Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
416 PM CST Sat Nov 22 2014
Short term...(this evening through sunday)
issued at 415 PM CST Sat Nov 22 2014
Another difficult forecast in the short term as low clouds and
stratus struggling to erode across the eastern half of the forecast
area. Where the stratus has eroded...temperatures have recovered
nicely in the 40s and 50s...but across the east under the blanket of
low clouds...temperatures have remained steady in the 30s. Expect
the east to fill back in with low clouds and fog tonight...and may
have to issue a dense fog advisory tonight for portions of the
forecast area. Best chances appear to be along and east of a line
from Vermillion to mashall and east. Expect visibilities to improve
as surface trough shifts east and winds become northwest drying out
the low levels. With the passage of the trough...could have a small
amount of precipitation with strong 700 mb frontogenesis coinciding
with weak divergence q aloft. Precipitation type could be a real concern through
the morning hours with temperatures at the surface near freezing.
Tough call in terms of precipitation type...for now...kept precipitation types
confined to rain or snow...but could be potential for freezing rain
with warm dry layer in the lowest 2000 feet of the atmosphere.
Expect precipitation to be all rain as it shifts further east into
locations where temperatures remain above freezing. High
temperatures a tough call on Sunday...with 925 hpa temperatures cooling
from near 10c down closer to 5c by 0z Monday. Further back to the
northwest...925 hpa temperatures hover in the lower single digits leading
to a relatively small diurnal fluctuation under a mostly cloudy sky.
Long term...(sunday night through saturday)
issued at 415 PM CST Sat Nov 22 2014
By Sunday night...northwest winds increase to 20 to 30 miles per hour with
higher gusts to 45 miles per hour. Colder air begins rushing back into the
region...with temperatures dropping back below freezing in the
evening and reaching the teens and lower 20s by daybreak Monday. As
the northern stream upper trough digs into the northern plains and
merges with the southern stream wave to our east...some scattered
light snowfall will track southeast across the forecast
area...however the better forcing remains to our east and without a
great deal of moisture...any snowfall will be on the light side.
Could see around a half inch to inch in southwest Minnesota...
otherwise a few tenths or trace at best. Additionally...with the
gusty winds...the light snow will blow around a bit...but amounts
will be too light for any blowing snow mention. Any existing snow
pack should have enough of a crust to not cause any blowing snow
With the deep upper level trough sliding across the western Great
Lakes and sharp ridging along the West Coast...upper level northwest
flow settles into the area. While not particularly a cold
airmass...temperatures remain below normal in the middle 20s to middle 30s
on Tuesday and Wednesday. Models track a clipper wave along the
upper flow...although the timing...track and strength of this fast
moving wave remains in poor agreement. The GFS is quite a bit faster
than the other models...followed by the European model (ecmwf) then the NAM. But it
seems the general consensus is for the best chance of light snow in
our northeastern counties on Tuesday night.
Uncertainty further increases Wednesday and Wednesday night as the
baroclinic zone sets up near the forecast area and potentially
becomes the focus mechanism of additional light precipitation as
warm air advection lifts into central South Dakota and the middle Missouri
Valley. Confidence is fairly low with where this will set up...but
will leave some isolated light snowfall mention Wednesday and
Models waver with bringing further waves along the flow...but not a
lot of confidence in the timing. Thanksgiving looks to be the
coldest day in the extended with highs only in the teens and lower
20s followed by Thursday night lows in the single digits. Friday
temperatures moderate some into the upper teens to near 30.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1140 am CST Sat Nov 22 2014
low clouds and fog will be somewhat problematic across portions of
southwest Minnesota into northwest Iowa. Expect ceilings to
improve slightly through the afternoon hours...though the
southerly flow should bring back the low ceilings and fog tonight.
As the trough moves through late tonight...boundary should flush
low level moisture out leading to improving conditions.