Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1157 am CST sun Dec 21 2014
Short term...(today and tonight)
issued at 325 am CST sun Dec 21 2014
Widespread stratus and areas of fog continue early this morning as
temperatures rise slowly into the lower 30s. Because temperatures
have been hovering near the freezing mark...dont think there will be
as many icing reports as there were yesterday when temperatures were
several degrees colder. Nonetheless...patches of freezing drizzle
are still possible early across central Minnesota especially where
readings are coolest.
The biggest change to the forecast for today was to introduce a
healthy burst of snow across the Wisconsin counties this afternoon.
Models are unanimous in showing a large area of snow developing
after 11 am near the Minnesota/WI border and advancing east northeast during
the afternoon. Forecast soundings are kind of impressive showing
full saturation through the column...some lift...and temperatures
below 5kt solidly below freezing except at the surface.
Thus...accumulating snow is a good bet...particularly as one heads
toward dun...Barron...Rusk and Chippewa counties where it should
persist the longest. Here...we expect at least 1 to 2 inches...but
an isolated 3 inch report cannot be ruled out. Pepin and Eau Claire
counties may mix with a bit of rain from time to time...keeping snow
The wave responsible for generating this batch of snow will push
east this evening. Little lift...but ample low level moisture will
not be favorable for steady precipitation this evening...but drizzle
will become more widespread as the evening GOES on when the main
upper low across the northern plains pushes southeast toward Sioux
Falls. Eventually the lift will be able to sustain steadier rain
after 06-09z across southwestern Minnesota...with widespread drizzle
persisting elsewhere. Strengthening southeast flow will keep
temperatures steady or slowly rise overnight into the middle/upper
30s...thus not anticipating icing issues.
Long term...(monday through saturday)
issued at 325 am CST sun Dec 21 2014
Long term concerns remain evolution of mixed precipitation event
through Tuesday. Models continue to trend colder in the wake of
One change in the deterministic models this morning was an
overall shift of the surface low associated with the closed upper
air circulation with the amplifying trough. This brings in a
little cooler air to the northern portion of the County Warning Area later Monday
into Tuesday. Followed a general NAM/GFS blend. Warmer air will be
drawn north across most of the County Warning Area Monday. A snow...rain snow mix
will be found over the northern Thursday of the County Warning Area...with rain
and possible morning sleet in the south. Will trend colder from
the west then Monday night and Tuesday...with mainly snow over all
bu the far east come Tuesday morning.
Another strong turn in the models has been the higher probability
of a change over to all snow Monday night across the central County Warning Area
as the upper trough axis pivots east and drives stronger enhanced
forcing over this area. If this does occur...we could end up with
a band of heavier snow accumulation across south central...into
east central Minnesota and a portion of west central Wisconsin...Monday
night and Tuesday. Will continue to monitor overall trends today
and determine if headlines become warranted for a portion of this
In the wake of this system...the models continue to generate
another short wave moving rapidly in from the west on Thursday
and Thursday night/Christmas day/. This generates a band of snow
across a portion of southern Minnesota. It looks to be light however and
the system will be fast moving. The parallel GFS was similar to
the the operational GFS and the European model (ecmwf) this evening...but a little
slower. Will indicate a slight chance of snow over the far south
developing Thursday...which continues into Thursday night.
Colder air drops over the area over the weekend. The models
diverge some on handling the short wave associated with this
Arctic air intrusion. Will keep the trend dry for now...but will
monitor todays trends and see if a snow chance has to be
introduced on Saturday.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1157 am CST sun Dec 21 2014
IFR ceilings forecast through the period with less than MVFR
visibilities. A moist southerly flow will persist through the taf
period as low pressure approaches from SD/NE. Extensive br/fog
through the period. Some improvement in the visibilities will
occur this afternoon...but visibilities drop back tonight to
between a 1/2 and 1 1/2sm at most locations. Complicating matters
is a weak area of low pressure aloft passing by this afternoon and
evening. This will aid in the generation of mainly light rain
across the Twin Cities and points southwest with snow mixing with
and changing to rain across west central WI by evening. Model
trends today continue what began last night and that is for the
low pressure system approaching from the west to pass south of US
on Monday. This means a colder profile with a rain and snow
mixture during the day. Just another degree or two colder will
mean most taf sites would be all snow by Monday afternoon along
with accumulation. Right...we are going with a changeover to all
snow by Monday evening with several inches of accumulation likely
across the area Monday night.
Kmsp...bouts of light rain/snow this afternoon. No accumulate with
temperatures holding in the middle 30s. In fact...slowly rising temperatures
tonight with upper 30s by daybreak Monday. Although a rain/snow
mix is forecast on Monday...it could become all snow in the
afternoon. Definitely all snow by evening.
/Outlook for kmsp/
Monday night...IFR. Snow. Several inches of accumulate. Winds southeast 5-10 kts.
Tuesday...MVFR/IFR. -Sn or -ra/-sn mix. Winds northwest 5-10 kts.
Wednesday...MVFR. Slight chance -sn. Winds northwest 10-15 kts.