Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Omaha/valley NE
526 PM CST Thursday Nov 20 2014
Short term...(tonight through sunday)
issued at 315 PM CST Thursday Nov 20 2014
Main forecast problems are precipitation chances/precipitation type from late Friday
afternoon through Sunday and temperatures through the period.
500 mb pattern this morning featured a broad longwave trough across
much the Pacific...a smaller scale trough off the West Coast of
Washington...Oregon and northern California and a blocking ridge
from western Canada northward to above the Arctic Circle. Our area
was in modest northwest flow...with a large trough extending
southward from Hudson Bay into the eastern United States.
Tonight into early Friday afternoon...we expect it to be mainly dry
as the ridge of surface high pressure currently over the area shifts
eastward. 12z NAM snowcover initialization was not very good...showing
higher amounts of snow on the ground in our area than reality. So
surface and boundary layer parameters from that model should be used
with caution in the short term. We will see some weak low level warm
air advection and an increase in low level moisture. Cloud cover is
going to be tricky. Band of stratocumulus clouds that extended
from northeast Nebraska into southwest Iowa should break up or
lift north overnight. Some middle and high level clouds will try to
spread in from the west. It is possible that some low clouds may
develop in the increasing southerly low level flow...but a more
significant increase in low clouds will occur Friday afternoon.
The shortwave trough currently along the West Coast will drop down
into The Four Corners region by Friday evening...while a weaker wave
moves over middle America. A few of the models are generating light
precipitation into far southeast Nebraska and southwest Iowa by late Friday
..so a small chance of light rain was added to the forecast for
that. Some patchy drizzle possible into the Lincoln and Omaha/Council
Bluffs areas middle to late afternoon. Surface temperatures at that
time should be at or above freezing.
It appears that the strongest forcing for precipitation will stay off to our
south and east...but we will keep some chances going for about the
southeast half of our forecast area Friday night into Saturday.
This unfortunately still looks messy as far as precipitation type GOES.
Surface temperatures will be the key...since precipitation falling will be
in liquid form. Right now it does not look like a big impact event
for most areas...since amounts if any will be light. But...it may
be enough to cause some problems...especially for areas within
about 20 miles of the Missouri border if they are below freezing.
Again...surface temperatures will be the key factor.
Energy in the northern stream tracking mainly across the Dakotas may
be enough to produce a bit of mixed precipitation in our far northern zones
early Sunday...so that was added. Current timing suggests that
light rain chances in the southeast part of the forecast area
will decrease a bit by Sunday afternoon. Trended probability of precipitation down a bit
by late afternoon ... did not go completely dry. A few models
suggest that it could end up being dry though with northwest low
level winds Sunday afternoon.
Long term...(sunday night through thursday)
issued at 315 PM CST Thursday Nov 20 2014
The 12z GFS/ECMWF/Gem and ensembles all start out in pretty good
agreement at the beginning of this period. They show a ridge
extending from near 30n/130w northward into the Yukon...with a
broad trough from Manitoba and Ontario southward into Texas. In
general...the ridge will try to build eastward early next week.
The models diverge considerably by middle week...and so confidence
does drop off by that time as well. This is similar to what
things looked like at this time yesterday. Highs will be in the
30s Monday into Wednesday but it appears they will slide toward
the 20s by Thursday. Lows are expected to be mostly teens and
lower 20s but possibly a little colder toward Thursday night. Will
keep the period from Tuesday into Wednesday dry for now...with a
slight chance of snow for at least parts of the area Wednesday
night into Thanksgiving day.
Aviation...(00z tafs for kofk...klnk and koma.)
Issued at 525 PM CST Thursday Nov 20 2014
A band of low clouds were just northeast of koma and kofk at 23z
and were expected to remain away from those sites...although
there is a small chance they could spread back in for a few hours
before winds turn more southeasterly later this evening/early tonight.
With light winds and fairly moist low levels...MVFR fog was
possible at taf sites later tonight before winds increase Friday
morning. A strengthening low level jet could provide low level
wind shear conditions at kofk later tonight and this was
maintained in forecast. On Friday...a continued srly low level
flow was expected to bring MVFR ceilings back across taf sites later