Scientific Forecaster Discussion
fxus63 kfsd 082058
Area forecast discussion
National Weather Service Sioux Falls South Dakota
258 PM CST sun Dec 8 2019
Short term...(this evening through monday)
issued at 258 PM CST sun Dec 8 2019
Upcoming forecast concerns will be fog today and tonight, then the
band of snow overnight and early Monday morning and then an extended
period of cold weather.
Fog and stratus lower and far more aggressive at pushing south than
the models have anticipated. Even the latest high resolution data
struggling with this push of cold and saturated air. Have opted to
issue a dense fog advisory through midnight. While some locations
that go down to near zero will improve this afternoon, all hints are
that many locations will either continue to trend downward or head
back in that direction once the sun GOES down. With surface winds
remaining from the east and northeast through the early overnight
and no source of drier air from that direction the fog should remain
in place until north to northwest winds increase later tonight and
light snow moves through.
As for the incoming snow chances, the combination of good saturation
and falling temperatures through the column of air, light snow can
be expected for most locations north of roughly a Gregory South Dakota to
Storm Lake Iowa line. The farther north the better the chance. Along
and south of I-90 late tonight and early Monday morning the snow
will likely fall for about 2 to 4 hours while closer to Highway 14
it may fall for about 3 to 5 hours. This will bring higher snow
amounts to those areas. Will aim for about a half an inch to an inch
and a half along Interstate 90 and and inch and a half to 3 inches
closer to Highway 14. Northwest IA, far southern South Dakota and northeast NE
should just catch a glancing blow. Temperatures will crash through
the night with a stronger surge of cold air coming in late tonight.
This cold surge will keep temperatures falling through Monday
morning and they may only steady out in the afternoon before falling
more at night. Most locations will see temperatures of about 5 to 15
degrees by 9 am with a fairly Stout north wind at 20 to 30 mph. This
will drop wind chill values to about 5 below to 15 below zero.
Long term...(monday night through sunday)
issued at 258 PM CST sun Dec 8 2019
Monday night into early Tuesday morning will see a small chance for
some additional light snowfall. The dendritic layer is from near the
surface to about 800 mb and hints are that it will be saturated.
Forcing weak but should be enough to at least get a few flurries.
Will not add any threat for accumulation but watch trends to see if
the threat increases.
Another weak warm advective shot moves through on Tuesday night and
will usher in additional cold air. Not out of the question for a
little more light snow but will likely remain flurries.
Not much change in temperatures Tuesday night into Wednesday as the
reinforcing shot of cold air keeps temperatures trapped in the
single digits and teens most locations.
Temperatures begin to modify going into Thursday. Broad warm
advection develops but thermally we start to lose touch with the
dendritic layer and saturation so not a real strong threat for
light snow. Will continue to monitor Wednesday night for potential
but appears that the higher threat will be north of the area.
The models diverge a bit Friday into next weekend on timing of next
shot of cold air. The good news is for now looking like a short term
shot of cold air and will not be in place for more than a couple of
days. So the basic trend is back to normal or a touch above Thursday
and Friday then back below normal Saturday and Sunday, but not as
cold as this early week Arctic outbreak.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1110 am CST sun Dec 8 2019
IFR and LIFR conditions spreading south more rapidly than most
models have anticipated. Seeing dense fog become much more
widepsread as well. Hints that we may have trouble shaking these
conditions until the snow and the wind shift come in later
tonight. Ksux will likely miss the brunt of the lower conditions
but khon already in them and kfsd should fall into them between
18z and 19z.
South Dakota...dense fog advisory until midnight CST tonight for sdz053>056-
Minnesota...dense fog advisory until midnight CST tonight for mnz080-081-089-
Iowa...dense fog advisory until midnight CST tonight for iaz001>003.