Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1048 PM CST Thursday Dec 12 2013
Short term...(this evening through friday)
issued at 351 PM CST Thursday Dec 12 2013
A weak wave will exit the area this afternoon but all that has
really come of this is some middle level cloud cover. Another weak wave
will move across ND into northern Minnesota this evening which will keep
some of the middle and high clouds in place as well as help give a
little push to the low level Arctic air to the north. Winds will
turn to the east and northeast overnight then gradually turn to the
north late tomorrow afternoon into Friday night. This will bring a
colder air mas into the area and far northern parts of the County Warning Area may
see temperatures fall a little earlier than normal while the
remainder of the County Warning Area stays something close to a normal diurnal
curve. Lows should mainly be in the single digits above zero.
The diurnal range on Friday will be limited with clouds and colder
air gradually filtering in so only expecting a temperatures rise of
about 8 to 10 degrees from the morning low. So highs mainly in the
teens with a few 20s in northwest Iowa. Upper level support and some
middle level moisture works into the northern County Warning Area late in the afternoon
and some patchy light snow will be possible...mainly north of
Highway 14. Not expecting much of any accumulation through sunset.
Long term...(friday night through thursday)
issued at 351 PM CST Thursday Dec 12 2013
Weaker northern end of troughing pushes across the area on Friday
night...with most of the deeper lift forcing expected in the 03z
to 09z window...tapering off from the northwest. Any meaningful
moisture is cut off by stronger southern stream...with moisture
mostly limited to Post frontal area...and toward minimal convergence
signal with weak inverted trough across eastern County Warning Area. Suggestion
that pretty much a shallow event overall...which does not Bode
well for getting any widespread measurable precipitation. Likely
will be dealing with a dusting of occasional flurries for the most
part...with best chance of measurable to the northeast parts of
the County Warning Area. Arctic air again filters southward Friday night...but
clouds likely to help keep temperatures in the single digits above zero.
Saturday...hard to Weed out the higher relative humidity associated
with the colder airmass from that which will legitimately produce
lower clouds. Tried to tie most to the exiting surface trough far
east...and also with secondary wave pushing toward northern Minnesota/surge
southward ahead of advancing surface ridge axis. Again have temperatures
that would favor fairly easy dendritic production...so kept some
very low probability of precipitation going at times...but any accumulation would be quite
minimal. Highs in the teens...and with north to northwest winds
will find wind chills mainly a bit either side of zero...coldest
early in the day.
Core of the coldest air push should remain just east of the area...
but surface ridge will drift slowly across on Saturday night. If
skies can remain clear for any length of time...would be wide open
for a quick fall...and have hedged some readings below zero in the
north and east. May yet be a few flurries around given the thermal
range and potential for lower clouds.
Thermal boundary which was driven far west will begin to return
eastward as a warm front on Sunday. Fairly decent wave drops
southeast on Sunday afternoon...with concentrated area of strong
warm advection. As moisture deepens up...could see the lower level
frontal zone activate to some light snowfall late day and through
the evening. For now...have focused on the northeast third of the
area...although the ultra progressive GFS would have all action
well clear to the northeast.
Another stronger wave looks to follow on Monday...pushing into the
Great Lakes. Dynamics for lift are removed to the northeast...but
will help to kick up northwest winds across the area...right through
Tuesday morning. 30 to 40 knots potentially in the mixed layer...
so upped winds in grids from model forecasts...and will have to
keep an eye on potential lag on increase with future forecasts.
Very little overall agreement by Tuesday through Thursday time
frame...and much lower confidence than normal on the forecast.
Some great difficulties appear in handling jet energy coming on to
the British Columbia coast...which feed back into multiple stream
timing and locations for phasing middle to late week. GFS much more
northern stream dominant and slower to couple up systems...showing
more of an Arctic middle level snowband Thursday into Thursday night.
European model (ecmwf) and Canadian global grossly different with a quicker phasing
and tendency to really warm up ahead of system...and wrapping up
a much stronger wave to the southeast. As a result of the
uncertainty...did not make many alterations from initialization
values. European model (ecmwf) continues to waver wildly in temperatures...now indicating
as warm as 10c Wednesday at 850 hpa...with GFS somewhat more
consistent run to run...but not so much within ensemble. Shaded
temperatures cooler a bit for Wed/Thu...and maintained chance for
precipitation on Thursday...somewhat favoring the GFS side of
things for now.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1045 PM CST Thursday Dec 12 2013
Light and variable winds through the early night time hours will
transition to northeasterly and pick up a bit after 09z as colder
air filters into the region. Models are suggesting that stratus
may develop by Friday morning as this colder air comes in...though
confidence is not high as models often overdo the low level
relative humidity with strong cold air advection...so kept with
scattered low clouds at this point.