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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
524 am CST Monday Dec 22 2014

Short term...(today through tuesday)
issued at 334 am CST Monday Dec 22 2014

A deepening upper level trough situated across the north central
Continental U.S. Very early this morning...and with strong moisture transport
out ahead the main upper level wave and a midlevel pv anomaly
swinging through the region...presently seeing a broad area of
showers lifting through the central portion of our County Warning Area. These
showers will continue to spread eastward through the day...with
models still pointing toward much of our area being dry slotted into
tonight as the upper level low slides across South Dakota and much
of the upper level energy dumbbells around our County Warning Area. Still very
mild across the area this morning with readings in the middle 30s to
around 40...and not going to see much change in temperatures
through the day. As of this morning there is still a warm layer of
air aloft...keeping the precipitation in the form of plain rain
with the relatively mild surface temperatures...and while there
could be brief period of mixed precipitation in our northwest this
morning as thermal profiles cool...by late morning all models have
temperatures aloft below 0 degrees c. So any precipitation that
occurs after that will be in the form of rain or snow dependent on
surface temperatures. Based on surface temperatures...most locales
should just remain plain rain through much of the day...possibly
changing to snow in our west during the afternoon as surface temperatures
cool. In any event...looking at minimal snowfall accumulation
today.

By tonight as thermal profiles continue to cool any precipitation
would be in the form of snow...though as related to the above
discussion...quantitative precipitation forecast amounts are not overly impressive as the better
amounts generally swing around our County Warning Area. Still...could see a couple
of inches of accumulation through the western James River Valley
during the night as a trowal begins to exert effects on that area.
Back to the east any snowfall amounts would be an inch or less. Do
begin to see northwesterly winds pick up late tonight on the
backside of the low pressure...which could result in some blowing
snow in our west dependent on snowfall amounts. While our neighbors
to the west have issued headlines for snow tonight...currently
forecasted snowfall amounts in our area do not justify an advisory
at this point...even with the increasing winds. If it ends up
looking like snowfall amounts will be higher over that area...future
shifts may have to consider an advisory taking in to account blowing
snow.

On Tuesday the aforementioned broad trowal drops across the area
with light snow persisting through the day. Temperatures will be a
little cooler...in the lower to middle 30s. It will also be breezy
through the day as the surface low winds up to our east...possibly
resulting in patchy blowing snow through the west.

Long term...(tuesday night through sunday)
issued at 334 am CST Monday Dec 22 2014

Any snow over the area will gradually move to the east and end by
sunrise on Wednesday as the upper level low moves into the Great
Lakes. There could be up to an inch of additional snowfall around
Marshall...tapering to less than a half inch moving southeast
toward the South Dakota and Iowa borders. Despite the snow
ending...clouds will once again be very slow to move east of the
area. Expect some clearing to approach the James River from the
west by sunrise. With the clouds in place and fairly strong
northerly winds east of the James River as well...low temperature
will remain well above normal in the lower to middle 20s. With skies
becoming partly cloudy late...and new snowfall...lows expected to
fall into the upper teens.

A weak ridge will move across the eastern Dakotas and into
Minnesota by Wednesday afternoon. Flow east of the ridge will turn
to the south or southwest during the day. This will help a few
breaks to develop in the clouds during the day over most of the
area. The winds will also bring mild air aloft back into eastern
South Dakota. The question is how much of this warm air will mix
down to the surface. Winds do not look very strong...generally 10
miles per hour or less...and there is new snow cover as well. While the
potential exists to reach the 40s along and west of the James
River...the new snow and light winds will likely keep temperatures
closer to isothermal from the upper 20s in much of SW Minnesota to the
middle 30s in south central South Dakota. On Wednesday night...expect light
southerly flow through the night. In addition...some middle clouds
will spread over the area during the night ahead of the next wave.
With the clouds and south winds...lows will not fall much
remaining in the lower to middle 20s.

In the extended...Thursday through Sunday...the main focus is on
the wave moving into the area late Christmas day and remaining
into Friday. The Gem...ECMWF...GFS...and parallel GFS...all agree
that there will be precipitation over the area...primarily
snow...from sometime late Christmas day into Friday afternoon.
The issue is the timing of the precipitation. The Canadian Gem is
the fastest and the operational GFS is the slowest with the European model (ecmwf)
and parallel run of the GFS fairly similar. Because of timing
uncertainty...cannot rule out precipitation beginning late
Christmas day but the most favored time based on model consensus
is from later Thursday night into Friday morning. All models have
also begun to focus the band of snow farther north toward i90 and
north. With this in mind...have highest probability of precipitation west of the James
River on Christmas night and then generally along and north of
i90 on Friday. Because of timing uncertainty...kept probability of precipitation closer to
40 percent at this time. Am a little concerned if the slowest
solutions occur there could be lingering snow in SW Minnesota and northwest Iowa
on Friday night but kept it dry for now. This will be a frontal
band but accumulating snowfall in any one location is expected to
be fairly short...6 to 12 h. So at this time thinking in the
heaviest snowfall will be around 2 inches. But a few solutions are
heavier so will to keep eye on this system as it could impact
travel...especially Thursday night and Friday.

The other concern is precipitation type. With temperatures
expected to be above freezing in most areas on Christmas...any
precipitation will likely be a mix of rain and snow or even all
rain. As colder moves in Christmas night...the expectation is
that precipitation will go from a mix of rain and snow to all
snow. There is a low probability that there could be a period of
freezing rain or sleet south of i90...but at this point...the
probability is so low that will stick with rain or snow for now.

After the system moves through...colder air will move in for the
weekend with highs below normal in the teens and lower 20s. There
are hints of light snow either late Sunday or more likely Sunday
night and Monday. For now...kept the weekend dry until there is
better agreement with this system.

&&

Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 524 am CST Monday Dec 22 2014

Stratus and precipitaiton are the main concern for aviation
today. With a couple of exceptions...most areas have ceilings below
1000 feet with visibilities generally 3 to 6 sm with fog and areas of rain.
Rain will gradually diminish this morning leaving areas of light
fog and stratus. As the inverted trough moves east through the day
and winds turn to the northwest...expect that areas of rain and
snow will redevelop...and gradually change to snow. This will be
later this morning in khon and late afternoon or evening in kfsd
and ksux. Expect visibilities to decrease to IFR after 06z due to snow.
There is the potential for visibilities below 1sm toward 12z but with lift
rather unfocused will not go any lower with visibilities at this time.
Winds will also increase after 06z as the low deepens over the
Great Lakes with sustained winds of 10 to 15 kts by 12z Tuesday.
Stronger winds and more light snow expected after 12z Tuesday.

&&

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

$$

Short term...jm
long term...schumacher
aviation...schumacher

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