Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 141131

Area Forecast Discussion
National Weather Service Sioux Falls SD
531 AM CST Tue Nov 14 2017

.SHORT TERM...(Today and Tonight)
Issued at 350 AM CST Tue Nov 14 2017

Main focus of the upcoming 24 hours will be the fog, stratus, and
the small potential for rain late Tuesday night. MVFR/IFR stratus
has taken hold across the forecast area and will continue for much
of Tuesday as approaching upper level shortwave continues to drive
warm moist air into the region. Thermal profiles remain favorable
for both fog and stratus. But, with the return of southerly winds
this morning, expect visibilities to increase a bit before clearing
out once the cold front swings through late in the afternoon. On
that note, have raised winds above guidance a bit in deference to
the return of a decent pressure gradient this morning through the
afternoon and then again after the surface boundary swings east of
the forecast area after 00z. Tough call on high temperatures with
the combination of widespread cloud cover and southerly flow. Have
largely gone near consensus guidance without anything to key on
which competing interest will win out.

Tuesday night brings a small chance of rain to areas along a Windom-
Storm Lake line as the shortwave translates eastward. Clear model
consensus is that the best forcing associated with the wave doesn`t
really get going until it is east of our forecast area. Have left a
few small chances for rain in deference to the few ensemble members
that are a bit less progressive with the wave.

.LONG TERM...(Wednesday through Monday)
Issued at 350 AM CST Tue Nov 14 2017

On Wednesday, short wave will be moving well east of our area with
an upper ridge over the Rockies. Behind the departing surface low,
Canadian high pressure will be filtering southward into the northern
and central plains. Increased wind speeds Wednesday morning by
eliminating any bias correction factors as a decent gradient exists
coupled with mixed layer winds of 25 to 30 knots. This will give 10
to 20 mph winds in our western zones, and breezy conditions in our
east. However by afternoon, the gradient and mixed layer winds
slacken off, so winds should decrease between 18Z and 00Z for many
locations. Highs are the other concern on Wednesday. There is a risk
that they could be a category cooler than forecast. Current forecast
highs are assuming a mixed depth through about 1km. If this fails to
materialize, values will be two to three degrees cooler. But the air
mass is fairly dry and these readings are below normal except for
our far southern zones near Sioux City where we are forecasting near
normal highs.

The high pressure system will move off to the east Wednesday night,
with another upper trough of low pressure carving out over the
western U.S., with a leading surface trough developing over the
western plains. Tight pressure gradient will exist on Thursday,
especially along and west of I 29, coupled with mixed layer winds
averaging 35 knots in the same areas. Thursday still looks quite
windy, again especially along and west of I 29, with 25 to 35 mph
winds common, perhaps even gusting toward 40 mph at times by midday
and afternoon in the James River valley and points westward. There
could be quite a thermal gradient across our forecast area on
Thursday. Areas east of I 29 are still under the lingering influence
of the departing Canadian high which should give highs only in the
lower 40s, but conversely temperatures will climb well into the 50s
approaching central SD. The stout winds will shift eastward across
the CWA Thursday night, but with some decoupling effect they will
relax into the breezy category. The one exception is the Buffalo
Ridge in southwest MN where we manually raised wind speeds into the
20 to 35 mph category through the evening hours with 40 knots not
too far off the surface. But they too will decrease after midnight.

Late this week, there is still about 6 to 9 hours of timing
discrepancy between the deterministic GFS and ECMWF with the GFS
continuing to be faster. Another round of stratus comes up Thursday
night and will certainly linger into Friday, so the timing of the
wind shift to the northwest is crucial in ushering the stratus
eastward. Obviously the slower ECMWF hangs onto the stratus on
Friday much longer than the GFS. This will make a difference not
only on highs, but also on wind speeds so for now just kept
superblend going by blending both models. Light precipitation
chances are still possible but it will fall as primarily rainfall.
In fact raised lows a smidge with the stratus and winds Thursday
night above superblend values by blending in consensus raw model
and weighted model values. If any precipitation still exists in our
northern and eastern zones Friday night, it could changeover to very
light snow as surface temps and temperatures aloft cool, possibly
cold enough to introduce ice into the top of the cloud layer. The
upcoming weekend looks dry with below normal temperatures. It could
be breezy on Saturday with a northwest flow.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 526 AM CST Tue Nov 14 2017

LIFR/IFR stratus will continue to affect the forecast area Tuesday
morning. Widespread IFR fog will begin to mix out as southerly
winds increase this morning. Winds will veer to northwesterly by
the later part of the TAF period. This will eventually filter out
our stratus, with VFR conditions expected at all TAF sites by
Tuesday evening.


SD...Dense Fog Advisory until 9 AM CST this morning for SDZ038>040-

MN...Dense Fog Advisory until 9 AM CST this morning for MNZ071-097.



SHORT TERM...Ferguson
AVIATION...Ferguson is the U.S. government's official web portal to all federal, state and local government web resources and services.