Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1018 PM CST Friday Nov 27 2015
Short term...(this evening through saturday)
issued at 353 PM CST Friday Nov 27 2015
Surface high pressure will remain across the area through the
overnight. Have sided towards the cooler guidance values over the
fresh snowcover...with more mild temperatures across the bare
ground. Regardless...should be a good radiational period across the
For Saturday...clear skies should gradually give way to filtered
sunshine as high clouds expand from southwest. With light
winds...should be a struggle to mix significantly limiting high
temperatures in the upper 20s-lower 30s.
Long term...(saturday night through friday)
issued at 353 PM CST Friday Nov 27 2015
The main forecast concern is the potential winter storm from late
Sunday night through Monday night.
Prior to the storm...a very weak wave will move northeast from the
High Plains into central South Dakota. The lowest levels of the
atmosphere are somewhat dry. But weak forcing for ascent persists
for much of the night. With saturation in the middle atmosphere...
there will be snow above the surface and if it can persist long
enough it may reach the ground. If snow does fall...it would be
much less than an inch Saturday night and Sunday. Farther
east...there is little forcing for ascent and the near surface
layer is expected to remain relatively dry. So at this point am
not anticipating any freezing drizzle or drizzle...just a lot of
stratus. Lows on Saturday night will range from the teens in
southwest Minnesota and adjacent areas of South Dakota and Iowa to the lower
20s elsewhere. Highs on Sunday will be within a couple of degrees
A very potent upper level low will begin move northeast into the
Central Plains on Sunday night. Strong forcing for ascent will
move into the Missouri River around midnight and spread toward
Interstate 90 by 12z on Monday. There is also a 850 mb
temperature gradient across the eastern plains. This results in
fairly strong warm advection and isentropic lift across most of
the forecast area by 12z Monday and continues across the forecast
area through the day on Monday. The only limiting factor to
snowfall late Sunday is dry easterly flow. So there will be a few
hours overnight where the lower atmosphere will need to saturate
before snow begins. At this point...expect snow to begin in the
Missouri Valley after middle night and around i90 either late Sunday
night or early Monday morning.
The heaviest snowfall is most likely to be on Monday. All models
show the pv anomaly moving northward from eastern Nebraska into
west central Minnesota. At the same time there will be weak
frontogenesis somewhere in eastern South Dakota or western
Minnesota and Iowa. The combination should result in an
enhancement in snowfall rates during the late morning and
afternoon in the vicinity of the 850 mb front. While current model
forecasts would put this near i29...uncertainty in the frontal
position means it could be anywhere from just west of i29 to near
Highway 71 in southwest Minnesota and northwest Iowa. Where this front
sets up could see over 4 inches during the day on Monday. Even
where snowfall is not enhanced...light snow is expected much of
the day so that most areas may see a couple of inches.
The upper low will begin to slowly move off to the east on Monday
night and Tuesday. Models indicate that a trowal will develop with
light snow continuing northwest of the upper low...especially east
of the James River. Snowfall amounts on Monday night will be
lighter than on Monday but at least a couple of inches will be
possible. The light snow will end on Tuesday. There could be light
accumulations in southwest Minnesota during the morning.
With the exception of the Canadian Gem...all deterministic models
are showing over 0.5" over most of the forecast area with maximum
amounts exceeding 1" where the heavier snowfall sets up on Monday.
At this point...expecting snow ratios from 9:1 early in the event
to 13:1 by late Monday night. This could mean a large portion of
the forecast area seeing over 6 inches of snow...with some areas
having the potential to more than 8 inches of snowfall in 24 h.
Because there is still some uncertainly on location...will not
issue a watch but continue to highlight the potential for
significant snowfall in social media and the severe weather potential statement.
Upper level low is slow to retreat into the Great Lakes region as
it is essentially cut off from the steering flow aloft. As system
shifts east...we return to zonal flow. With the northern branch of
the jet focused well to the north...have fairly seasonal
temperatures forecast...but sided with cooler guidance Wednesday
and Thursday with fresh snowpack across the region. Friday flow
becomes southwesterly aloft...and with 925 hpa temperatures approaching 5
to 7 c...may see temperatures rise above freezing.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1018 PM CST Friday Nov 27 2015
VFR through the period.