Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1106 am CST Sat Nov 28 2015
Short term...(today and tonight)
issued at 412 am CST Sat Nov 28 2015
Broad middle level warm advection will begin to spread north this
afternoon as the flow at 700 mb and elsewhere around that height turns
southwesterly. This will result in a lot of middle clouds hugging the
southern part of our area today starting to spread north/northwest.
This is in response to the first short wave coming up put of the
southwestern upper low. This wave will provide some lift for a
little light snow over the western part of the area tonight...
possibly starting far southwest late this afternoon. Although the
lift will be there...it looks fairly shallow and far from the
dendritic area in relatively warm temperatures. Add the dry air at
lower levels into the equation and there could be an excuse for
taking the light snow mention out. However feel it best to leave low
pop mention in for now...most likely some areas will get the
proverbial dusting. Although the cloud increase will affect all of
the area...there is really no hope for measurable snow east late
tonight. Even any light snow or flurries west will probably
dissipate by the end of the night as this fist wave moves on north
and starts to pull back west around the upper low.
Temperatures are not much of an issue with readings warming to 30 to
35 today...and doing limited cooling to around 20/lower 20s tonight
with the cloud cover. Winds will be pretty light in the flat surface
high pressure regime...tending to a light southerly puff by tonight.
Long term...(sunday through friday)
issued at 412 am CST Sat Nov 28 2015
Focus for the medium-long range portion of the forecast is on
impending winter storm set to affect the region early next week.
Have trended a bit drier/slower to spread probability of precipitation into the area late
Sunday/Sunday night...in part due to need to overcome relatively dry
low-level easterly flow across northeast portions of the area...but
also due to lack of moisture above 700mb...especially in temperature
range needed to introduce sufficient ice into the cloud layer to
produce measurable precipitation. For this reason...despite weak lift from
broad warm advection ahead of the upper wave...will continue to hold
slight chance probability of precipitation west of the James River through the day Sunday...
with only chance probability of precipitation into the Missouri River valley through 06z
Sunday night. Sky cover expected to be abundant however...and should
see relatively small diurnal range with highs Sunday a degree or two
either side of freezing...and lows Sunday night in the middle 20s.
28/00z GFS has become an outlier regarding the track of this
system...shifting both the upper level and surface lows farther
south...keeping the surface low well south in Kansas/northern
Missouri while the remaining models continue to swing the surface
system northeast into Iowa. Operational GFS also among the farthest
south with the upper low within its own ensemble. For this reason
have generally favored NAM/ECMWF/Gem combo for primary details of
this forecast. With this in mind...expect fairly rapid increase in
precipitation late Sunday night/Monday morning as upper low moves
across Nebraska...strengthening and becoming slightly negatively
tilted as it does. By midday Monday...pv anomaly pushes through
Nebraska with a hint of some middle-level instability above frontal
boundary working into our southern County Warning Area around Sioux City. This
swings northeastward into southwest Minnesota through the afternoon
as trowal begins to develop. This should focus period of heavier
snow near/east of I-29 through the day Monday...with snow then
lingering longest across the northeast half of the forecast area
through Monday night as the system moves east into the upper
Could still see a brief period of sleet mixing in with snow over the
far southeast County Warning Area as precipitation begins late Sunday night...but otherwise
looking at primarily a snow system...with snowfall rates generally
beginning around 9:1 or 10:1...increasing to 14:1 as colder air is
pulled into the backside of the system. Given this along with the
prolonged period of snowfall...currently looking at broad area of 6
or more inches of snowfall throughout the forecast area Sunday night
through Tuesday...with potential of 8+ across the east given the
chance of locally heavier snow during the day Monday. Given that the
event is still a couple of days out...and that much of the snowfall
will be spread over 24-36 hours...will hold off issuing headlines
just yet. But will continue to highlight in severe weather potential statement/weather graphics.
Temperatures will likely see little fluctuation during the event...
generally varying from lows in the middle-upper 20s and highs around
30. After the storm exits...will see colder air work into the area
as we clear out midweek. Temperatures aloft begin to warm up later
in the week...but with expected snowfall...could be tough to see
this warming realized at the surface and will only slowly warm highs
through the latter part of the week.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1100 am CST Sat Nov 28 2015
VFR conditions are expected through the taf period. Middle-level clouds
will gradually build northward through the night...however are not
expected to drop below 3000 feet above ground level. Winds will remain light and
variable...which could lead to patchy ground fog.