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Area forecast discussion 
National Weather Service North Platte NE
650 am CDT Wednesday Oct 1 2014

issued at 359 am CDT Wednesday Oct 1 2014

500 mb analysis this evening had a very active pattern across western
portions of North America with numerous shortwaves and closed lows
noted. Upper level lows were noted over the Yukon...northwestern
portions of the northwest territories of Canada and another low off the
northern coast of Alaska. Across the lower 48...a tandem of upper
level lows were noted. The first was located over northern South
Dakota with a secondary shortwave over northwestern Montana. Further
east...ridging extending northward from the Ohio Valley northward into Ontario
with a closed low noted over the eastern Great Lakes. As of 2 am
CDT...upper level low pressure had moved north into southern
Manitoba...while the secondary shortwave had moved south into southwestern
Montana. Closer to western and north central Nebraska...a nice
shortwave was noted over central Nebraska. This feature has led to
the development of some isolated showers...generally along and
east of a North Platte to Bassett line over the past hour. At the
surface...a surface trough of low pressure extended from roughly
just west of just east of Broken McCook. Dew
points along and east of this feature were in the 50s to lower
60s...while west of this trough...dew points were in the 40s to
around 50. As of 3 am CDT...under mostly cloudy
skies...temperatures ranged from 51 at Valentine to 63 at Oneill.


Short term...(today and tonight)
issued at 359 am CDT Wednesday Oct 1 2014

Today and tonight...over the next 24 hours...the main
forecast challenge is precipitation chances early this morning...and
later tonight as the upper level disturbance...currently over southeastern
Montana...approaches the High Plains. For this morning...weak shortwave
energy...currently over central Nebraska...will lift into northestern
Nebraska by middle morning. Have introduced some isolated probability of precipitation in vicinity of
the surface trough of low pressure to better reflect current radar
trends. In addition...will include a mention of the far
eastern counties through middle morning with weak srly flow and
moisture advection present. Behind the exiting shortwave this
morning...temporary subsidence will lead to clearing into the early
afternoon hours. This will allow highs to reach the lower to middle
70s across the eastern 2/3rds of the forecast area. Further
west...especially over the northwestern County Warning Area...increasing cloudiness
in association with an approaching shortwave...will hold highs into
the 60s. Precipitation chances late this afternoon will be limited
primarily to the far western County Warning Area with the main threat for showers
and thunderstorms arriving later tonight. In addition to the threat
in the west...locations in eastern Nebraska may see an increased
thunderstorm threat this afternoon in vicinity of the surface trough.
At this time...this threat appears to be just off to the east of the
forecast area where the surface trough is forecast to be. With this
in probability of precipitation are forecast in the eastern forecast area this afternoon.
By tonight...the upper level trough will lift into the central
rockies. In advance of this feature...a decent cold front will track
from the Panhandle into central and eastern Nebraska from middle
evening into the overnight hours. At this time...middle and upper level support
for precipitation will be strongest across central and southern portions of the
forecast area. Inherited forecast had likely probability of precipitation from the southern Sand
Hills south into southwestern Nebraska and this forecast remains on

Long term...(thursday through tuesday)
issued at 359 am CDT Wednesday Oct 1 2014

An active second half of the week is expected...while the weather
pattern quiets down and warms up for next week. Models are in good
agreement with how things will play out so have fairly good
confidence in the forecast this cycle.

An upper level trough will moved through the plains Wednesday
night and as it pushes eastward on Thursday Post frontal showers
are expected to linger across the southeast half of the forecast
area through Thursday morning. Models are indicating a deep layer
of moisture remaining over these areas through middle-day and with
strong upward motion seen through this layer felt it was needed to
keep precipitation chances continuing through the morning hours.
One thing of note...the lift in the low levels does begin
diminishing after 12z Thursday morning...and with instability
minimal if there is any seen in soundings...the precipitation
should steadily decrease in intensity through the morning hours.
Cooler air will advect into the region behind the upper level
trough which should lead to cooler temperatures for the day
Thursday. Believe there will be a period of sunshine from late
morning into the afternoon which would help warm the surface so
kept temperatures in the 60s. Locations where the rain continues
into the afternoon will be east of the local area...but will
likely only see highs in the low 60s.

