Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 141746

Area Forecast Discussion
National Weather Service Sioux Falls SD
1146 AM CST Thu Dec 14 2017

.SHORT TERM...(Today and Tonight)
Issued at 331 AM CST Thu Dec 14 2017

Current water vapor image shows the next short wave diving southeast
near where ND, SD and MT meet. Deterministic models are all in good
agreement in tracking this wave primarily along and west of the
James River valley today when following the PV and mid and upper
level QG forcing. There is not much in the way of surface features
to follow with this wave. A broad area of weak low pressure exists
in central ND which weakens as tries to move southeast. At any rate,
moisture depth is still pretty robust across our western zones where
saturation exists through about 5km, and a thermal profile cold
enough to produce ice crystals. High pops for a low QPF event are
warranted in our southwest corner where around a half inch of snow
may fall this morning before warming up a bit this afternoon.
Heading to the east, pops rapidly drop off heading toward the I 29
corridor this morning. However, another weak mid level wave which
shows up in the 700-500mb QG forcing field dives southeastward this
afternoon and evening impacting the eastern half of our forecast
area. Just enough instability and a favorable thermal profile of
-12C at the top of the cloud layer may produce a few light snow
 showers/flurries, or sprinkles depending on temperature.
 Therefore some locations in east central SD, parts of southeast
 SD, extreme northwest IA and southwest MN could receive a dusting
 of snow this afternoon although some of it may melt. These light
 snow showers or flurries will likely linger into the evening
 hours as the wave departs. Concerning temperatures, highs today
 will be in the 30s, followed by lows tonight mainly ranging from
 20 to 25 degrees.

.LONG TERM...(Friday through Wednesday)
Issued at 331 AM CST Thu Dec 14 2017

In the longer range, the next system to watch continues to be on
Saturday night. Basically a three pronged upper wave takes shape
with the northern stream wave moving into the Dakotas, a second
wave near the four corners area, and a closed low off the
California Baja. The latest ECMWF is a little less aggressive with
the hookup between the southern and northern stream waves and is
in closer agreement to the NAM and GFS. There may be a bit of
light snow Saturday night in our west when initially, there is
some saturation in the confluent region of the northern and
southern stream jets. But as the northern stream takes over on
Sunday, the moisture supply cuts off and basically leaves our
forecast area dry. The surface low and associated inverted trough
angling from northeast to southwest in the central and southern
plains is fairly robust, but just does not have enough saturation
to work with this far north. At this time, next week looks
seasonably mild. In fact we may not be warm enough on Monday and
Wednesday, especially in our western and southern zones if some of
these 925mb temperatures pan out. Winds could be pretty strong on
Tuesday following a very weak cold frontal passage, with strong
subsidence and drying present for that day.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1144 AM CST Thu Dec 14 2017

Scattered light showers are expected this afternoon and evening
across the area, with MVFR ceilings likely. South and west of the
James River, mid-day temperatures will be warm enough to allow
falling snow to partially melt and create a rain snow mix. North and
east of the James River valley, snow will be the dominant
precipitation type, with only light accumulations under half an inch
expected. Most will see only a dusting to a couple tenths of an
inch. Winds will be light today, with precipitation ending around
15/0300z. Friday looks dry with breezy southwest winds developing
during the afternoon, gusting in the 20 to 25 kt range.




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