Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
259 PM CST Sat Feb 13 2016
Short term...(this evening through sunday)
issued at 300 PM CST Sat Feb 13 2016
Focus this afternoon is on impending snow development across the
northern plains ahead of wave currently swinging through Montana.
Current seeing band of light snow trying to develop from central South Dakota
into the lower Missouri River valley...though eastern portions of
the band running into dry layer per forecast soundings and not much
more than flurries if anything reaching the ground in our area as of
20z. High-res models shift this band northward through the late
afternoon and early evening...though continues to encounter the dry
layer until more sufficient lift from approaching wave reaches the
area late this evening. By that time...even coarser NAM/GFS shift
the focus of this band north of the previous forecast position...
primarily north of our forecast area or just brushing areas north of
Highway 14 with some locally higher amounts. This results in lower
amounts than previously forecast in our northern County Warning Area...but still a
widespread storm total snowfall of 3 to 5 inch range expected over
the northeast half of the forecast area.
With the northward shift...expect locations south of I-90 to see a
break in snowfall for a few hours during the evening/early in the
overnight period...though difficult to determine precise timing
and/or location...so will keep at least chance probability of precipitation going across the
south through the evening. Another trend seen in the models today is
to bring a wrap-around area of snow farther south into south central
South Dakota and far northern Nebraska late evening/overnight...as a
stronger piece of energy digs into the base of the upper trough. 12z
run of NAM was very robust with amount of precipitation produced in this
area...while most other models indicated the precipitation wrapping in but
at a much lighter rate. 18z NAM has since backed off on the bullseye
of quantitative precipitation forecast/snow in our southwest quadrant tonight...but still enough
signal that this area will see around an inch of snow or a little
more that we bumped probability of precipitation into categorical range.
Based solely on snowfall amounts...contemplated dropping advisory
from some counties along the southwest edge of the highlight in our
area. However with added complication of the gusty southerly wind
and likelihood of areas of blowing snow across the east-west I-90...
opted to keep the headlines in place for now.
Beyond the snow...long-advertised warming trend is still on track.
Temperatures will be steady to slowly rise throughout the forecast
area tonight...with readings in the teens east/20s west expected by
daybreak. Warming will continue to edge eastward tonight...with
highs from the middle 20s far east to around 40 in our far southwest.
Long term...(sunday night through saturday)
issued at 300 PM CST Sat Feb 13 2016
Low clouds over the eastern/northeastern part of the area should
continue to decrease Sunday night. Meanwhile a weak wave is
expected to bring a few middle clouds from west to southeast during
the night with no support for light precipitation. Skies may become
mostly clear/mostly sunny for a while in some areas Monday before
clouds start to increase from the west/northwest with the next
approaching wave. After a modest drop to around 20 or in the 20s
Monday night...warming Tuesday will bring temperatures mostly into
the 30s...with some 40s southwest corner.
The precipitation event with this wave looks to be almost all on
Monday night...with perhaps a little light precipitation lingering
far southeast early Tuesday morning. Broad warming ahead of the
wave presents the threat of some rain/freezing rain/sleet before the
air cools later in the night as the wave is passing. With most of
the precipitation concentrated in the east and more during the
latter part of the night...snow accumulations of one or two inches
are expected mainly east of a Delaware smet to Parker South Dakota to Sioux City
line. A little glazing is possible over the same area early Monday
night. As usual...prospects of greater accumulations of snow or ice
will need a strengthening of the Canadian wave over how the models
currently depict it...and models seem to be fairly close to each
other now...whipping the system across fairly quickly and showing
more digging as it moves closer to the upper trough axis to the
east. A slower and stronger system would certainly have some decent
instability to work with but with less thermal support than with the
current system. The fast movement should provide at the worst a
fairly brief period of modestly strong north/northwest winds.
Temperatures will be fairly mild and significant blowing snow is not
expected at this time.
Wednesday through Saturday will be marked by decent warming then
temperatures remaining mild. Strong upper ridging will bring the
quick warmup. A upper wave denting the ridge about Friday looks even
shallower than it did yesterday and this leads to going with the
extended guidance and leaving precipitation out. If anything...the
threat would have to be a few April showers arriving two months
early. The shallow nature of the wave seems to preclude much cooling
behind the system as next weekend nears...and given a resurgence of
ridging over The Rockies after the forecast period...we could be
looking at above normal temperatures lasting the rest of this month.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1215 PM CST Sat Feb 13 2016
VFR conditions over most of the area at the start of the period
will deteriorate to MVFR ceilings/MVFR-IFR visibility after 21z
as snow develops across the region. Southerly winds gusting
25-30kt could result in periods of LIFR visibility in snow and
blowing snow 00z-12z. Gradual improvement to VFR visibility is
expected west to east after 12z Sunday...but anticipate MVFR
ceilings persisting through the end of the taf period.
South Dakota...Winter Weather Advisory from 9 PM this evening to noon CST
Sunday for sdz040-055-056-061-062-066-067.
Winter Weather Advisory from 6 PM this evening to 9 am CST
Sunday for sdz038-039-052>054-057>060.
Minnesota...Winter Weather Advisory from 9 PM this evening to noon CST
Sunday for mnz071-072-080-081-089-090-097-098.
Iowa...Winter Weather Advisory from 9 PM this evening to noon CST
Sunday for iaz001>003-012>014-021-022.