Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 272025
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
325 PM CDT Thu Apr 27 2017

.SHORT TERM...(Tonight through Sunday)
Issued at 324 PM CDT Thu Apr 27 2017

Rain and potential for snow this weekend are the primary forecast
concerns.

Broad mid level trough covered most of the western two-thirds of the
CONUS this afternoon, with strong low noted over the Northern Plains
into the Great Lakes. More of note for our area was shortwave
extending from southern Wyoming into western Kansas that was already
triggering scattered showers into western Nebraska and central
Kansas. This is the feature that will bring most of our weather over
the weekend.

Shortwave is forecast to lift northeast into Nebraska and deepen,
closing off a low in southwest Nebraska for Friday. Low then
migrates southwest to the Four Corners region by Saturday morning,
and eventually swings into the Plains for Sunday. There is fairly
decent agreement between models on this scenario, with some
divergence coming on Sunday when GFS brings core of closed
circulation into southwest Iowa by the afternoon, and ECMWF/Canadian
solutions place low in south central Kansas. NAM is more closely
aligned with faster GFS solution. This difference in placement of
low and associated cold air aloft will play a big role in regards to
precipitation type in our area as NAM/GFS would favor more snow in a
more dynamic cooling regime, while ECMWF/Canadian would favor a rain
or rain/snow mix.

Until Sunday, low level temperature profiles suggest rain will be
the most likely precipitation type, save for parts of northeast
Nebraska in the late night and early mornings when low level
atmosphere cools enough to maintain snowflakes to the ground. Little
if any accumulation is expected before Sunday as ground temperatures
and near-surface warm layer melt falling snow.

As closed low moves closer Sunday, forecast soundings suggest a good
chance for a complete changeover to snow in northeast Nebraska, and
at least a mix in all of eastern Nebraska and southwest Iowa. If GFS
and NAM position wins out, dynamics would tend to cool to below
freezing deeper column of air just above the surface, leading to all
snow in more of our area. Will have to see model trends the next
couple of runs to really pin this down, but given widespread QPF
totaling over an inch in eastern Nebraska, if a change to snow does
occur over a wider area, expected duration and rates will lead to
some significant accumulations of heavy and wet snow. For now, the
uncertainty in all of the above factors leads us to maintain only
minor accumulations of snow in northeast Nebraska, with a mention of
rain snow mix over the rest of the area on Sunday.

The evolution of precipitation is expected to begin later this
evening as shortwave rotates into Nebraska and western Iowa
overnight. Wave will gradually work north of our area as it
dissipates Friday afternoon, at the same time low begins its
southwest migration. Will have fairly high rain chances later
tonight into Friday before support wanes. We should likely see a dry
period Friday night into Saturday before broad area of upper
diffluence overspreads the Plains Saturday afternoon ahead of
ejecting Four Corners upper low. Chances increase Saturday night as
low moves closer, and continues all day Sunday with expected change
or mix with snow as outlined above. Total precipitation amounts in
the inch and a half to two and a half inch range are likely across a
good part of our area from tonight through Sunday night.

.LONG TERM...(Sunday night through Thursday)
Issued at 324 PM CDT Thu Apr 27 2017

Progression of upper low out of the Plains will bring an end to the
more significant precipitation Sunday night. In fact, models re-
converge on low position over Iowa by Monday morning, then lift
system into the Great Lakes. Mid level troughing behind this system
sharpens into the middle of the week, keeping our area under a cool
temperature regime. A better chance of some sunshine suggests we
will be a bit warmer than the weekend. Precipitation chances will be
sporadic, relying on minor impulses riding through the flow. A more
significant wave should remain to our south, keeping precipitation
there for Thursday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 107 PM CDT Thu Apr 27 2017

Patchy MVFR conditions early on at KOFK...then the trend will be
for increasing mid clouds this afternoon and tonight with showers
developing. As the low level moisture and lift increases
tonight...introduce showers then MVFR/IFR CIGS. There may be hit
or miss showers at KLNK in the evening, however have the bulk of
the steadier precipitation after 10Z. Forecast soundings show the
potential for a rain/snow mix at KOFK Friday morning. Surface
winds will be variable mostly under 10kts...however as the surface
low pressure system moves into western Nebraska...the easterly
component to the wind increases to 10 to 15kts or greater
generally after 09z.


&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&

$$

SHORT TERM...Dergan
LONG TERM...Dergan
AVIATION...Zapotocny


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