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Area forecast discussion...updated 
National Weather Service North Platte NE
1122 PM CST Friday Dec 19 2014

Synopsis...
issued at 250 PM CST Friday Dec 19 2014

Strong pv anomaly moving across the Pacific northwest ahead of large cyclone
over the Gulf of Alaska. Downstream of this feature ridging exists from
the Great Basin into the central Canadian rockies. Further west
across the Pacific...strong nearly zonal flow exists which is an
extension of the east Asian jet. This identification is important in
understanding the eventual amplification of this pattern later in
the period.

Closer to home...a quick moving compact pv anomaly is diving
southeast across South Dakota however very little precipitation is associated with it
as low levels are dry to the south and west of the feature. Enhanced
flow south of this pv anomaly is responsible for clearing low clouds
across much of western Nebraska...though clouds and some fog remain
across eastern areas of Nebraska.

&&

Update...
issued at 959 PM CST Friday Dec 19 2014

Konl has been at 1/2 mile and a 1oo foot ceiling for about 3 hours
and it looks this area north and south is in prime fog region. A
dense fog advisory is out for the rest of the night for
Holt...Garfield and Wheeler counties.

&&

Short term...(this evening through saturday)
issued at 250 PM CST Friday Dec 19 2014

Through tonight the biggest forecast challenge continues to be fog
reformation in the eastern portion of the area. Stratus line extends
from near lxn to onl this afternoon and although visibilities look better
based on ndor web cams...konl still reporting less than 3sm at the
moment. Believe based on short range model forecast soundings that visibilities
will improve through early evening from eastern Custer into eastern Holt and
Boyd counties...but after sunset visibilities will decrease again on the
western edge of stratus formation...similar to what happened across the
bbw and lbf areas last night. Min temperatures will be greatly affected by
cloud cover and fog in those areas...but strong radiational cooling
potential exists further west. Have gone with a cooler than most
guidance solution. Some chance of fog development in river valleys
due to snowmelt this afternoon being trapped in the boundary layer
with light winds and good radiational cooling tonight. Temperature forecast
very tricky in all locations tonight.

Fog dissipation is expected by middle morning in the eastern Custer to eastern
Holt County areas previously mentioned. End result is plenty of
sunshine through the day most areas and warmer temperatures as a transitory
ridge moves into the Central Plains. Again...maximum temperatures will still be
affected by existing snow cover. Next weak pv anomaly will move
across the Dakotas through the day Saturday...but the best forcing
looks to stay north of the forecast area based on the latest short range
models.

Long term...(saturday night through friday)
issued at 250 PM CST Friday Dec 19 2014

A front and upper trough pivoting through Nebraska Sunday night will
bring a chance of precipitation to central and north central
Nebraska. System relative upglide and frontogenesis in the 295-305k
layer will lift nearly saturated air into a conditionally unstable
layer. During the evening...low level temperature profiles indicate
rain rather than snow. As the lowest level cools...there is a
possibility of snow after midnight and into Monday.

As the cyclone continues across Minnesota and Iowa Monday and Monday
night...another front and upper trough swings through. System
relative upglide and frontogenesis in the 290-300k layer continues
lifting saturated air into the dendritic growth layer. With
temperatures of 35-40f most areas...it could be either rain or snow.
The lift cuts off by 12z Tuesday.

For Thursday...the gfs40 indicates a surface low pressure center
crossing the Central Plains Thursday evening with an inverted trough
extending into western Nebraska and southwest South Dakota. A cross
section through the area shows that the lift is primarily in the
700-500mb layer with only a marginally moist boundary layer. This
would limit the coverage and extent of precipitation.

The European model (ecmwf)...however...shows a closed 700mb low coming out of the
northern U.S. Rockies and dipping south of McCook and turning
northeast into Iowa. This track would favor significant snow across
central and western Nebraska. The track is also similar to the one
taken by the cyclone that went across this past weekend.

In an attempt to blend the two primary extended models...we end up
with 25-35 percent probability beginning in the northwest Thursday
and 25-40 percent over a large part of the middle part of the state
Thursday night. The probabilities then decrease during the day
Friday.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1121 PM CST Friday Dec 19 2014

LIFR is expected continue overnight along and east of Highway 281.
VFR/local MVFR in fog is expected elsewhere through 15z Saturday.
VFR is expected west of Highway 281 from 15z Onward. MVFR/IFR may
continue into the afternoon near and east of konl...perhaps into
Saturday evening as little upper level support will arrive until
Sunday morning to scour out the Arctic air and stratus.

&&

Lbf watches/warnings/advisories...
dense fog advisory until 9 am CST Saturday for nez010-028-029.

&&

$$

Update...cdc
synopsis...jws
short term...jws
long term...Springer
aviation...cdc

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