Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 190454

Area Forecast Discussion
National Weather Service Sioux Falls SD
1054 PM CST Wed Jan 18 2017

.SHORT TERM...(This evening through Thursday)
Issued at 346 PM CST Wed Jan 18 2017

Very mild afternoon under brilliant sunshine across the area, with
readings varying from near 40 along Highway 14, to around 50 in the
lower Missouri valley.  Quite a bit of melt leading to fairly high
dewpoints across areas east of the James Valley.  Main forecast
issue for the short term is predicting the advance of lower clouds.

Overall, moisture depth is quite shallow prior to at least later in
the night, so prospect for fog would seem to be low despite the high
near surface moisture until this time with some decent amount of
southerly gradient remaining. Trajectory for slightly deeper
moisture does get fairly close to the far southeast locations around
KSLB, and could see a bit of stratus/fog threat there somewhat
earlier. Main advection of stratus somewhat slower toward 09z into
the southern CWA and streaking north/northeast through the remainder
of overnight and morning hours.  As a result of cloud trends, likely
to see lower temps during the mid to late evening in the south/east
CWA, while more typical trends to coldest readings closer to 20 in
snowcovered mid James valley.

Clouds will become much more widespread during Thursday, backing
west of the James River. Much of the model moisture field depth
remains 2000 ft. or less, suggesting an insufficient depth for a
significant drizzle threat with at best indication of perhaps a bit
of shallow weak warm advection forced lift.  At this time, have
backed off drizzle development until very late in the day, again
timing out with a little better depth to low level moisture and
development of more coherent lift at the top of the layer.  Diurnal
range will generally be quite small for most locations under dreary
gray skies, while a decent rise of 15-20 degrees could occur from
morning lows along the far western edge of the CWA.

.LONG TERM...(Thursday night through Wednesday)
Issued at 346 PM CST Wed Jan 18 2017

Main challenge in the intermediate term is tracking a succession
of upper lows moving northeastward through the central plains
Thursday night through Saturday. These are spawned basically from
long wave troughing throughout the intermountain west.

First concerning temperatures, due to the abundant stratus,
continued the trend of relying on consensus raw model temperatures
which had the effect of lowering the diurnal temperature range.
Although superblend was not too out of whack, it looked as though
its diurnal range was a bit too much. Therefore consensus raw model
values raised night time lows, and lowered daytime highs Thursday
night through Saturday night. Therefore this said, it appears most
of the accumulating precip will fall as some sort of liquid variety.
Despite the first short wave moving northward Thursday night and
Friday, the depth of moisture is still rather shallow, generally in
the lowest kilometer, coupled with isentropic lift around 290K which
is extremely weak. In fact even the soundings do not exhibit any
sort of warm air advection wind profile which will limit lift. So
expecting only drizzle with the probability that fog will also
exist. However left fog out of the grids for now as am not sure how
widespread it will be in terms of coverage. It could be quite
widespread, at least along and east of the James River, and we
encourage the media to go ahead and mention it for their broadcasts.

On Friday night and Saturday morning as the next short wave advects
into our area, the soundings show a definitive increase in moisture
depth with saturation through the mid levels. Drizzle will give way
to decent chances for primarily light rain, albeit the surface
temperatures and temperatures aloft our just cool enough in our
western zones approaching central SD that there could very well be
areas of light snow. But expecting little to no accumulation due to
a lack of measurable QPF. Interesting how the ECMWF 850mb
temperatures picked up a little cool layer aloft in our western
zones by 12Z Saturday of sub freezing air, which should aid in light
snowfall or flurries. But again, very minimal accumulations if any.

After Saturday, the weather quiets down until next week when the GFS
shows a major upper low with temperatures aloft cold enough to
support snow, perhaps rain or snow around Sioux City next Tuesday.
Precipitation would be all snow Tuesday night through the CWA,
lingering into Wednesday. This solution is not backed up by the
ECMWF which is much stronger with the northern jet stream, thus
cutting off the moisture laden southern stream. However, the GEM
Global at 144 hours (12Z Tue) is closer to the GFS solution.
Therefore boosted pops east of the James River Tuesday and Tuesday
night blending in some GFS and GEM, but not going extremely high on
the pops yet due to the ECMWF.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1045 PM CST Wed Jan 18 2017

Advection of MVFR to IFR stratus will commence from the south
starting around around 12z in the Missouri Valley, spreading
rapidly north and east through the early morning. Lower
visibilities associated with patchy fog and drizzle will also be
possible Thursday and again late in the TAF period approaching 06z
Thursday evening, however confidence is too low for dz mention at
this time.




SHORT TERM...Chapman
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