Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1123 am CST Sat Jan 31 2015
issued at 1052 am CST Sat Jan 31 2015
Per latest model runs have updated the forecast to increase quantitative precipitation forecast
amounts...and in turn snowfall amounts...through areas generally
from the Interstate 90 corridor and southward. Related to
that...upgraded portions of the Winter Weather Advisory to a
warning for most of northwestern Iowa...northeastern Nebraska and
extreme southeastern South Dakota where 6 to 8 inches of snow is
likely through Sunday morning. Will most likely have to refine
current headlines a bit more for the afternoon package based on
12z model runs. Also removed rain from the forecast in our far
south this afternoon in anticipation of wet bulbing which would
quickly result in any rain changing to snow.
Short term...(today and tonight)
issued at 332 am CST Sat Jan 31 2015
This snow system has become stronger with each run since yesterday
and now is expected to bring significant snow accumulations to most
of our forecast area...with the greatest amounts in northwest Iowa.
Despite initially mild low level temperatures...steady cooling is
starting to call in question an initial mixture of rain with the
snow in the far south. Given the expected rapid development of
precipitation today...with keep a mixture over a small area far
south into this. Temperatures of course will not get too far today
before cooling...given how mild they are early this morning.
Models have very good agreement of precipitation pattern and
amounts. Think here the amounts are in general slightly overdone.
However...the certain cooling and a lot of the lift destined to be
in the dendritic zone should make for some decent accumulations.
Amounts should stay less than 8 inches in northwest Iowa but not
very much less. Given the moderate winds and some drifting but
little or no blowing snow expected...plus an event duration well
over 12 hours in that area...will keep the highlight to just an
advisory for now. The advisory will extend north and west to part of
southwest Minnesota...the fsd area...Mitchell and the far southwest corner
of the area in Gregory County. Am starting the advisory late this
afternoon/ 3 PM CST/ far south and at the start of the evening/ 6 PM
CST / further north. Snow should be tapering off far west late
tonight but will keep the advisory through tonight there...and into
The cold air coming in will drop temperatures to the single digits
northwest to the teens southeast tonight. Winds may get to the
breezy category but not strong enough to blow the snow higher than
Long term...(sunday through friday)
issued at 332 am CST Sat Jan 31 2015
Progressive northwest flow will allow active weather to continue
through the middle of next week...followed by quiet conditions for
the second half of the week.
Storm system impacting the area Saturday night will push off to the
east on Sunday. Moderate snowfall will be mainly confined to the
eastern half of the forecast area on Sunday morning before
diminishing from west to east in the afternoon. Northerly winds
increase to around 15 to 25 miles per hour with some higher gusts to 30 miles per hour.
This should allow some blowing and drifting of snow...particularly
in open areas. Am concerned that the blowing snow may be of greatest
concern in northwest Iowa where the heaviest snowfall is expected.
This May Point more towards needing a Winter Storm Warning instead
of an advisory in our far southeast...but think that starting with
an advisory and seeing if winds look stronger in future runs is a
good start for now. Definitely something for the day shift to look
Much colder temperatures rush into the region on Sunday...which will
result in nearly steady or falling temperatures through the day.
Clearing skies on Sunday night...fresh snowpack and winds sharply
decreasing will bring a much colder night. Lows will drop into the
single digits below zero...but with light winds the wind chills
stay out of advisory territory.
A middle level shortwave slips across the northern plains on Monday.
Temperatures moderate slightly ahead of the wave with return flow
briefly returning. Stratus will accompany this wave and while
support is pretty weak...could see a few flakes develop in the our
far east Monday afternoon and evening. Temperatures warm further on
Tuesday with highs recovering into the middle 20s northeast to near 40
in the Lower Brule. Another middle level wave dropping along the
northwest flow will bring another chance for some light snow in the
afternoon...closely followed by another wave Tuesday night. Another
shot of cold air advection follows...with highs only in the middle
teens to lower 20s on Wednesday.
Upper level ridging finally begins to rebound to our west on
Thursday and Friday. Temperatures moderate some on Thursday...but
Friday readings improve into the 30s and even some 40s in south
central South Dakota.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1110 am CST Sat Jan 31 2015
Current observations show IFR conditions over most of the area
with patches of visibilities below 1 sm due to fog. Generally expect most
areas along and south of i90 to keep at least MVFR visibility for fog
prior to snow beginning late this afternoon and evening. In
addition ceilings will be IFR through the afternoon. Large area of
snow is expected to develop from south to north from late
afternoon into the evening. Expect that visibilities will gradually fall
from 3-5sm in the late afternoon and early evening to 1 sm or less along
and south of i90 by 03z. In addition...ceilings will be below 500
feet as snowfall increases. From 05z to 12z...there may be periods
of 1/2sm snowfall and possibly even 1/4sm snowfall at kfsd and
ksux. Since duration and timing of the 1/4sm snowfall is very
uncertain have not included in taf but did include 1/2sm snowfall
late tonight at both locaions. Snow will gradually diminish in
intensity after 12z but ceilings likely to remain below 1000 feet
through 18z. North of i90...with drier air moving
southward...snowfall will not be as intense and therefore...ceilings
and visibilities will both be a little better...generally 1 to 3sm with
ceilings between 500 and 1000 feet overnight. Snow is expected to end by
early Sunday morning around khon with visibilities improving to 6sm by
15z and ceilings rising above 1000 feet.
South Dakota... Winter Storm Warning from 3 PM this afternoon to noon CST Sunday
Winter Weather Advisory from 3 PM this afternoon to 6 am CST
Sunday for sdz050-063-068-069.
Winter Weather Advisory from 6 PM this evening to noon CST
Sunday for sdz062-066-067.
Winter Weather Advisory from 6 PM this evening to 6 am CST
Sunday for sdz059>061-064-065.
Minnesota...Winter Weather Advisory from 6 PM this evening to noon CST
Sunday for mnz089-090-098.
Iowa...Winter Storm Warning from 3 PM this afternoon to noon CST Sunday
Winter Storm Warning from 6 PM this evening to noon CST Sunday
NE...Winter Storm Warning from 3 PM this afternoon to noon CST Sunday