Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231208

Area Forecast Discussion
National Weather Service Sioux Falls SD
708 AM CDT Fri Mar 23 2018

.SHORT TERM...(Today and Tonight)
Issued at 325 AM CDT Fri Mar 23 2018

Primary forecast concern is the potential for heavy snow over
portions of southwestern Minnesota from later today into Saturday

Current conditions show a light band of rain from southwestern North
Dakota into south central South Dakota. Only light precipitation has
been observed at observation sites which are all above freezing. At
the same time, dew points across western Minnesota and western Iowa
have been slowly falling through the night as easterly flow brings
in drier air. Dew points in southwestern Minnesota are now between
28 and 32 and between 30 and 34 in northwestern Iowa. With mid
20s dew points in south central Minnesota and easterly flow only
expected to increase through the morning, expect dew points to be
between 25 and 30 prior to precipitation beginning. With
temperatures only expected to be in the mid 30s, temperatures
should fall to freezing or even below freezing 1-3 hours after
precipitation beginning.

As for the storm, the well-advertised upper level wave/PV anomaly
has moved into the lee of the Rockies and is expected to move east
across Nebraska today and tonight. This wave has resulted in strong
mid level warm air advection/isentropic ascent over south central SD
which has led to the development of the area of light rain. The warm
advection region is expected to move east through the morning -
reach FSD and BKX by 15Z and into southwestern Minnesota and
northwestern Iowa by early afternoon.  The warm advection appears to
outrun the upper wave so expectations are this band will slowly
weaken as it moves east of I-29 early this afternoon. As the band
weakens, strong 850 to 800 mb frontogenesis will set up near I-29 as
the upper level wave moves into eastern Nebraska. All models show
elevated instability so expecting rapid development of precipitation
in the vicinity of this front between 21Z and 00Z which will
quickly spread into western Minnesota and northwestern Iowa.
Models continue show elevated instability for upright convection
during the evening. Cape is less than 100 J/kg so not sure there
will be enough charge separation to get thunder but this will
certainly enhance precipitation rates and are expecting over 0.5"
of precipitation between 00Z and 06Z over portions of southwestern
Minnesota and northwestern Iowa. With the wave a little slower
heavier precipitation is expected to continue well past midnight
in southwestern Minnesota and northwestern Iowa when an additional
0.25 to 0.5 in is expected. Light snow or rain will gradually end
on Saturday morning although clouds will hang around the
remainder of the day on Saturday.

The big change with this forecast is that all models but the NAM
have trended colder both at the surface and in the low levels.  The
NAM keeps a warm layer through 00Z which would actually result in
sleet or freezing rain for a period late this afternoon and evening
in southwest Minnesota and rain in most other areas.  However, the
GFS, Canadian and ECMWF are all colder with temperature from 925 mb
and above at or below zero over most of southwestern Minnesota the
entire event and temperatures falling below freezing along I-29 in
SD as well as most of northwestern Iowa between 03Z and 09Z tonight.
Have sided with the colder model consensus. The combination of drier
surface air advecting in from the east allowing for evaporative
cooling and colder temperatures aloft means that once the heaviest
precipitation arrives toward 00Z, any rain will quickly change over
to snow. In addition, locations farther south and west such as Sioux
Falls and Spencer may see more accumulating snow later in the night
if temperatures aloft fall below freezing after midnight. With the
warm temperatures expect snow ratios from 6:1 to 10:1 and this
results in 5 to 9 inches of snow over southwestern Minnesota except
for Rock County. And with an earlier changeover the Iowa Great Lakes
could see 2 to 4 inches with less than 2 inches expected in Storm
Lake, Sioux Falls and De Smet. During the period of heavy snowfall
overnight, this heavy, wet snow may accumulate on trees and wires as

With this package will be upgrading the entire winter storm watch in
southwestern Minnesota to a winter storm warning.  Have expanded the
winter weather advisory to include Dickinson County Iowa, Rock
County MN and Moody County Iowa where 2-6 inches of snow is
expected. Considered upgrading Brookings, Nobles and Pipestone
Counties to a warning given snowfall could be around 6 inches but
with uncertainty in low level temperatures am not confident that
these areas will reach 6 inches. These locations will have to be
watched to see if colder air arrives sooner.

.LONG TERM...(Saturday through Thursday)
Issued at 325 AM CDT Fri Mar 23 2018

Stratus looks to hang tough across the area on Saturday night into
Monday.  Could see some drizzle develop in Central South Dakota on
Sunday morning. With surface temperatures at or just below freezing
east of the James, could be some spotty freezing drizzle if drizzle
is able to develop that far to the east. Forecast soundings indicate
lift through the stratus layer, and it appears thick enough to get
the collision and coalescence process going.

Sunday night into Monday morning more robust warm air advection
develops ahead of an inverted trough that works through the area
Monday night.  Once again, could see drizzle through the evening
hours with stratus deepening to near 8-10K feet across the area.
Biggest concern will be across southwest Minnesota, where
temperatures are expected to fall a few degrees below freezing.
Depending on what occurs tonight in terms of snowfall, the snow may
keep the near surface layer cool enough making precipitation
problematic for a time Monday morning in southwest Minnesota. Have
left freezing rain mention in the forecast across southwest
Minnesota. Elsewhere, expect surface temperatures to be warm enough
to support rain, but do expect enough qpf that it could cause
exasperate spring melt/flooding concerns along area creeks/streams.

Thereafter, less precipitation is expected through the remainder of
the week as the Hudson Bay low re-establishes itself leaving the
northern plains in prevailing northwest flow.  This will likely lead
to at or below normal temperatures through much of next week.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 703 AM CDT Fri Mar 23 2018

A storm system will move across the area over the next 24 h. This
morning a band of light rain will move from the James Valley to
I29. Precipitation is light with this band and ceilings are well
above 3000 ft with visibilities over 6 miles. Later this
afternoon, rain is expected to rapidly develop across southeastern
South Dakota and slowly move to the east. Rainfall will be heavier
such that visibilities will fall below 6 miles at times. As rain
changes to snow after 03Z at HON and FSD, the visibility will be
less than 3 miles. Visibilities will also lower to less than 300
feet late this afternoon and below 1000 ft toward midnight.


SD...Winter Weather Advisory from 7 PM this evening to 7 AM CDT
     Saturday for SDZ040-056.

MN...Winter Weather Advisory from 7 PM this evening to 7 AM CDT
     Saturday for MNZ089-097-098.

     Winter Storm Warning from 4 PM this afternoon to 10 AM CDT
     Saturday for MNZ071-072-080-081-090.

IA...Winter Weather Advisory from 7 PM this evening to 7 AM CDT
     Saturday for IAZ002-003.



SHORT TERM...Schumacher
AVIATION...Schumacher is the U.S. government's official web portal to all federal, state and local government web resources and services.