Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
935 PM CST sun Dec 21 2014

Update...
issued at 935 PM CST sun Dec 21 2014

Have overhauled probability of precipitation and weather some mainly to slow down the
spread of precipitation spre4d over the northern part of the area
and to lower the overall chance far west/northwest. Still...light
rain at middle evening was spreading over the Missouri River area and
should continue to develop northeast through the night. Slight
adjustments to temperatures and to the nature of any sleet/snow
mixture far north and west later tonight. Overall however not too
much change....with most of the precipitation tonight still being
light rain.



&&

Short term...(this evening through monday)
issued at 323 PM CST sun Dec 21 2014

Clearing line has made some progress east this afternoon. However by
late this afternoon and evening expect the low stratus to return
northwest and cover much of the area yet again. Hrrr and rap
guidance have been doing pretty well the past few nights...and
suggest dense fog will probably not be as big of an issue tonight. A
strong upper wave will dive southeast into the plains tonight. Still
quite the spread among model solutions. NAM and GFS are further
south with the system than the European model (ecmwf). European model (ecmwf) has been the most
consistent and the parallel upgraded GFS is more in line with the
European model (ecmwf) solution. Thus decided to weigh heavily in that direction for
this forecast. Lift ahead of the wave will likely generate
precipitation tonight...developing in the west this evening...and
spreading east through the night. Based on model soundings and
consensus raw temperatures...believe this will mainly be rain.
Although could see some sleet mixed in...mainly west of the James
River.

Monday is when the model differences begin to have a bigger impact
on the forecast. The favored European model (ecmwf) and parallel GFS solution results
in a dry slot over much of the County Warning Area. The majority of the wrap around
snow associated with the developing trowal should stay northwest of
the County Warning Area. Do think locations from Huron to Chamberlain could get in
on some of this activity though. Soundings suggest that by Monday
any warm layer is surface based...suggesting either rain or snow
depending on surface temperature. Raw model consensus of
temperatures suggests ptype stays predominately rain east of the
James River into the afternoon...with the rain snow mix getting to
Interstate 29 by 0z. But as mentioned...much of the County Warning Area should see
very limited quantitative precipitation forecast through the day hours...with just some rain in the
far east and an inch or two of snow from Brule to western Beadle
County. Will need to monitor model trends...because if the NAM and
operational GFS further south solution verifies...more of the County Warning Area
would see precipitation on Monday. But even then the warm low level
temperatures would likely keep most areas rain for a majority of the
day keeping any snow accums down.

Long term...(monday night through sunday)
issued at 323 PM CST sun Dec 21 2014

Monday night into Tuesday will be a period where the system has
slowed down just a bit and a fairly persistent trowal sets up.
Strong north to northwest winds will gradually develop through the
day so it is starting to look like we may need and advisory for snow
and blowing snow Monday night into Tuesday. Models still have just
enough disagreement and the worst conditions look to be late third
period into 4th period so will hold off on issuing any headlines but
if this evenings models come in similar will likely need something.
Monday evening will start off relatively mild in the southeast with
temperatures above freezing so a better chance for rain in northwest
Iowa before midnight but by about 11pm to midnight should be cold
enough for snow. No concerns right now of mixed precipitation Monday
night as the warm layer is at the ground so should be either rain or
snow. Went ahead and increased probability of precipitation tonight into Tuesday and
increased quantitative precipitation forecast amounts some so that by Tuesday evening so that much
of the area should see 2 to 4 inches with the James Valley west
seeing the best chance at approaching 4 inches. Again...with 2 to 4
inches of snow and 20 to 35 miles per hour winds on Tuesday and Tuesday night
hazardous travel is looking more likely Tuesday into Wednesday.

By Wednesday everything should be to the east of the area and winds
will diminish through the day. Cloud cover should also break up a
bit...at least getting down to mostly cloudy. With fresh snow on the
ground expecting highs mainly in the 20s.

In the outer periods (thursday through sunday)...still an
interesting system coming through on Thursday night into Friday. The
12z GFS seemed a bit out to lunch but the 12z European model (ecmwf) and the 12z
parallel GFS both agreed well and showed some consistency from
yesterdays runs. Unfortunately the superblend is polluted with the
12z GFS so probability of precipitation came in much lower. Having trouble collaborating
higher probability of precipitation but right now anticipate that probability of precipitation will need to be
increased for especially Thursday night. After this starting to look
like a period of colder weather will settle in as a large scale
trough sets up northerly flow aloft into the northern rockies and
northern plains. So anticipating highs in the teens and 20s Friday
into Sunday with lows in the single digits...possibly dipping below
zero assuming parallel and European model (ecmwf) are more on target. So went ahead
and dropped superblend lows about 2 to 4 degrees in most locations
Saturday morning and Sunday morning.

&&

Aviation...(for the 00z tafs through 00z Monday evening)
issued at 548 PM CST sun Dec 21 2014

IFR to LIFR ceilings will persist along/east of the James River...with
stratus backing into khon at the start of the period. Will see rain
develop later tonight...possibly mixed with some sleet over far
western/northern portions of the forecast area...including khon taf
location toward daybreak. Ceilings could bump up into IFR or low end
MVFR as rain starts...but confidence in this is low and will
stick with IFR-LIFR persistence with generally MVFR-IFR visibility.
Rain will slowly transition to snow near/west of the James River by
18z. Farther east the transition may be more elevation dependent
through the afternoon...with higher elevations in east central South Dakota
and southwest Minnesota transitioning first...while valleys including kfsd
and ksux taf locations see rain through most of the taf period.



&&

Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&

$$

Update...
short term...chenard
long term...08
aviation...jh

National Weather Service Glossary of Abbreviations