Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Omaha/valley NE
642 am CDT Wednesday Aug 27 2014
Short term...(today through friday)
issued at 318 am CDT Wednesday Aug 27 2014
Forecast challenges for today continue to be heavy rain location/timing
and potential for flash flooding along with headlines.
The 00z oax sounding had increased to 1.74 inches of precipitable water with
elevated instability around 1000j/kg. At 06z...a look at the WSR-
88D radar mosaic showed the the bulk of the convection was in the
upslope region of the Western Plains in southwest Nebraska and
northwest Kansas. There have been isolated echoes...locally. By
08z...convection had increased over southeast Nebraska and
southwest Iowa. This was where precipitable waters were the highest along with
The 00z upper air maps showed the closed 500 mb low over Utah with some
leading shortwave trough energy in Colorado. 700 mb dewpoints were pooled
over eastern Colorado and western Kansas...with some 4 degree c dewpoints into
eastern Nebraska. The 850 mb dewpoints at koax was 10...but 15 degree c
Over the next 24 hours...the 60kt 300 mb jet on the backside of the trough
is forecast to round the base of the trough and move across Colorado.
There is some the upper level support over the forecast area this
morning...however the best is over the Western Plains. The upper
level support/large scale lift continue to increase into the day
and overnight. 700 mb warm air advection...increases this afternoon
and lifts north and east after 06z. We are lacking the steep lapse
rates and instability is weak...but increases this morning this
afternoon. The 850 mb warm air advection is slower to evolve and less
pronounced than forecast 24 hours ago...thus precipitation evolving
slower locally and more focused to the west.
Currently have a Flash Flood Watch in effect for those areas that
have seen locally heavy rain over the last week and where 2 inches
inches of rain in an hour could lead to flash flooding. Although
slow to develop...the hi resolution models do increase the
coverage of rainfall through the morning hours. Although do not
expect widespread flooding...the airmass is capable of producing
locally heavy rain...thus will leave the watch in effect. The
heavy rain threat will continue through tonight as multiplerounds
of thunderstorms are expected and precipitable water values around 2 inches with
deep warm cloud depths will make for efficient rain production.
Forcing increases this evening and expanding the Flash Flood Watch
into tonight for northeast Nebraska can be assessed depending on
how the morning activity develops.
Thursday and Thursday night...the organized heavy rain should be
lifting north into Minnesota...however with the upper level system
and the surface front are forecast to still in the area. Will
maintain likely probability of precipitation for parts of the area. Friday...the NAM is
the odd model out...as the GFS/ec hold on to some scattered
precipitation...thus have kept the rain mention in.
High temperatures will be comfortable...in the 70s/80s each day.
Long term...(friday night through tuesday)
issued at 318 am CDT Wednesday Aug 27 2014
A few showers linger into Friday night...but Saturday looks mainly
dry. There is still some pockets of moisture and elevated
instability so will need to keep an eye on this. Lift increases
Saturday night through Sunday night as another shortwave trough
affects the area with a good chance for rain Sunday night.
Aviation...(12z tafs for kofk...klnk and koma.)
Issued at 636 am CDT Wednesday Aug 27 2014
Scattered thunderstorms will affect all eastern Nebraska taf sites
through the morning...bringing occasional IFR conditions. Areas
of MVFR ceilings will also develop outside of thunderstorms. While
tafs indicate a dry forecast for the 18z to 00z period...there is
a slight chance for more activity firing during that time.
Otherwise a stronger low pressure system moving out of the High
Plains will increase rain and thunder chances again after 00z.
Occasional storms will affect taf sites 00z to 12z...with a
downward trend for ceilings to IFR levels during that time.
NE...Flash Flood Watch until 10 am CDT this morning for nez042>045-
Iowa...Flash Flood Watch until 10 am CDT this morning for iaz055-056-