While the Thursday activity is ongoing...a stronger system from
the northern stream will be making its way towards the northern
plains. The system now over the Gulf of Alaska and moving into
northern British Colombia will strengthen significantly through
Friday as it moves towards the north central Continental U.S.. again...the
models are in good agreement with the track and strength of the
system through the weekend so think it is applicable to say the
upper low will become closed by Friday morning over the Dakotas
before digging deeper into the trough as it moves into the lower
Ohio River valley. This system is quite potent...and brings a
large magnitude of cold air down with it as 850mb temperatures
fall down below 0c for a good portion of the area by 12z Friday.
As a weak pv anomaly rounds the base of the trough and moved
across western Nebraska Thursday night lift will strengthen over
southwest Nebraska so have added chances for rain showers to the
forecast. This trend is fairly new in from the models suites so
will continue to monitor closely the potential for rain with this
passage. If it does rain...looking at minimal amounts...if even
any measurable as there is a lack of moisture and lift and/or
instability to work with to develop showers. In some locations
winds will stay up across the forecast area Thursday night with
15kt winds looking likely. The winds and probable higher cloud
cover could present an issue with low temperatures. Took these
into account and kept temperatures a bit warmer than other
thoughts mights suggest if just looking at the temperatures aloft.
Although there is question on frost formation...kept it in the
forecast for locations over the northwest part of the forecast
area as lighter winds are expected over those areas...and skies may
become clear overnight to improve radiational cooling.

Friday will be a cool day...with temperatures well below normal.
The entire system will progress east however the colder airmass
will remain over at least a portion of the forecast area. The
stronger winds will continue from the overnight hours and will be
breezy through the day. With good confidence on the colder signals
dropped high temperatures Friday quite a bit...with highs now
being forecast in the 50s...with a few locations in the southwest
still having around 60 in the forecast. Friday night/Saturday
morning will see high pressure build into the plains...leading to
a diminishing trend in the winds. The lower winds and clearing
skies due to strong subsidence on the back side of the system will
create a good rational cooling set-up for much of the forecast
area for Saturday morning. So believe this will be the coldest day
of the week and again...dropped temperatures significantly. Lows
in the forecast are now in the low to middle 30s for much of the
forecast area. Model statistical guidance is giving lows for some
locations in the 20s...but at this time there is a slight bit of
uncertainty so not ready to go that cold yet but will watch how
the models trend in the next few runs. Due to the cold
expected...and with higher confidence did add frost to the
forecast over much of western and north central Nebraska.

As the system moves through Friday night...ridging will be
building over the western part of the country. There will be a
period of a few days when the pattern stays fairly
stagnant...leaving the local area in northwest flow with
temperatures staying near normal through the weekend. Also...ample
dry air will continue to push into the area so a dry forecast is
in place through the weekend.

Then into next week...the pattern begins to shift as the longwave
trough over the eastern Continental U.S. Moves east allowing the western
Continental U.S. Ridge to build into the plains. This will bring about
another period of above normal temperatures...and with dry air in
place a dry forecast.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 649 am CDT Wednesday Oct 1 2014

An upper level trough of low pressure and cold front will approach
and pass through western and north central Nebraska tonight. This
will increase the threat for thunderstorms and MVFR ceilings overnight.
For today...expect scattered high cloudiness across western and
north central Nebraska with ceilings ranging from 12000 to 20000 feet above ground level.
By evening...ceilings will lower to around 7000 feet above ground level as the front
approaches. The threat for precipitation and thunderstorms will be highest
from 03 to 09z Thursday for the klbf terminal and from 04z to 10z
for the kvtn terminal. With some timing differences between the high
res models as to when thunderstorms will begin to impact the klbf
and kvtn terminals...will handle the mention with a prob30 group
at this time.


Lbf watches/warnings/advisories...


short term...clb
long term...Brooks

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