Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201744
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1144 AM CST Mon Feb 20 2017

.SHORT TERM...(Today and Tonight)
Issued at 343 AM CST Mon Feb 20 2017

Full latitude trough in lee of the Rockies this morning containing
several weaker impulses shearing north and northeast through the
Plains. Airmass based on evening raobs is generally above the 90th
percentile for warmth at most levels, and southern plains moisture
content is near record. Strong meridional flow transporting this
moisture northward, and lift forcing is forecast to enhance toward
morning as jet with southern plains impulse drags rapidly northward.
Some of the better mid-level lapse rates through the predawn hours
will continue to lift toward northeast south Dakota, as additional
destabilization aloft spreads toward northwest Iowa.

So far, CAMS have been performing adequately with respect to small
area of showers/isolated thunder northeast of KHON, as well as the
more organized convection moving through far southeast Nebraska.
Have generally followed trends suggested in models this morning,
taking organized convection to the south north and northeast mainly
through northwest Iowa and into southwest Minnesota during the early
morning, with additional development arcing back toward east central
SD on nose of jet, but may be a little more of a struggle to get
much more than isolated activity to develop back west of I-29 as
flow begins to veer aloft as wave shears past early morning.
Other showers are scattered back in a narrow zone along the mid-
level trough axis across western SD, and this area will work
eastward through midday, but as narrow zone slides into the far
western CWA later in the morning, shearing features should begin to
take toll on coverage, with only an isolated threat around midday.
Surface front will progress east of I-29 midday, with a small
potential for destabilization near the boundary around mid to late
afternoon before sliding out of the CWA, and have maintained just a
small chance for showers and isolated thunderstorm as larger scale
lift forcing will have outpaced the boundary eastward long before.

All in all will be a challenging temperature forecast with
increasing moisture and clouds this morning along with precipitation
likely holding temps in the 50s, with cold advection behind the
front perhaps driving down several degrees after passage. Sharp
drying coming in behind the boundary, and will see better mixing on
northwest flow as well as increasing sunshine through areas
near/west of I-29 in the afternoon with 925 hPa temps suggesting a
rebound toward lower 60s.

Tonight, gradient mainly east of the James valley early in the
evening will keep winds up as well as temperatures, but dry and
mostly clear conditions as weak ridge axis settles into the I-29
corridor and eastward by midnight should allow temps to fall off
fairly quickly. Behind the ridge, low-level warm advection will
rapidly develop with weak low-level jet establishing over the heart
of the area by daybreak.  Locations of elevation will certainly see
an increase in both winds and temperature later in the night,
perhaps 5-8 degrees warmer than low elevation sites returning to the
lower 40s by daybreak after an earlier dip to mid to upper 30s.

.LONG TERM...(Tuesday through Sunday)
Issued at 343 AM CST Mon Feb 20 2017

Primary story in the medium and extended forecast deals with extreme
warmth through Wednesday, and then a potential winter storm Thursday
and Friday.

Tuesday will be the warmest day of the week as flow turns zonal and
boundary layer winds are southwesterly. A minor shortwave passing
through the area could add a bit of mid-level clouds, but generally
mostly sunny skies are anticipated. Temperatures should have no
issues surging into the 60s and even approaching the 70s in the
Missouri River valley regions.

A slightly stronger shortwave moves through the Dakotas on
Wednesday. Cold advection aloft will begin early during the day, but
temperatures should still climb into the 50s and perhaps lower 60s
along Highway 20.  An increase in mid-level moisture results in a
risk of sprinkles or very light showers Wednesday morning.

The other big story of the extended forecast is the potential winter
storm Thursday and Friday.  Continuing a theme of the past days,
models have narrowed down the risk areas, but persistent oscillations
from run to run continue.  This is typical given the primary wave is
still 2500 miles away over the East Central Pacific this morning.
The second area models are struggling with is the depth of the upper
trough over the central Canadian provinces.

The general consensus continues to point to increasing precipitation
risks by Thursday afternoon, with light rain quickly cooling surface
temperatures to levels more suitable for snow along the Missouri
River Valley in the afternoon. Have adjusted the hourly trends to
try to better capture this evaporative cooling process. This change
results in minor snow accumulations in south central South Dakota
Thursday afternoon.

the upper trough will swing across the area Thursday night into
Friday, with differences in just how much this wave will deepen as
it crosses the Tri-State area. ECMWF/GEM are slightly stronger with
the Canadian trough and deeper with the Plains trough as it moves
across Iowa Friday morning.  GFS maintains a weaker more open wave,
never quite maximizing the full potential of a double jet structures
of the EC/GEM.  Ensemble spreads continue to vary wildly and are
another sign that this system is far from being locked in. For now,
blended preference from the GEFS/GFS and slightly deeper ECMWF to
maintain forecast continuity. This idea continues to favor the
highest snow amounts along and south of I-90, and potentially more
focused in Northwestern and northern Iowa.

High pressure will slide southward behind this system and should
leave Saturday very chilly.  Likely our first below normal
temperatures in quite some time. A second system could impact the
region late Sunday and into Monday but right now, wouldn`t put any
preference any one direction.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1131 AM CST Mon Feb 20 2017

MVFR and IFR conditions will hang tough east of a cold front
moving through the region. Could see a few showers and fog across
southwest Minnesota into northwest Iowa early this afternoon, but
conditions should improve to VFR as the front moves through.

Gusty northwest winds will decrease this evening and become
southerly overnight.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...BT



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