Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
615 PM CDT Tuesday Sep 1 2015
Short term...(this evening through wednesday)
issued at 323 PM CDT Tuesday Sep 1 2015
The pattern aloft featured southwest flow across the Central Plains
with ridging to the south while an upper low pressure syste was
just off shore near British Columbia. A middle level shortwave trough
was slowly translating across the Central Plains and generated
hit or miss convection earlier this morning. The middle level
shortwave trough is expected to lift out this evening...then
another round of isolated to scattered convection is possible
tonight as the low level jet strengthens to 40kts. The orientation of
the jet axis places the better chances for convection along our
southern and eastern zones. The elevated instability is rather
meager...near 1000 j/kg and although expect thunder just not
looking for anything to be severe.
Wednesday could see a few lingering showers or storms in the
morning...then conditions dry out for the remainder of the day.
Southerly wind speeds ramp up as mixing deepens and the surface gradient
tightens. Model forecast soundings indicate mixing above 850 mb with
winds at the top of the layer near 30kts...and resultant surface winds
are expected to be southerly at 20 to 25 miles per hour with gusts of 30 miles per hour.
The strongest winds are forecast for our southern and eastern zones
where gradient will be tighter...while the western areas will be
closer to the western surface trough boundary. High temperatures
will again be above normal with readings in or near the 90s...and
also expect some degree of cloud cover from morning convection and
also as upper jet streak crosses.
Long term...(wednesday night through tuesday)
issued at 323 PM CDT Tuesday Sep 1 2015
Primary forecast concern through the long term lies with
Models showing zonal/slight southwesterly upper level flow across the area
at the start of the period Wednesday night...with high pressure
centered over western Texas into the Southern Plains...with an area of low
pressure over the southwestern coast of Canada and accompanying trough
extending down the West Coast of the US. At the surface...the County Warning Area
continues to sit with srly winds in place...as a trough of low
pressure extends through the High Plains. Inherited a slight chance
mention of a few thunderstorms in the 06-12z period Wednesday night/early
Thursday morning...and debated removing the mention...but a few models
continue to show the potential with the passage of a very weak wave
and an increased low level jet nosing into the region. Did trim back the area
of probability of precipitation to far southeastern portions of the County Warning Area. Any thunderstorms that do
develop are not expected to be severe.
During the daytime hours on Thursday...did keep the forecast
dry...but models are showing the upper level pattern becoming more
amplified across the Continental U.S....as that upper level low and trough dig
further south into the Pacific northwest and West Coast. The Main Ridge axis
pushes further north...reaching from the Southern Plains into the western
Great Lakes...leaving sharper southwesterly flow in place across the Central
Plains. At the surface...there doesnt look to be a lot of change...with
the County Warning Area remaining on the eastern side of low pressure/trough
axis...keeping winds srly.
Through the rest of the long term period...Thursday night through
Monday...the potential is there for more of an unsettled pattern.
The more amplified pattern looks to remain in place well into the
weekend...as that Pacific northwest low digs a little further south...ending up
roughly over southern Idaho by 00z sun. Sun/Mon...models show this system
then taking a more nerly turn...passing through The Rockies and by
Monday morning is located near the ND/can border. Periodic shortwave
disturbances out ahead of this system...and with this system
itself...will bring opportunities for precipitation. Have probability of precipitation going
in each period Thursday night through Monday...but kept them in the 20-
30 percent range...as confidence in timing/location/overall coverage
is not high...but in the coming days expecting there to be some
period dried out.
As far as temperatures go...no significant changes were made...with
most days in the middle 80s to middle 90s. The end of the period on Monday
has more uncertainty...as models trying to show a cold frontal
passage with the main upper level system pushing through...dropping
things into the 70s/80s.
Aviation...(for the 00z kgri/kear tafs through 00z Wednesday evening)
issued at 608 PM CDT Tuesday Sep 1 2015
Expecting VFR conditions for the forecast period. Main issue
appears to be the possibility of a low level jet setting up after
dark. Models not in total agreement on strength and location but
it does look like it will set up over southern central Nebraska. With
decent surface pressure gradient continuing...south winds should
stay around 10 knots overnight and then increase with mixing
occurring after 15z Wednesday.