Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
440 am CDT sun Mar 9 2014
Short term...(today and tonight)
issued at 436 am CDT sun Mar 9 2014
Very mild day in store today as warm front moved across the forecast
area overnight. 925 hpa temperatures are around 5 c this morning...but are
expected to warm further into the 10-15 range throughout the day.
Dense cloud shield across the area will dilute warming potential
some...along with remaining snow cover. Sided closer to warmer
guidance in areas that are snow free...but closer to isothermal from
925 hpa where four plus inches of snow remain across the forecast
Should have a moist boundary layer tonight...however...expect strong
enough winds along with remaining cloud cover to keep fog from
developing overnight. With warm 925 hpa temperatures...expect lows to
generally remain above freezing through the overnight.
Long term...(monday through saturday)
issued at 436 am CDT sun Mar 9 2014
Another warm day expected across the area on Monday. Question is how
much cloud cover...which should be thicker than today...and snow
cover will impact temperatures. Will continue to go in the 40s to
around 50 over the deeper snowpack areas...and a bit cooler than
today over most of the snow free areas due to the cloud cover and
slightly cooler low level thermal profiles. Attention then turns to
our next system Monday night into Tuesday. Still looks good for a
banded precipitation event...with very strong frontogenesis
interacting with a wave ejecting out of The Rockies. Thus should be
a period of pretty good lift over the County Warning Area. NAM seems like an outlier
with its quicker progression...lower quantitative precipitation forecast and further south axis.
This seems to be caused by it digging The Rockies wave further
south...and thus the front over US becomes more sheared off and
causes the band to weaken quicker. Since the NAM seems to struggle
with large scale dynamics...will not use it for this forecast.
The Gem and GFS are pretty similar...with the European model (ecmwf) a bit further
north with everything. All three seem reasonable...so will go close
to a blend of these for quantitative precipitation forecast and timing. Precipitation should enter
our west around midnight and the Sioux Falls vicinity in the predawn
hours. Precipitation will then exit from northwest to southeast
through the day Tuesday.
Precipitation type is a concern with an initially warm boundary
layer. This layer should cool as lift increases and we see the
effects of evaporative cooling. However seems likely we will see
rain initially...then it mixing with and changing over to snow as the
heavier band moves in. Thus the heaviest of the precipitation should
end up as mainly snow. The dendritic zone is rather small...but lift
is strong and equally spread through the profile suggesting fairly
good snow growth. However given the warm lower levels...think ratios
will struggle to get much above our climatological 12 to 1 or so.
For now thinking a general 1 to 3 inches across the County Warning Area...highest
north of the Missouri River corridor. However quantitative precipitation forecast values should be
high enough to potentially support slightly higher totals if we can
change to snow quicker. Will have to monitor trends...as
unfortunately this will be a close call.
Behind the system we see ridging begin to build back into the region
on Wednesday...and with that we should see warming temperatures. Decent
ensemble spread on Wednesday suggesting some uncertainty on how
quickly the warm air returns...and how much impact any new snow will
have on temperatures. For now looks like 30s east and 40s west.
Pretty high confidence we warm into the 40s and 50s by
Thursday...withe even some 60 degree readings again possible in south
central South Dakota. A wave passing to our north in the northwest
flow will drag a cold front through the County Warning Area Friday. Thus models
trending a bit cooler...although 40s and 50s still seem probable.
European model (ecmwf) and GFS in good agreement that another wave in the northwest
flow will move across on Saturday...with this one potentially
producing a period of precipitation. Although a large spread among
the European model (ecmwf) ensembles suggests confidence of this happening is lower
than the model agreement would suggest. High temperatures off these
latest runs would only be 30s...but given the uncertainty will stick
with the warmer allblend for this forecast package.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1017 PM CST Sat Mar 8 2014
VFR through the period. A small chance for a little low level wind shear late
tonight and early Sunday morning as winds at about 1200 feet above ground level
increase to 30 to 35 miles per hour from the southwest while the surface
winds should stay around 8 to 12 miles per hour. Should be enough wind at the
surface however to avoid any official low level wind shear.