Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 131212

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

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

As expected, fog has mostly stayed away tonight thanks to the return
of light southerly gradient winds. Will need to keep an eye on
things early this morning as RH values will remain high as warm
moist air advects into the forecast area. This 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 particular our
northwestern IA zones, by late this afternoon. 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

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


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 612 AM CST Mon Nov 13 2017

Messy aviation forecast for the next 24 hours to say the least.
Widespread stratus is approaching from NE/IA and will mix out
west to east through the day Monday before building back westward.
Have broken out the discussion separately for each airport.

KSUX - The leading edge of the stratus, currently located along a
GRI-CBF line, features LIFR ceilings and occasional closure level
visibilities in the fog underneath. Current thinking is that KSUX
will avoid the closure level fog as mixing begins to ramp up this
morning. LIFR ceilings are expected by 16z, with eventual
improvement to high end IFR/low end MVFR ceilings. Highest
uncertainty pertains to how quickly this improvement will occur.
Improvement has taken about 5 to 6 hours in upstream sites. Have
cut an hour off this time due to mixing as the winds increase.
Some patchy fog is possible Monday night.

KHON - The greatest overall uncertainty relies in the TAF for
HON. A surging blob of stratus currently seen at ANW is inbound
for HON. The saving grace may be the mixing from gusty winds,
which have already begun west of the James River. Current
expectation is that stratus will slide just east of the terminal
and thus have kept out any ceiling reductions this morning.
Stratus builds westward and impacts HON late in the TAF period,
with a risk for patchy fog.

KFSD - Thanks to the aforementioned mixing, FSD is only expected
to be dealing with MVFR stratus during this TAF period beginning
at 18z. There is a risk for patchy fog Monday night.




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.