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Area forecast discussion 
National Weather Service Omaha/valley NE
322 PM CDT sun Aug 30 2015

Short term...(tonight through wednesday)
issued at 318 PM CDT sun Aug 30 2015

Potential for one more morning of fog...then warmer temperatures and
a couple small rain chances through Wednesday are the primary
forecast concerns.

Once again this morning we saw areas of fog and stratus across a
good portion of eastern Nebraska and southwest Iowa with light flow
and trapped moisture under strong inversion. Fog had lifted but
clouds were slower to clear at early afternoon...but stronger
southerly flow should scour those out a little more efficiently than
Saturday. Stronger low level flow in response to shortwave moving
across the northern plains will make fog less of an issue especially
west of our area...but weak surface winds tonight/Monday morning
suggest much of southeast Nebraska and western Iowa will again
see at least patchy areas of fog.

Overall middle level pattern will begin amplification process
through the middle of the week. Low/trough in Alaska/British
Columbia is expected to settle south along the Pacific northwest
coast as downstream ridge begins to pump up across the eastern
two-thirds of the country. During this transition...modest
southwest flow over the plains will provide a couple of rain
possibilities as shortwaves eject through our area.

The first will come Monday night into Tuesday morning. This appears
weaker than previous model runs...but will still induce area of
isentropic upglide across northeast Nebraska. No strong consensus
among models or their ensemble will maintain a slight
chance for convection there Monday night.

The next shortwave is scheduled into our area Tuesday night into
Wednesday. Again this is not a strong impulse by any means...but low
level jet and thus Theta-E advection is stronger. And again there is
no real consensus between models as far as quantitative precipitation forecast is concerned. However
we will see decent isentropic upglide spreading from central into
eastern Nebraska overnight...leading to modest elevated instability.
And a smattering of model output converts this into thunderstorms
over a good part of eastern Nebraska and southwest Iowa late Tuesday
night and Wednesday. Thus am inclined to retain a small chance for
thunderstorms during both of those periods...though coverage is not
expected to be widespread.

Otherwise a general increase in temperatures is forecast Monday
through Wednesday as lower atmosphere warms due to persistent
southerly flow and better afternoon mixing regime. Have highs into
the upper 80s for the most part each day...with some areas seeing
lower 90s. Higher dew point temperatures should also
hold...suggesting overnight lows will remain well into the 60s if
not lower 70s.

Long term...(wednesday night through sunday)
issued at 318 PM CDT sun Aug 30 2015

A slow progression of longwave pattern is expected during the longer
term period. GFS/European model (ecmwf) and their ensemble means are fairly similar
in showing this...with a few smaller scale details still needing to
be worked out with time. We will start out the longer term with
western trough axis extending from British Columbia into northern
California...with an eastward shift to a Saskatchewan to Nevada line
by Sunday afternoon. Meanwhile...broad ridge from the plains to the
East Coast will initially amplify from our area into the Great Lakes
before stronger middle level flow on front side of trough works into
the plains for the weekend. Thus a mainly dry and warm pattern is
forecast through the end of the week...with increasing rain chances
for the weekend.

GFS differs from European model (ecmwf) this weekend in that it shows a much more
pronounced sub-tropical moisture plume extending from off the
northern Baja California coast into the plains. Thus it is much more bullish on
precipitation both Saturday and Sunday. There is still a lot of
uncertainty on how moisture profiles will pan out...but with a
majority of model output suggesting decidedly higher rain
chances...feel fairly confident we will see scattered storms
Saturday and Sunday. Otherwise look for highs close to 90 before
clouds/precipitation keep temperatures in the 80s for the weekend.


Aviation...(18z tafs for kofk...klnk and koma.)
Issued at 1214 PM CDT sun Aug 30 2015

While fog has cleared out of the area low clouds continue to
linger this afternoon. Expect skies to clear at kofk within the
next hour...with clouds gradually lifting at koma and klnk by
around 22z...however confidence in exact timing remains low.
Southerly winds will continue with a few stronger gusts possible
at kofk this afternoon. Expect lower visibilities once again
tomorrow morning as more fog develops.


Oax watches/warnings/advisories...


Short term...dergan
long term...dergan

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