Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
555 PM CST Wednesday Nov 25 2015
Short term...(this evening through thursday)
issued at 250 PM CST Wednesday Nov 25 2015
Messy forecast for tonight into tomorrow as a series of weak
impulses move across the area ahead of large trough across the west.
On the large scale...the right entrance region of jet streak
positioned north of the Great Lakes region will create large scale
upper level divergence across the region tonight through at least
the first half of Thanksgiving. Thereafter...the jet streak starts
to work east and forecast area is on the left entrance region of
that jet streak.
Model soundings show a warm layer aloft overnight that has trouble
saturating at least initially. As the isentropic lift increases from
the south...expect deeper saturation to lead to light precipitation
and drizzle expanding north across the region. With thermal
structure aloft...have forecast icing potential...freezing rain
and/or sleet from Ponca Nebraska to just north of Spencer Iowa and
south. With icing amounts around a tenth of an inch...will issue a
Winter Weather Advisory for northeast Nebraska into northwest Iowa.
As cold air nudges southward through the day and the atmospheric
column cools...expect precipitation to change over to snow. Have
heaviest snow falling immediately north of the icing band...with up
to 2 inches in the band. With no complicating factors in the snow
band...will leave this area out of the advisory for now.
Temperatures tonight will fall slowly with fair amount of stratus
around. May also see some areas of fog develop across northwest
Iowa into southwest Minnesota as saturation deepens throughout the
night. Temperatures will be very steady throughout the day tomorrow
with poor mixing conditions. Winds will generally be in the 15-20
knot range throughout the day.
Long term...(thursday night through wednesday)
issued at 250 PM CST Wednesday Nov 25 2015
Thanksgiving evening will see lingering light snow in northwest Iowa
quickly end in the early evening. Skies Thursday night should
steadily clear from the north-northwest...am looking for this clearing to be
faster than what we had been going for before. Skies will then stay
pretty clear Friday and Friday night before starting to increase
slowly Saturday as the western upper low approaches...however the
Saturday increase in clouds will also be slower in this forecast
than in the earlier one.
Temperatures will be on the modestly cold side for the end of
November. Lows Thursday and Friday nights will be mostly 10 to 15
except upper teens far east and southeast Friday night. Highs should
be in the upper 20s Friday...warming a rad to just below freezing
Saturday as the cold air moderates a little. Winds Thursday night
and Friday will still be a bit brisk but am keeping the daytime
increase from the loaded grids a bit below the breezy mention.
Sunday through Tuesday will bring the threat of snow with light
accumulations as short waves rotate around the southwestern upper
low as it moves east northeast toward our forecast area. Timing of
the best chance of snow and greatest accumulation threat is still
hazy with model and run to run differences continuing. The consensus
seems to be hazily centered on about Monday with the initial wave
coming up Sunday lacking enough strength and moisture to do much.
Even if a stronger wave drives the upper low across the area around
Monday...as the models are suggesting...modest moisture and weak
thermal contrast seem to Blunt any threat os the snow assuming
winter storm levels...which it would apparently have to do without a
lot of wind. Temperatures during this period will not do much
changing...just slightly warmer low temperatures under decent cloud
Accept the model consensus of this system to move on east in time
for a dry Tuesday night and Wednesday...with temperatures showing
perhaps a very slight warming on highs with the heating.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 539 PM CST Wednesday Nov 25 2015
Several concerns for the aviation forecast this evening. MVFR and
IFR ceilings will continue overnight through much of the day
Thursday throughout the region. Additionally...areas of fog and
drizzle will persist through the evening hours in northwest Iowa
and parts of southwest Minnesota...followed by a light wintry mix
of rain...sleet...freezing rain and snow developing overnight.
Light snowfall will be possible late tonight through much of the
day Thursday elsewhere...however the wintry mix will become more
steady Thursday morning before gradually switching to entirely
snowfall in the afternoon. Expect low ceilings and reduced
visibility in mixed precipitation...with visibility dropping below
2 miles once the switch to snowfall occurs.
South Dakota...Winter Weather Advisory from 4 am to 6 PM CST Thursday for
Iowa...Winter Weather Advisory from 4 am to 6 PM CST Thursday for
NE...Winter Weather Advisory from 4 am to 6 PM CST Thursday for