Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 042113

Area Forecast Discussion
National Weather Service Sioux Falls SD
313 PM CST Sun Dec 4 2016

.SHORT TERM...(This evening through Monday)
Issued at 259 PM CST Sun Dec 4 2016

Subtle upper ridge of high pressure will move rapidly eastward
across the forecast area tonight. A westerly surface flow of air
will become southerly tonight. With a low level jet increasing to 25
to 35 knots across our southern and eastern zones between 06Z and
12Z Monday, coupled with a tightening gradient, surface winds could
become a bit breezy late tonight generally east of the James River
valley. In addition, strong mid level warm air advection will help
to usher in mid level cloud cover. Therefore after evening,
temperatures should begin to steady out, then likely warm a bit
after midnight. The main question is how far temperatures will fall
this evening as the winds become light when they are shifting to the
south and skies are still clear. Despite the very mild temperatures
this afternoon, trended toward the cooler bias corrected readings
this evening before modifying the temperatures after midnight. So
lows should be in the 25 to 30 degree range. Concerning fog, between
the advancing mid level cloud cover and increasing southerly flow,
fog should not become a major concern.

On Monday, upper trough will advance into to the Dakotas. A surface
wind shift to a brisk northwesterly direction will occur with this
trough and will begin to usher in cooler air across our western
zones by early to mid afternoon. Despite the aforementioned mid
level clouds moving from west to east, our eastern zones should once
again be quite mild ahead of the wind shift. Therefore highs should
warm into the upper 40s near Sioux City, progressively cooling off
to the upper 30s toward Huron and Chamberlain. There is enough mid
level frontogenesis aiding ascent in our extreme southeast near
Storm Lake that a light shower could occur late in the afternoon.
Some hookup exists between moisture near the surface and the mid
levels as exhibited by both the NAM and GFS soundings that a bit
of rain could reach the ground.

.LONG TERM...(Monday night through Sunday)
Issued at 259 PM CST Sun Dec 4 2016

A potential post frontal light rain/snow band should move quickly
southeast of the area Monday evening. Will continue to carry slight
chance POPs in the Storm Lake area, south of Spencer and east of
Sioux City. POPs will not be higher for now because the
precipitation will come from frontogenesis and lift that will be
fairly high in the mid levels, above 700mb. No snow accumulation is
expected. Otherwise, Monday night will bring an influx of cold air
and decreasing clouds. Lows will be in the teens to low 20s Tuesday

Tuesday through Thursday will be cold, breezy, and probably dry.
Initially mostly clear skies will cloud up some Wednesday through
Thursday. A secondary west coast wave driving into the developing
broad trough is expected to pass well south of the area. Meanwhile,
light snow could threaten from a decent Canadian wave rotating
around the upper low which will be over western Ontario. But current
guidance keeps measurable snow north and northeast of the area and
is accepted. The wave could easily spawn some flurries here, but
with timing questions on such a low impact event, and after
coordination, will leave out for now.

The models look a little too weak on wind fields, and with decent
low level mixing expected from model soundings, have raised speeds
a bit for Tuesday and Wednesday daytimes. Anticyclonic flow and
slowly rising pressures are factors here. These days should be
breezy to marginally windy. With daytime temperatures dipping to
the teens and lows in the single digits, wind chills will go below
zero. This is a pretty cold air mass, though it would have more
bite if we had snow cover.

Models have been shifting on strength and path of a decent wave set
to approach Friday and Saturday. Will keep a chance of light snow
spreading into the area during this time. Models today, including
the latest EC, are suggesting that precipitation will be mostly
north of the area, but will keep the half decent chance because
further changes in model solutions are expected. The light snow
should be out of the way in time for a dry Sunday.

The cold temperatures will warm just a little by the weekend, but
will stay below normal, especially on daytime highs.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1141 AM CST Sun Dec 4 2016

The 18Z TAF set reflect mostly VFR conditions at this time. Clear
skies will be giving way to a lot mid level cloud tonight as a
short wave moves across this area. It is anticipated that this
cloud cover will keep fog formation at bay. However, would not be
surprised to see some MVFR fog near sunrise Monday around KHON as
the mid level clouds move off to their east coupled with southerly
winds decreasing at the same time. Concerning winds, the low level
jet will increase rather dramatically between 06Z and 12Z Monday
primarily over KFSD and KSUX. The winds aloft are strong enough to
mention non convective low level wind shear late tonight and early
Monday at KSUX.




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