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Area forecast discussion 
National Weather Service North Platte NE
327 PM CDT Thursday Sep 3 2015

Synopsis...
issued at 326 PM CDT Thursday Sep 3 2015

Anomalous upper trough over the Pacific northwest continuing to dig further
south as evidenced by water vapor imagery and 50m height falls at 500 mb
at Medford or. Downstream ridge over the plains also continuing to
build into southern Canada with weak height rises there...however more
subtle 500 mb height falls in asssociation with a pv anomaly moving in
southwesterly flow aloft across western Colorado. Some lightning associated with this
feature as it interacts with higher terrain and new lightning
developing into the northestern Colorado High Plains early this afternoon. Temperatures
have climbed quickly where strong insolation is prevalent...with middle
to upper 90s already in NC Nebraska.

&&

Short term...(this evening through friday)
issued at 326 PM CDT Thursday Sep 3 2015

Primary forecast issue will be coverage of rain showers/thunderstorms and rain as the
aforementioned pv anomaly moves northeastward into southern Wyoming and western nebr this
evening. Weak height falls associated with the pv anomaly will
overspread the western portion of Nebraska this afternoon. As this
occurs...pressure falls in the High Plains will increase southeasterly flow
at low levels and serve to tighten the dew point gradient across the
western portion of the area...as deeper mixing lowers dew points in the
west and more moist air works in from the southeast. In addition...southwesterly
flow aloft is contributing to the deeper moisture as evidenced by
the increase in mixing ratio at 320k. As a result the 1.11 inch precipitable water
on this mornings lbf sounding is near the 90th percentile and well
above the mean amount for this time of year. However...the strongest
upper forcing remains further west of the area and as a result
effective shear lags initial development this afternoon. Therefore
expecting isolated thunderstorms and rain to develop along/near the convergence line
from SW Nebraska into the western sandhills...and much like the last
couple days will struggle initially as the convective towers fight
dry air entrainment...but since better cape is available above the
level of free convection today...and with the arrival of better effective shear as the
upper forcing progresses...isolated to scattered thunderstorms and rain should sustain and move
northeastward. As the low level jet develops in response to the upper forcing...and
then is later enhanced as is typical on the High Plains as the drier
air to the west cools quicker tan the moist air to the
east...upscale development should occur this evening and allow thunderstorms and rain
to increase in coverage across western Nebraska...where rainfall has been
scarce the last two weeks. Despite better dynamics eventually...severe
thunderstorms and rain not expected because of insufficient shear...though wouldn/T be
surprised if small scale storm interactions cause a couple to become
strong.

As the pv anomaly progresses northeastward overnight...precipitation should decrease
in coverage and intensity through the night and especially as low level jet
wanes toward 12z Sat. Decent sunshine should occur by late morning most
locations and allow temperatures to climb to above Ave again
Friday...though not as warm as the last couple of days. Next short
wave trough embedded within southwesterly flow will work into the central
rockies and out onto the High Plains late Friday.. there is some
disagreement among the models as to the forcing associated with it
but regardless any precipitation chances would come beyond 00z Sat.

Long term...(friday night through thursday)
issued at 326 PM CDT Thursday Sep 3 2015

Middle and long term models in fair agreement. Trough axis will lie
over western Nebraska Friday night and into the day Saturday and
Saturday night with 40kt low level jet. Slight chances for thunderstorm retained
Friday night into Saturday morning and then increasing through the
day Saturday as the negatively tilted upper low moves into the
northern plains. With the atmosphere mixed and increasing warm air advection and
moisture advection ahead of the cold front Sunday morning have
increased to chance probability of precipitation along the frontal boundary Saturday
morning and then shifting east through the day. Lows Friday and
Saturday night in the 60s and 70s with the fairly strong low level
jet. Highs on Saturday in the low 90s. Cooler on Sunday with the
passage of the cool front and precipitation ending over north
central Nebraska. Strong northwest flow through the day on Sunday.
Lows in the 40s and 50s Sunday night. Frontal boundary lifts north
back into western Nebraska and becomes the focus for thunderstorms
through the day mainly over the eastern nhalf of the forecast
area. Highs in the 80s Monday. Cloudy and cool through the
remainder of the forecast period with highs in the 70s as the
upper low moves slowly east out of south central Canada. Slight
chances for precipitation Tuesday with weak waves moving through
zonal flow aloft pushes a weak cold front across the area.

&&

Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1235 PM CDT Thursday Sep 3 2015

Main forecast challenge for aviation will be thunderstorms and rain chances this
afternoon and evening. Short term models as well as cams
identifying a convergence zone near the dry line boundary that
will be the focus for development in an isolated sense this
afternoon...then growing slowly upscale as the low level jet kicks in this
evening and overnight. Shear remains weak however so widespread
organized convection doesn/T seem likely at this point. Cloud
bases will be quite high initially in inverted-v type sounding
profile this afternoon...and some gusty wind potential exists near
thunderstorms and rain. Some lingering rain showers potential in portions of NC Nebraska so have
carried that potential as a prob30 group for kvtn.



&&

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

$$

Synopsis...jws
short term...jws
long term...power
aviation...jws

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