurs behind the cold front which will move through the
entire forecast area by 12z. There is currently a decent warm layer
above the surface...but will erode fairly quickly overnight in the
wake of the front and as surface temperatures will see a slow trend
downward...with lows by 12z in the low the time surface
temperatures drop below freezing...the warm layer should no longer
be an issue. Therefore...have removed the chances for freezing rain
and should instead see a transition from west to east from
a rain/snow snow. The transition east will be fairly slow
as there remains at least a mix in the eastern portions of the
forecast area through middle afternoon Monday. By 21z...forecast
soundings show all areas with the entire column below
freezing...perhaps with the exception of the surface which would
still lead to snow. The last concern will be strengthening winds on
Monday. A strong pressure gradient develops behind the cold front
with northwest winds increasing to 20 to 30 kts...especially in
areas west of Highway 83. Winds at 850mb in this time frame will be
upwards of 50s...and while not expecting mixing to this level...any
vertical mixing will bring down some stronger winds so thinking
gusts to 35kts are not out of the question. Although the snow
falling on Monday is not expected to be heavy...the strong winds
will likely cause areas of blowing snow which could create hazardous
travel conditions across portions of the area. Therefore will be
issuing a Winter Weather Advisory for the western sandhills into the
Pine Ridge for the day and evening hours Monday.

Long term...(monday night through sunday)
issued at 348 PM CST sun Dec 21 2014

Models having some difficulty with middle term system that will
affect the northern plains Monday Tuesday time frame. Latest European model (ecmwf)
has moved north into South Dakota while NAM and GFS continue trend
of closing off upper low over Nebraska Monday. Will use a blend of
GFS and NAM for the middle term solutions. Monday night negatively
tilted closed off system over eastern Nebraska. Strong jet
170-190kts+ will dig upper low further south and deepen it
rapidly. Baroclinic with little progression east over night.
Northern Nebraska will be in deformation zone with good lift and
deep layer moisture. Saturated to nearly 500mb. Precipitable water
at 144% this morning and looking upstream into Montana precipitable waters
150%-200% of normal. Have gone with higher hpcqpf guidance.
Temperatures will be falling into the 20s for overnight lows with
all snow in the forecast. Very tight pressure gradient with strong
winds..areas of blowing snow reducing visibilities to a half mile. Strong
winds continue through the day over western Nebraska. Snow ending
over north central zones Tuesday afternoon. Highs only in the 30s.
Have lowered temperatures over western Nebraska Tuesday night with
snow cover. May have to be lowered more. Warming Wednesday and
Wednesday night ahead of next system. Have kept temperatures
somewhat cooler with snow cover. Thursday cold air advection
through the day with falling temperatures. May be a mix of
precipitation early but have kept it all snow for now. Cooling
after the front on Christmas day and cooler through the end of the


Aviation...(for the 06z tafs through 06z Monday night)
issued at 1109 PM CST sun Dec 21 2014

A band of rainshowers between anw and tif will move across bub and
onl 10-12z. As an upper system pivots across northern Nebraska...
snow is possible from vtn to onl and north of bbw 12-18z. Wind
310-340 at 14-16g23-25kt about 12z and will increase to
18-20g26-32kt by 19z. This will produce some blowing and drifting
snow in central northern Nebraska which includes mhn...tif...vtn and


Lbf watches/warnings/advisories...
Winter Weather Advisory from 6 am CST /5 am MST/ Monday to
midnight CST /11 PM MST/ Monday night for nez004-005-023>025-



short term...Brooks
long term...power

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