Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 131707
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1107 AM CST Mon Nov 13 2017

.UPDATE...
Issued at 622 AM CST Mon Nov 13 2017

Updated to add zone code for dense fog advisory in the Storm Lake
area. Fog and low stratus moving northward into the Storm Lake Iowa
vicinity, so thus updated for a dense fog advisory. At this time,
the headline is not needed for Sioux City with lighter fog
expected there.

&&

.SHORT TERM...(Today and Tonight)
Issued at 334 AM CST Mon Nov 13 2017

Warm moist advection will drive the main foci of the short term
forecast, stratus and drizzle potential. Stratus currently located
over Kansas will advect northward today, eventually impacting the
CWA in northwest Iowa first. It looks like another day to soar
above guidance high temperatures with sunshine and mixing west of
the James River. At the same time, will hold back highs to around
consensus in our southeastern zones due to the expected afternoon
sky cover.

After 00z Tuesday, the warm moist advection will lead to a chance
for patchy drizzle in southwest Minnesota and northwest Iowa. At
this time, given the marginal surface moist layer depth of 1km or
less and the poor vertical placement of the most favorable ascent -
sometimes above the moist surface layer - will limit mentions of
drizzle for now.

.LONG TERM...(Tuesday through Sunday)
Issued at 334 AM CST Mon Nov 13 2017

On Tuesday, a short wave will move into the northwestern high plains
by afternoon. Ahead of this wave, a southerly flow of air will
continue to bringing up ample low level moisture ahead of a surface
wind shift to the west. Low level RH is generally very high east of
the James River valley early Tuesday, then shifts slowly eastward
through the day as the surface wind shifts to the west ushering in
drier air. The stratus will likely hang on through the entire day in
our far southeast zones around Spencer and Storm Lake. This creates
a challenge in terms of high temperatures. This area may be a couple
of degrees too warm. However the moisture advection is very strong
which can overcome a lack of mixing. So either way, temperatures
will be above normal. In our western zones where stratus will have
no impact and a westerly flow of air takes over, raised highs by
mixing to about 900mb, giving a lot of upper 50s, even lower 60s in
south central SD. Concerning precipitation, thought about drizzle
chances Tuesday morning, especially in northwest IA. However the
stratus depth is not overly thick with moisture mainly only through
900mb. There appears to be no ascent in the cloud layer and although
there may be some slight cooling of cloud temps early Tuesday
morning, the cloud tops begin to warm again after 15Z. Therefore
left out the mention of drizzle for now but certainly cannot rule
out some very light patchy drizzle in our southeast zones. Fog may
be limited too due to the gradient, but this will also need to be
watched.

The aforementioned short wave passes across our forecast area
Tuesday night giving everyone a northwest flow of air coupled with
clearing skies from west to east. Lows will be pretty mild as the
air mass is still of Pacific origin. Wednesday will follow with
somewhat of a breezy day east of I 29, and highs will be cooler.

High pressure moves off to the east Wednesday night producing a
stout southerly flow of air by Thursday. The gradient and mixed
layer winds are maximized west of I 29 where it will be quite windy,
with 20 to 35 mph common in our SD zones. Wind speeds will be a
little less in northwest IA and southwest MN but temperatures will
be cooler when compared to our western zones.

From Friday onward, there is quite a bit of discrepancy between the
medium range models with the details. At 12Z Friday, the GFS shows a
short wave passage moving into the northern plains on the south side
of an upper low near Lake Winnipeg. The ECMWF is absent this
feature, although both models are in good agreement over the
intermountain west with the next short wave trough. Because of the
lead short wave, the GFS is a full 12 hours faster with a surface
wind shift bringing the winds around to the northwest on Friday when
compared to the ECMWF. Precipitation chances with the various waves
will therefore exist this week, but with warm temperatures aloft,
tried to keep snowfall to a minimum. It also looks like a windy
period, especially on Saturday, and potentially on Friday if the GFS
is closer. By the weekend, highs will once again be back below
normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1105 AM CST Mon Nov 13 2017

IFR ceilings will slowly lift north through the afternoon, but
should accelerate north this evening and overnight. The deepest
low cloud layer should be found in parts of northwest IA and
southwest MN. This may even allow some drizzle to develop in these
areas.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

UPDATE...
SHORT TERM...Ferguson
LONG TERM...MJ
AVIATION...08



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