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Area forecast discussion 
National Weather Service North Platte NE
624 PM CDT Tuesday Sep 1 2015

Synopsis...
issued at 318 PM CDT Tuesday Sep 1 2015

Low pressure aloft has migrated southeast from southeastern
Alaska into the southern Alaska Panhandle over the past 24 hours. A low
amplitude trough of low pressure extended southward to northern California with
broad west southwesterly flow extending from the trough eastward across
the northern tier of states and southern Canada. High pressure was present
north of the Bahamas with a ridge extending into southeastern Canada. Other
features across the lower 48 included a strong shortwave over
central Indiana and a second over the arklatex. Across the Central
Plains...a weak disturbance continues to lift from central portions
of Nebraska into southern and southeastern South Dakota. This
disturbance was responsible for showers and embedded thunderstorms
earlier today. At the surface...a weak surface trough of low
pressure extended south from western South Dakota through the
Nebraska sandhills into western portions of Kansas. Given the weak
nature of this feature...winds were light across western and north
central Nebraska this afternoon with most locations either reporting
nearly calm winds or southerly winds at around 10 miles per hour. Skies were
mostly cloudy in the northeastern County Warning Area...with mostly sunny skies
elsewhere. Readings as of 3 PM CDT ranged from 81 at Oneill to 93 at
Valentine.



&&

Short term...(this evening through wednesday)
issued at 318 PM CDT Tuesday Sep 1 2015

Rain showers exiting the northestern portion of the County Warning Area as upper pv anomaly pulls
northeastward across NC Nebraska. Steady stream of weak pv anomalies noted in
southwesterly flow aloft as seen in water vapor will anchor a surface trough
across the eastern Panhandle into Wednesday afternoon before it begins to wash
out. This occurs as the upper ridge builds over the plains in
response to heights fall in the Pacific northwest. For tonight...some upward
forcing as next weak pv anomaly interacts with the surface trough in the
eastern Panhandle tonight as seen in cross sections showing shallow
circulations along the trough. However Storm Prediction Center mesoanalysis Page showing
low relative humidity in the local-level of free convection layer suggesting that dry air entrainment will
inhibit much development. Models not suggesting actual quantitative precipitation forecast with this
process either...but have included an area of high-based isolated
rain showers/thunderstorms and rain in and near the surface trough. Any development should
dissipate with sunset. Classic plains nocturnal low level jet will develop
overnight as well...however any development should be east of the
area closer to the upper forcing associated with the aforementioned
pv anomaly.

As mentioned upper ridge continues to build tomorrow which will
allow for strong insolation...and some warming enhancement in the
eastern Panhandle and north central zones with weak wrly component to
winds to push temperatures to between 95-100 in most places. Mixing in eastern
areas looks to not be as deep where more moisture remains in place
along with southeasterly winds which are not conducive to deep mixing in
this setup. Some fire weather concerns in the northwestern zones as most
models suggest relative humidity values in the upper teens. In addition...much of
this area /fire zones 204 and parts of 208/ have had very little
rainfall in the last 7 days and little in the last 2 weeks in the
southern half of 204. However model soundings suggest a deep layer of
light winds /less than 20 kts below 400mb at Merriman/ so even with
deep mixing winds will likely not reach criteria. This will have to
be something to be watched however. Have kept the forecast dry as
increasing heights and warm temperatures aloft will inhibit thunderstorm
development.

Long term...(wednesday night through tuesday)
issued at 318 PM CDT Tuesday Sep 1 2015

Wednesday night through Friday...in the middle term...temperatures and
precipitation chances will be the main forecast concerns. For
Wednesday night...a surface trough of low pressure will remain
anchored across the eastern Panhandle and northestern Colorado with a
decent pressure gradient extending into eastern Nebraska. H925 winds
are in the 10 to 20 knots range across the eastern County Warning Area through 12z
Thursday. With winds not totally decoupling overnight...mild temperatures
are expected. Lows will range from the middle 50s in the eastern
Panhandle where dry air and lighter winds exist...to around 70 in
the far east where mixing will be more prevalent. Shifting to
Thursday...another day of hot temperatures are expected across
western and north central Nebraska. Albeit...not quite as hot as
Wednesday. Was concerned yesterday about possible critical fire
danger conds Thursday afternoon with the higher winds expected.
However...the NAM solution this morning...does have an slight easterly
component to the low level winds during the afternoon hours. This
will transport addtl bl moisture westward and limit temperatures to the
lower to middle 90s. Further west...along the surface trough...minimum
relative humidity may touch twenty percent thus afternoon...however winds will be
lighter along the trough axis curtailing any critical fire danger
threat. Late Thursday afternoon into Thursday night...thunderstorms
will begin to develop in vicinity of the surface trough over the eastern
Panhandle and southwestern Nebraska. The latest NAM solution effectively weakens
the cap after 21z along the trough which is expected to lie across
far southwestern Nebraska and the eastern Panhandle. Thunderstorms are
expected to initiate along the surface trough during the late
afternoon hours and track northeast Thursday night as a nice shot of
middle level warm air advection spreads over western and north central
Nebraska. The latest NAM solution tracks this convection northeast
into central and north central overnight along the nose of a 40 to 50
knots low level jet. On Friday...lingering convection will exit the
northeastern County Warning Area during the morning hours. By afternoon...the threat
for thunderstorms will increase once again along and east of a
surface trough anchored across western portions of the forecast
area. Middle level warm air advection will approach from either western Kansas or northestern
Colorado Friday afternoon/night with the GFS/NAM/ECMWF differing on
the track of this feature. Compromising between solutions tracks
this lift through the middle of the forecast area Friday afternoon
into Friday night. Inherited forecast has low probability of precipitation mentioned in all
areas Friday night and see no reason to modify this forecast at this time.

Saturday through Tuesday...in the extended...a decent 500 mb trough of
low pressure will track from the Pacific northwest into the northern
rockies...then south central Canada by Monday. The European model (ecmwf) and GFS
model solutions have diverged somewhat with their intensity of this
feature. The latest European model (ecmwf) is much stronger with the 500 mb low. If this
were to verify...a prefrontal trough would be pushed well east of
the area Sunday...which would lead to a dry forecast. The GFS solution is
weaker with the 500 mb low and further west with the surface
trough...which would facilitate some mention of probability of precipitation in the central
and eastern County Warning Area Sunday/Sunday night. A compromise between solutions
was utilized...ie. Having low probability of precipitation in the east Sunday/Sunday night
and not eliminating them altogether as the European model (ecmwf) would suggest.
Both model solutions do drop a decent cold front into the forecast
area Sunday night into Monday with a decent cool down noted for
Monday. After widespread highs in the upper 70s Monday...temperatures will
begin to migrate upward into the lower 80s in the west Tuesday as
southerly winds increase on the back side of exiting high pressure.
Eastern areas will hold onto the 70s for one more day.

&&

Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 614 PM CDT Tuesday Sep 1 2015

High resolution short term projections show the possibility of
showers or thunderstorms in southwest Nebraska south and west of a
bbw-Oga line and even in north central Nebraska south and east of
an onl-bbw line. Confidence in either scenario is not high. The
only scenario in which we have moderate to high confidence is that
they are very unlikely north and west of an anw-tif-aia line. For
the southwest...the likelihood of anything at lbf is quite low.

Otherwise...unlimited ceiling and visibility is expected in
central and western Nebraska.

&&

Lbf watches/warnings/advisories...
none.
&&

$$

Synopsis...clb
short term...jws
long term...clb
aviation...Springer

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