Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 160825

Area Forecast Discussion
National Weather Service Sioux Falls SD
325 AM CDT Mon Apr 16 2018

.SHORT TERM...(Today and Tonight)
Issued at 321 AM CDT Mon Apr 16 2018

Pesky stratus on the western edge of the large upper trough,
continue to slowly sink southward.  Stratus has really prevented our
temperatures from falling overnight, and should prevent records from
being reached.

Today: Models really holding onto stratus into the morning hours,
with 900 MB RH plots keeping clouds through mid-day in some areas.
Condensation pressure deficit plots suggest that while stratus west
of I-29 dissipates, a new area of cloud cover will pivot into the
eastern zones today. Nevermind the stratus though, any sun that
peaks through today will help get us through the fact that highs
today will only reach the lower to middle 30s, nearly 25 degrees
below normal and close to record coldest highs in some locations.
Winds will be light, so at least we won`t have to deal with any
extremely cold wind chills.

Tonight: High pressure will begin to slide eastward, which should
allow temperatures to again plummet east of I-29.  Models continue
to struggle with overnight lows, given such an abnormally cold air
mass, so have manually trended towards colder guidance.  The one fly
in the ointment could be an increase in mid and upper cirrus moving
over the ridge to our west.

.LONG TERM...(Tuesday through Sunday)
Issued at 321 AM CDT Mon Apr 16 2018

The medium range forecast once again calls for the dreaded four
letter word in Spring, snow. A fairly large and somewhat progressive
upper trough pivots into the Northern Plains late Tuesday night.
Models continue to have some difficulty resolving the progression
and depth of this wave moving through, likely the response to the
more uncertain and fluctuation impact of  blocking mechanisms

The GFS remains a fairly inconsistent outlier to the other models
(although all have been bouncing around slightly), with a 700 mb
low further north than most guidance. This model has also been on
either extreme of it`s ensembles over the past day, owing to a bit
less confidence in the spread. Overnight preferences have fallen
towards a blend of the 00Z ECMWF/NAM/GEM, which matches the
thoughts of WPC. This solutions all have a bit less warm air
advecting northwest into this system, reducing the mix/rain
potential. They also have a bit more compact 850mb surface low
that tracks near or just north of I-80 Wednesday morning.

That said, precipitation should begin to break out quickly across
western SD/NE Tuesday evening as the upper low and attendant low
level warm advection develops.  With the wave taking a bit of a
negative tilt as it enters the Plains, moisture will actually advect
to the northwest quadrant of the 850/700 low, with a minimal
eastwards warm advection wing. Speaking of moisture, have some
concerns about the potential interaction of a moisture plume
emanating out of the Pacific and over the Southwestern US this
morning, clearly visible on water vapor.  This interaction would
support the higher QPF values we`ve been seeing on recent model
guidance runs.

By early morning Wednesday, an elongated area of precipitation from
eastern Nebraska northwest into North Dakota is anticipated, with a
more intense area of precipitation forming northwest of the 850mb
along the MO river valley through the predawn hours.  As the wave
moves through, and deepens, we should see enough dynamic cooling to
limit the overall mix and rain potential, but wouldn`t be shocked to
have sleet and a touch of freezing rain before soundings completely

The adjusted QPF and potential low track favors the development of
moderate snow with over Northwest Iowa Wednesday morning. With some
elements of frontogenesis within the 700:650 mb layer, some banding
could be possible.  Given the broad synoptic lift and divergence
aloft, snow is expected to spread well north and northwest
Wednesday, although amounts are expected to be lighter than those in
NW Iowa and adjacent areas of MN/SD.

Broad accumulations of 2 to 6" will be possible, but a concentrated
areas of 6 to 10" of snow may find itself across Northwest Iowa
and into Southern Minnesota by time snow ends Wednesday night. The
greatest wildcard of the event is how quickly rain/mix can turn
to snow Wednesday morning. Faster turnover equals higher amounts,
slower turnover will mean lower amounts. Given the potential of
this system, along with the fact that we`re just 48 hours from our
previous storm and want to give folks enough time to prepare again,
decided to issue a winter storm watch.

Behind this system, really not seeing any big warmup in sight.
High pressure once again moves into the region, holding
temperatures well below normal into the end of the week.

There may be at least some good news on the horizon though.  The
second storm system we have been monitoring may now take a more
southern trajectory for the end of the week, as a trough moving into
the Pacific Northwest induces a stronger downstream ridge, stubborn
East Coast troughing moves offshore, and heights over the US in
general begin to rise. This trifecta could signal potential for
dry weather and warmer temperatures at the very end of the


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1037 PM CDT Sun Apr 15 2018

Flurries over southwestern MN will end around 06Z. Skies will
slowly clear, and with light winds cannot rule out patchy fog
development tonight. Guidance continues to suggest the better
potential for fog over the James River Valley, including KHON.


SD...Winter Storm Watch from late Tuesday night through Wednesday
     evening for SDZ071.

MN...Winter Storm Watch from late Tuesday night through Wednesday
     evening for MNZ081-089-090.

IA...Winter Storm Watch from late Tuesday night through Wednesday
     evening for IAZ001>003-012>014-020>022-031.

NE...Winter Storm Watch from late Tuesday night through Wednesday
     evening for NEZ014.



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