Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 131731

Area Forecast Discussion
National Weather Service Sioux Falls SD
1131 AM CST Wed Dec 13 2017

.SHORT TERM...(Today and Tonight)
Issued at 401 AM CST Wed Dec 13 2017

Strong short wave showing up very well on water vapor imagery
currently diving quickly southeastward in central and western SD. At
the surface, low pressure is positioned over central MN with the
wind shift through southwest MN and extreme northwest IA. The PV
surge at the 1.5 pressure surface is quite robust early today,
before moving to the southeast later this morning. In the meantime,
strong surface pressure rises are noted this morning coupled with
cold air advection in the surface to 925mb layer greatly adding to
wind speeds along the PV surge behind the initial wind shift. Wind
speeds are already gusting over 40 mph in our western zones for
instance, and these types of winds will move quickly eastward early
today. Therefore no changes needed to the wind advisory. Some of the
higher terrained areas, such as the Buffalo Ridge in southwest MN
and lower Brule areas, will experience gusts of 45 to 50 mph at
times late this morning and early afternoon, when deep mixing for
this time of year is coincident with low level cold air advection.
By mid afternoon, the strongest gusts will begin to subside as the
gradient relaxes a bit, coupled with less wind in the mixed layer
and an absence of pressure rises.

Measurable precipitation will be hard to find with this system.
Stratocu will enter our forecast area this morning. However the
moisture depth is thin, with the deepest moisture in southwest MN
midday and early this afternoon at about 100mb deep. Soundings are
dry adiabatic due to low level instability, so would not rule out
scattered flurries or light snow showers once the temperatures both
at the surface and aloft cool this afternoon. But measurable
snowfall will be hard to find, especially with the windy conditions.
This morning, any light precipitation will be primarily of the
liquid variety. Concerning temperatures, they are a challenge today.
Conditions have warmed up dramatically as expected right along the
strongest winds, with lower 50s currently at Chamberlain and
Mitchell. Therefore temperatures will warm elsewhere as the winds
increase early today, then will begin to slowly cool off before
steadying out as the mixing potential battles the cold air
advection, likely to a stalemate. At any rate, highs today will
likely be realized early in the day during the initial surge of
wind, noting that the models really struggle with the degree of
warming as the winds increase.

Tonight, the winds will decrease dramatically. There could be some
leftover flurries over parts of southwest MN and northwest IA in the
evening under lingering stratocu and a favorable thermal profile
aloft. But again measurable snowfall will be virtually non existent
due to a lack of deep moisture. Skies will then become partly cloudy
in our eastern zones helping to produce lows in the teens.
Conversely, another round of increasing mid and high level cloud
cover will put the brakes on temperatures from falling very much in
our western zones, with lows in the upper 20s likely in our south
central SD zones.

.LONG TERM...(Thursday through Tuesday)
Issued at 401 AM CST Wed Dec 13 2017

On Thursday, a weaker short wave will move southward. Mid and upper
level QG forcing as well as PV are consistent amongst the
deterministic models in showing the track of the wave primarily
along and west of the James River valley, mainly through the midday
hours. Moisture with this wave is quite robust, through about 5 km,
and the thermal profile shows a steady cooling through this layer
with no inversions to worry about. Chances for light snow appear
likely in our extreme west toward central SD with amounts possibly
between a half inch and one inch. Snow amounts wane to a dusting at
most toward I 29 moving too far east of the wave influence. The
isobars and winds aloft our fairly relaxed on Thursday except
conditions could become quite breezy Thursday afternoon in south
central SD behind low pressure which is moving southeast through
eastern Nebraska, and building high pressure over the northwestern
plains. Therefore the gradient is a bit strong toward central SD.
Blowing snow Thursday afternoon in south central SD should not be a
concern as the precipitation rate decreases and temperatures warm
into the upper 30s producing a light rain or snow mix.

On Thursday night, QG forcing shows another weak short wave moving
southeastward across Minnesota. For now, left out the mention of
flurries in our far eastern zones as the stratus depth is quite thin
once again. The next system to watch is Saturday night. The various
models are in pretty good agreement with the large scale in showing
a split wave. The GFS is strongest with the northern stream
throughout the northern plains and cuts off the moisture supply,
whereas the ECMWF and GEM Global show some hookup with the southern
stream energy. Therefore the ECMWF and GEM develop low pressure over
the central and southern plains, elongated from northeast to
southwest, with light snow developing across the favored northwest
quad throughout much of the forecast area as mid level temperatures
are cold enough to produce ice. For now, compromised between the
models and kept low pops confined to our western and southern zones,
noting that the GFS is totally dry in our area. Otherwise highs this
weekend will likely to continue to trend above normal, with
seasonably mild weather continuing into early next week.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1122 AM CST Wed Dec 13 2017

MVFR conditions are expected to continue across the region before
perhaps lifting to lower end VFR ceilings by later this afternoon
or evening. Confidence in the details in these relatively small
fluctuations and eroding MVFR deck are currently low. A resurgence
of more widespread MVFR ceilings is again expected by Thursday
morning. Northwest winds continue to gust in the 30-40 kt range
through the afternoon hours today before beginning to diminish
this evening.


Issued at 514 AM CST Wed Dec 13 2017

The grassland fire danger is elevated today due to the strong
winds. Most locations will experience fire danger in the high
category, with very high toward south central South Dakota.
Despite chances for very light snow showers or sprinkles today,
the precipitation will not be enough to moderate the fire danger.


SD...Wind Advisory until 6 PM CST this evening for SDZ067-070-071.

     Wind Advisory until 3 PM CST this afternoon for SDZ038>040-050-

MN...Wind Advisory until 6 PM CST this evening for MNZ072-080-081-089-

     Wind Advisory until 3 PM CST this afternoon for MNZ071-097.

IA...Wind Advisory until 6 PM CST this evening for IAZ001>003-012>014-

NE...Wind Advisory until 6 PM CST this evening for NEZ013-014.



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