Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1138 am CST Sat Dec 7 2013
Short term...(today and tonight)
issued at 241 am CST Sat Dec 7 2013
Some initial patchy middle clouds will spread across the area today
with the more consistent increasing cloud cover starting up this
afternoon. Am slowing the development of the snow into the area this
evening with the chance increasing starting far southwest about
sunset and snow becoming likely south of Interstate 90 by midnight.
Models indicating initial broad but fairly weak isentropic lift
spreading over the area from the S/SSW...then an enhancement of the
lift southeast late tonight as short wave approaches from the SW.
Any accumulations should be quite light until this enhancement
happens. Still...the probability of measurable snow will go to at
least likely over all except a far northwest strip...and increase to
certain southeast late tonight where the upper support combines with
the thermal advection. Guidance quantitative precipitation forecast grids look fairly good for
tonight. For now will go for a 15 to 1 snow/liquid ratio. At best
marginal dendritic snow expected southeast late tonight but there
will also not be much wind...so this seems like a good compromise.
By late tonight expect an inch to a little more south central and
southeast...to a little below an inch near Interstate 90 east of the
James River...to a half inch northeast...with less far west.
Temperatures will warm...if you call it that...to the zero north to
12-13 southeast that we have been going for...drop very slightly
early this evening...then steady off and slowly rise the rest of
Will keep the Wind Chill Advisory going for now for this morning but
we should be able to kill it a little early at least south. We have
already been stretching it and allowing it to stay for the extreme
cold though winds are going pretty light...so will leave it now for
consistency. This is not bad given that it is our coldest shot so
far and a little early int he season to boot.
Long term...(sunday through friday)
issued at 241 am CST Sat Dec 7 2013
The crux of the short wave energy moves across our forecast area
on Sunday...exiting rapidly Sunday evening...quicker then previous
model runs. The entire forecast area is deeply saturated on Sunday
with a fair amount of warm air advection in the 900mb to 700mb
layer. Then the moisture makes a quick exit from west to east
Sunday night. Northwest Iowa still appears to be the primary focus
for the highest amounts of snowfall. That area does have a decent
amount of ascent...coupled with a surge of pv at the 1.5 pressure
surface and even a bit of middle level frontogenesis. The snowfall
across the rest of the forecast area will be lighter and primarily
due to broad isentropic lift. Furthermore concerning northwest
Iowa...dendricity finally becomes established and this could also
help to enhance snow amounts. Temperatures across the rest of our
area appear too cold to get much in the way of dendritic growth.
With the latest model runs not showing really anything in the way
of major changes on Sunday...snow amounts have remained about the
same. In northwest Iowa...new projected snow amounts are just a
bit less due to the faster exit Sunday evening. But still...
northwest Iowa should get 4 inches or slightly more of snowfall.
Coordinated with oax and dmx about a possibly Winter Weather
Advisory. But due to a lack of wind and a long duration event...
decided to hold off for now. Speaking of wind...this will need
watched as the event gets close or is upon US as sometimes snow
amounts will be greater than thought. This is due to a lack of
wind tearing at the snow crystals and less compaction on the
ground. Something to watch for.
Concerning temperatures...they will begin to moderate late
Saturday night and continue moderating into Sunday. Given the
925mb temperatures and cloud cover holding in what heat there is...
highs should average about 12 degrees north to middle teens around
Sioux City and Storm Lake.
As mentioned above...the wave quickly exits Sunday night with
modest cold air advection. The wind will increase with the
subsidence which will help to hold the air temperature up
relatively speaking. Not anticipating lows colder then 10 below
anywhere...but this will be followed by another cold day on
Monday. When the time comes...we will likely need another Wind
Chill Advisory from about 06z to 18z Monday.
Further out...the models are in excellent agreement in bringing
another short wave through our area Monday night as it dives down
the northerly upper flow from Canada. Mixing should actually warm
temperatures just behind the wave passage Monday evening before
getting colder later Monday night. This wave passage is dry...but
model consensus does show another wave diving through our area
Tuesday night. The latest European model (ecmwf) does have light snow spread across
our forecast area with this wave as low and middle level baroclinicity
is strong...surging in another shot of cold air. Right now only
have small probability of precipitation from Yankton to Gregory County but this will need
to be watched in future model runs. Latest consensus raw model
values for lows Tuesday night are quite cold...so blended them
into the previous forecast to notch down lows.
Behind Tuesday nights wave...Wednesday will be another cold
day...with highs about 5 to 10 degrees colder then tuesdays.
Temperatures will then finally moderate into the 20s on Thursday.
However late this week...both the European model (ecmwf) and GFS are in excellent
agreement in giving our forecast area a glancing blow of Arctic
area due to another very cold high moving southeastward from
Manitoba to the Great Lakes. If this does indeed pan out...we
will have to begin a trend of lowering highs on Friday in future
forecasts by about 5 to 10 degrees.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1130 am CST Sat Dec 7 2013
VFR conditions expected through the day...with snow moving in from
the southwest tonight. Expect visibility to quickly drop into the IFR
category as the snow moves in. Could be periods of LIFR visibility as
well...with the best chance across northwest Iowa and adjacent areas
of northeast Nebraska and southwest Minnesota...although even kfsd
could see visibilities briefly below a mile. Expect this snow to
persist through the remainder of the taf period...ending from west to
east after 18z.
South Dakota...Wind Chill Advisory until noon CST today for sdz038>040-050-
Minnesota...Wind Chill Advisory until noon CST today for mnz071-072-080-081-
Iowa...Wind Chill Advisory until noon CST today for iaz001>003-012>014.