Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 101109
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
609 AM CDT Tue Oct 10 2017

.SHORT TERM...(Today and Tonight)
Issued at 347 AM CDT Tue Oct 10 2017

Skies have cleared for areas north and west of a Yankton, SD to
Jackson, MN line, allowing freezing temperatures for much of the
area. Freeze and frost headlines are on track, and will remain out
until after sunrise, when sunlight will quickly warm temperatures
above freezing.

Clouds south of this line have kept temperatures notably warmer, in
the mid 30s to low 40s. Areas around Sioux City to Storm Lake, IA
will continue to see cool autumn temperatures and on-and-off showers
throughout the day. Rain accumulations should be light, as a dry
layer below cloud base helps evaporate falling precipitation.

Southeast SD and other locations north of the precipitation will
also have another cool day, with light north and northeast winds
keeping the dry Canadian air mass in place. Highs remain mostly in
the 50s, which is several degrees below seasonal averages. Models
have trended a bit cooler for Wednesday morning, as skies remain
mostly clear. However, a shift to southerly winds and a moderating
air mass should keep overnight temperatures above readings seen
this morning.

.LONG TERM...(Wednesday through Monday)
Issued at 347 AM CDT Tue Oct 10 2017

The various deterministic models are in good agreement from
Wednesday through 12Z Friday. A large upper trough begins to dig
throughout the western United States placing the plains in a
southwest flow aloft. On Wednesday and Wednesday night, both the nam
and gfs continue to show a lot of stratus trapped in the lowest
kilometer, with moisture maximized near 900mb extending from our far
eastern zones to locations well east of our forecast area. The
models have been showing this moisture for a couple of days now, and
the stratus depth is deep enough on the soundings that the high RH
is likely not an artifact of cool air. After blending the nam and
gfs, raised sky cover from Windom MN to Storm Lake IA, with Marshall
MN to Sioux City IA appearing to be on the edge of the stratus
Wednesday and Wednesday night. This is all logical in that moisture
is trapped behind a departing mid and upper low, and begins to move
northward with a southeast low level flow. The stratus could impact
highs in our far east, but superblend and other guidance values are
already rather chilly in those locations so did not adjust down
further yet. Elsewhere on Wednesday, it still looks windy west of I
29 with 20 to 30 knots in the mixed layer. Therefore increased wind
speeds a bit above superblend values.

Both the nam and gfs exit the stratus east of our forecast area
pretty rapidly Thursday morning as the winds become more true
southerly and increase as well. For highs, soundings mix to about
900mb which produces a lot of upper 60s for highs in our CWA.

Model agreement continues Friday and Friday night. Upper QG forcing
moves across the northern plains with a very weak short wave, and
mid level frontogenesis is noted across much of the forecast area
coinciding with a strong mid level baroclinic zone. Only slight
chance to chance pops are warranted at this time because the lowest
1 to 2km are dry.

This upcoming weekend is when the deterministic models begin to
seriously diverge. Because of this, just went with the superblend
solution. The gfs and gem global are in very good agreement in
taking a strong short wave quickly eastward Saturday and Saturday
night, pushed by a 100 to 120 kt jet. These models produce decent
chances for light rain, possibly lingering in northwest IA and
adjacent locations Sunday morning. The ECMWF is an outlier at this
time, as it is a full 12 to 18 hours slower, and much stronger with
the short wave. This solution would give a drier Saturday, followed
by a wet Saturday night and Sunday. There is enough frontogenetic
forcing and trowaling behind the surface low and cold air in the 850-
700mb layer to produce accumulating snowfall in our forecast area
later Saturday night and Sunday morning. So this solution will
certainly need watched in the coming days. However at this time
since it is kind of odd man out, do not really want to hit the snow
scenario too hard since it is still 5 days away. But something to
keep an eye on.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 607 AM CDT Tue Oct 10 2017

Clear skies and light winds have allowed many areas to fall below
freezing, with widespread frost being observed. Areas around KSUX
are the exception, as cloud cover and showers here have kept
temperatures warmer. Intermittent light showers continue
throughout the day for NW Iowa, while areas north and west of KSUX
remain mostly clear. Mid-level cloud ceilings around Spencer and
Storm Lake, IA will begin to lower Wednesday morning causing MVFR
conditions.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Frost Advisory until 9 AM CDT this morning for SDZ070-071.

     Freeze Warning until 9 AM CDT this morning for SDZ038>040-050-
     052>069.

MN...Frost Advisory until 9 AM CDT this morning for MNZ081-089-090.

     Freeze Warning until 9 AM CDT this morning for MNZ071-072-080-
     097-098.

IA...Frost Advisory until 9 AM CDT this morning for IAZ002-012.

     Freeze Warning until 9 AM CDT this morning for IAZ001.

NE...None.
&&

$$

SHORT TERM...VandenBoogart
LONG TERM...MJ
AVIATION...VandenBoogart



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