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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
954 PM CST Wednesday Nov 26 2014

Short term...(this evening through thursday)
issued at 356 PM CST Wednesday Nov 26 2014

Skies will continue to rapidly clear overnight east of the James
River as high pressure builds south. Winds will only slowly decrease
over SW Minnesota as the gradient remains fairly tight through 06z. But
winds will drop off quickly along and west of i29. At the same time
warm advection above the surface will bring clouds into south
central South Dakota during the evening and to the James Valley after
midnight. This will mean no location will have ideal radiational
cooling conditions. In the James and Missouri valleys...the
increasing clouds will limit cooling and could even see temperatures
begin to rise toward dawn in south central South Dakota as southeast winds
increase. East of the James River...winds will stay up longer.
However...these winds are advecting in fairly cold air as it is
already near zero in eastern ND. So expect that advection itself
will get lows near zero. Then as winds diminish after
midnight...expect that lows will fall below zero over southwest
Minnesota and much of southeast South Dakota north of i90 and east of the
James River.

The high will slowly move east on Thursday. Southeast winds will
gradually increase through the day and warmer air will work north
into central South Dakota. Clouds will also spread from west to east through
the day as strong warm advection combines with an approaching weak
upper level wave to enhance lift. While the near surface layer is
not saturated early in the day...there may be enough lift and
saturation from above to produce light snow or flurries over east
central South Dakota into southwest Minnesota during the late afternoon. There
will also be a strong temperature gradient across the area as the
warmer air moving into central South Dakota allows highs to reach the middle
30s...while cold air over southwest Minnesota keeps highs in the
teens. In fact...many areas east of the James River will likely see
highs in the evening as increased cloud cover and strong warm
advection allow for rising temperatures into the evening.

Long term...(thursday night through wednesday)
issued at 356 PM CST Wednesday Nov 26 2014

Weak tropopause undulation will ripple southeast during the
evening and enhance lift potential along the tight baroclinic zone
as it lifts northeast through east central South Dakota and
southwest Minnesota. Areas of light snowfall will increase for
a short time during the evening...then shift east with isentropic
lift band. Behind this...the moist layer seen to become quite
shallow...1000 to 2000 feet of depth as deeper saturation pulls
east...with decent shear at the top of the layer. This would
bring some concern for development of fog/stratus/drizzle mainly
near/east of I 29 later at night. GFS not quite as aggressive with
holding on to the extent of low level moisture. While typically
overdone by the NAM...will deserve some close monitoring over the
next day or so. Strong southerly winds...increasing moisture...
and plenty of clouds should allow for temperatures to have a markedly non
diurnal trend middle and east...with quicker drop west and more
steady later on in the night as warming temperatures aloft and winds
balance clearing skies.

Friday will find a great moderation to temperatures. Fly in the
ointment would be potential for some lingering lower clouds across
the North/East...north of a surface quasi-warm frontal boundary
will exist somewhere across the area. This should aid in
decreasing surface wind speeds through the day...and lowering
pressures across the Western Plains backing winds more toward
southeast with time which could pull these clouds gradually
westward. Inversion continues to increase in strength through the
day...and will limit ability to mix out such clouds.

At this time...the decrease in gradient by Friday night...along
with the pooling of a great deal of snow melt moisture beneath
the inversion would suggest that Friday night is much more ripe
for fog/stratus development. For the time have mentioned some fog
chance by late evening...with areas of fog overnight and into
Saturday morning. Would be a slow dissipation on Saturday with
mainly advection required to remove given formidable inversion.
Have shaded quite a bit warmer on lows to be consistent with
clouds/fog...with much of the night likely in the upper 20s to
lower 30s...if not even warmer.

Consensus on Arctic boundary now quite a bit slower for Saturday...
as driving jet takes about 6 hours longer to move the entrance
region into position across the region. The slowed front leaves
the southern half of the County Warning Area set up to be quite warm...especially
so through the lower Missouri Valley. Expect very little snow
cover to remain by Saturday midday. Pattern is almost an exact
low to middle level match for the local extreme event climatology for
warmest maxes. Given raw model solutions... temperatures and even
the most shallow mixing would have Sioux City record high of 63
degrees not out of reach. With timing of the front changing so
much over the last day or two...have kept temperatures a bit more
conservative from the upper 30s far north...to upper 50s in the
Missouri Valley.

Front blast southward later Saturday through Saturday night. Temperatures
probably will not stop the fall with the end of the normal diurnal
cycle...and steady to slow fall their way through Sunday. Likely
that should be a few flurries for several hours well after the
passage of the boundary later Saturday night into Sunday as temperatures
fall into the ice favoring zone. Arctic ridge settles into the
region on Monday...but without snow cover should be in the process
of modifying slightly. Monday morning will still find lighter
winds and Prospect for more clear skies...and have nudged temperatures
down a couple degrees from initialization grids. Temperatures will start
to rebound heading into Tuesday...which should be a brief stay on
the warmer side of the baroclinicity before next time boundary
waffles southward.

Agreement harder and harder to come by in longest ranges of
the forecast period. GFS is somewhat faster bringing wave into
confluence zone across the area on later Wednesday and Wednesday
night...with European model (ecmwf) looking to accomplish a similar task with
system just a day or so later. With uncertainty in longest
ranges...have not altered the lower chance probability of precipitation across the
southeast third...and mentioned potential for mixed precipitation
with signal of strong high to the north concerning in terms of
locking in low level cold air despite some warming aloft.

&&

Aviation...(for the 06z tafs through 06z Thursday night)
issued at 950 PM CST Wednesday Nov 26 2014

Anticipate MVFR stratus to remain west of the taf sites...through
south central South Dakota overnight. Otherwise...VFR conditions
expected through the taf period with light and variable winds late
tonight picking up slightly out of the southeast on Thursday.

&&

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

$$

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

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