Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
324 PM CST Friday Feb 12 2016
Short term...(this evening through saturday)
issued at 324 PM CST Friday Feb 12 2016
Will leave the Wind Chill Advisory alone as new grids show the 20
below to 30 degree below wind chill values still in the same area
east of I 29. Main challenge tonight are low temperatures. Many
locations have an inch or two of new fluff to contend with...
therefore even the coldest guidance values look too mild. In
addition...the warm air advection really does not begin in earnest
at least along and east of the James River Valley until well after
midnight. This will give plenty of time for temperatures to bottom
out overnight. But eventually as middle and high clouds move in from
west to east...and as the surface winds turn to the southeast...
temperatures will begin to climb upward...first in our western
zones...and then finally east of I 29 toward morning. In Gregory
County however...actually went the opposite and trended warmer then
guidance readings for lows as their cloud cover may not totally
erode. And their warm air advection regime will begin quite early
Warm air advection is quite strong on Saturday ahead of the next
short wave. First of all...light snow chances begin in our southwest
zones later tonight along the leading edge of the warm air
advection. These chances then drift eastward on Saturday but snow
amounts should stay confined to a dusting or less. By late Saturday
afternoon...there will likely be a high pop/low quantitative precipitation forecast event in our
western zones as the short wave gets closer to the forecast area.
There could be some blowing snow in our far western zones by late
afternoon...however chose not to include those in the grids at this
time instead choosing to focus on Saturday night when the stronger
winds will hit.
Long term...(saturday night through friday)
issued at 324 PM CST Friday Feb 12 2016
Snow will increase quickly over the area from the northwest Saturday
night as a strong short wave arrives from the northwest. Models show
a fairly general snow with the concentration on most models being
over the northeast half of the forecast area. The Canadian model and
more so the NAM show a precipitation axis further south and feel as
do some others that they are outliers on this. In any event given
that the air will be quite unstable from 700 mb on up...the moisture
availability not copious...and the presence of frontogenesis in the
warm air advection...the snow could be banded given the decent thermal gradient
with possibly locally heavy snow transitioning to much lighter over
a short distance. However with the model variations it is hard to
pinpoint at this time where the best narrow snow band/S would be so
for now am going with the more general idea of 3 to 5 inches
northeast half and 1 to 3 southwest. The only question on
precipitation type is in the far west...with the GFS showing most of
the warm air upstairs and most other models cooler. After loading a
blend on maximum temperatures aloft and getting only a little warm wedge
over the far southeast early evening...decided to keep all
precipitation snow...allowing for cooling if precipitation develops
in that chance pop area.
Winds will be stiff with the start of this system and expect blowing
snow to develop with the falling snow. Winds will be strong enough
to threaten blizzard conditions especially at the start through the
middle of the night...with winds dropping off from the west late.
After coordination...and since winds are not a slam dunk for
blizzard...have opted to go with a Winter Storm Watch for the area
with the greatest accumulations...basically from north of Sioux
Falls into southwest Minnesota.
Temperatures of course will be warming slowly Saturday night with
lows early. Readings should reach the teens northeast to lower 30s
southwest be the end of the night.
Lingering snow early Sunday morning east of the James River should
end steadily from the west during the day. Decreasing clouds will
lag a little with fairly cloudy skies likely over southwest
Minnesota into Saturday night....otherwise skies will become partly
cloudy. As the wave passes...the warm advection pattern will weaken
and die...with Sunday being warmer but not warming...if that makes
sense...highs in the 20s northeast to lower 40s southwest.
Monday will see the approach of another northwesterly wave and the
return of some warming. With the lack of the current cold air to
help out on thermal support and the wave further to the north than
the Saturday night one...the snow threat Monday night and Tuesday
should be quite a bit less. The wind pattern should be more typical
with the strongest winds Tuesday on the back end of the system...but
not being real strong. Very modest cooling will rule Tuesday and
Wednesday as we go to dry Wednesday ahead of massive upper ridging.
Thursday will be dry and warmer with this ridging. Friday then
brings questions as a Pacific wave climbs across the upper ridge and
approaches. Will for now go with the extended guidance and leave
precipitation out the system seems likely to lack very much moisture
and will be mild...even if the stronger ec turns out. We might have
to eventually introduce a few February rain showers.
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1137 am CST Friday Feb 12 2016
The current infrared satellite picture shows a band of MVFR stratus
close to 2000 foot above ground level that has been developing slowly eastward
toward khon. Not convinced that it will actually reach khon as the
dry air could evaporate it before it reaches khon. But wanted to
hedge it in there with a tempo group early this afternoon as a
heads up. There is also some MVFR stratus near ksux which could
impact that site early this afternoon. Otherwise VFR conditions
will prevail from late this afternoon through tonight...with MVFR
moving back into khon by middle morning Saturday.
South Dakota...Winter Storm Watch from Saturday evening through late Saturday
night for sdz039-040-056.
Minnesota...Wind Chill Advisory from 7 PM this evening to 9 am CST Saturday
Winter Storm Watch from Saturday evening through late Saturday
night for mnz071-072-080-081-089-090-097.
Iowa...Wind Chill Advisory from 7 PM this evening to 9 am CST Saturday