Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1138 am CST Thursday Feb 11 2016
issued at 1039 am CST Thursday Feb 11 2016
Overall the large trend of the forecast still is in good shape
with the snowfall this afternoon and early evening. The snow is
lined up along a band of 750-700mb frontogenesis...with the
primary wave exiting this evening via our southeast zones. The
problem initially is a lack of saturation in the low levels.
Therefore slowed down the onslaught of the snowfall in our western
zones. But it will saturate up from northwest to southeast
eventually as the afternoon progresses...so the general trend of
the snow forecast and amounts still looks good after examining the
12z NAM and some of the hires models.
Short term...(today and tonight)
issued at 359 am CST Thursday Feb 11 2016
Little has changed in the overall pattern across the area with
persistent northwest flow over a tight low to middle level baroclinic
zone. As a result...ingredients again in place for a light snow
event later morning through early evening across the County Warning Area...with next
lift forcing boost courtesy of a wave/strong 130 knots jet streak noted
riding over shortwave ridge with strong height falls on evening
analysis across southern Alberta. With event yesterday...wave was
deeper into the cold air compared to system today which will dive
from water vapor defined location in northeast Montana at 09z...to
southwest Iowa by 00z. Progress of a nice exit region from northwest
to southeast across the area...primarily late morning through very
early evening. Again...thermal profiles are less than inspiring to
get higher snow to liquid ratios...with temperatures through much of
the layer residing several degrees to the warm side of the best
dendritic growth...and lift largely centered in this slightly warmer
layer. If could get some higher level lift into the eastern
County Warning Area...then potential to tap a little of the 600-500 hpa instability
above the higher frontal surface...but battle with drier air again
to limit efficiency.
As was the case yesterday...models in high degree of agreement...
both lower and high resolution...in timing and location of better
snowfall period. Concern again that precipitation may be favored too
low in the profile and biased toward the warmer side of the
baroclinic boundary...but with pre existing dry push into the
northeastern County Warning Area this morning...will give a little more leeway to
idea of keeping main axis of snowfall around or a bit more than an
inch just south of khon to kfsd to kslb. Potential that Lower Brule
areas could mix with a bit of liquid early afternoon as warming near
surface to just aloft.
Precipitation forcing drops off quite rapidly by middle evening...and
have accelerated the departure of lingering measurable precipitation
in the 03z-05z range. An earlier exit could result in a small window
for some freezing drizzle during the evening before temperatures begin to
chill again. Will get a renewed push of cold air starting southward
as Arctic ridge drops in behind exiting surface wave to the south.
Band of shallow 950-900 hpa frontogenesis on the leading edge of the
cold surge...and with temperatures settling into a very dendritic
range...may see another brief period or flurries as winds increase
to 15 to 20 miles per hour by daybreak in much of the area.
Long term...(friday through wednesday)
issued at 359 am CST Thursday Feb 11 2016
Cold...dry Arctic air settle sin from the northeast on Friday which
will bring sunny but cold conditions to most locations. Parts of
central South Dakota may hang on to clouds into the afternoon. Lowered highs
just a bit as the average 925mb temperatures is running about -15 to
-20 degrees c...which may make getting out of the single digits in
southwest Minnesota very difficult.
Friday night into Saturday cold high pressure will settle in but the
potential for a very cold radiational night will be offset by an
increase in cloud cover. The far eastern County Warning Area will see the best
potential to really bottom out and will shoot for around -15 degrees
f in those areas. Cold air sill still be in place on Saturday so
highs will struggle into the single digits east and to around 20 in
south central South Dakota. As a wave approaches from the west the threat for
snow will increase from west to east through the day into Saturday
night. Increased the chance for snow Saturday night into Sunday and
increased snowfall amounts a bit. Will need to keep an eye on
amounts. If the system can dig a little more than forecast some 4 to
5 inch amounts will be likely but if the wave comes in a little
weaker the better chance for a couple of inches will likely be in
the northern County Warning Area with not much snow in the southern County Warning Area.
In the outer periods(sunday through wednesday)...low pressure will
move through Sunday morning with a chance for some snowfall in the
eastern County Warning Area. The better chances should be in the morning.
Temperatures will be on the cold side as the low level cold air is
still in place. Northwest flow aloft develops behind this wave so
mainly dry conditions expected Monday through Wednesday. Along with
this drier regime will come moderating temperatures with a very good
chance for above normal temperatures. Right now aiming for lows
mainly upper teens to upper 20s and highs in the 30s with 40s along
and west of the James River Valley. One caveat to the forecast is
that confidence not super high as we are still dealing with a fairly
tight thermal gradient in the low levels so any unforecast ripple in
the atmosphere could shift the colder air south or the warmer air
north bringing some potentially big changes at times.
Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1133 am CST Thursday Feb 11 2016
The primary near term concern is when snow will start at the taf
sites this afternoon. All hires models and saturation profiles are
still showing snow to work its way from northwest to southeast as
the afternoon progresses...and timed the beginning of the snowfall
in the taf sites based on the saturation of the 850-700mb layer...
which by the way is currently still fairly dry. But when the
snowfall does start...am expecting IFR visibilities based on what
is upstream in central and northern South Dakota. The system rapidly exits
early this evening through northwest Iowa...and that will end the
snowfall but lower end MVFR to upper end IFR ceilings will likely
linger. Drier air will then advect southward very late tonight and
Friday morning producing VFR conditions from north to south.