285 
FXUS63 KFSD 181657
AFDFSD

AREA FORECAST DISCUSSION
National Weather Service Sioux Falls SD
1057 AM CST Wed Dec 18 2019

.SHORT TERM...(Today and Tonight)
Issued at 347 AM CST Wed Dec 18 2019

Coolest air has scraped far eastern parts of the area early this 
morning, but those areas beginning to warm as band of warm advection 
mid-level clouds slides eastward into western MN/far northwest IA. 
Question for today is how much of the warming aloft will mix to the 
surface, as light winds turn southerly through the afternoon. Large 
spreads between the majority/blend of raw models vs MOS guidance for 
most of the area, especially through western and southern parts of 
the forecast area with lesser discrepancy across our northeast. In 
looking at soundings, raw models having difficulty letting go of 
cold surface air beneath strengthening inversion, mixing to less 
than 0.5km despite low level southwest-west flow which typically 
favors good mixing. Over fresh snow cover this may make sense, but 
given aging snowpack and relatively mild temperatures/sunshine for a 
few hours yesterday, albedo effects should be lessening. With these 
factors in mind, and lack of early morning fog to slow warming, have 
favored a nudge toward warmer MOS guidance for today. This brings 
readings into the lower 40s across south central SD, while southwest 
MN holds in the lower-mid 20s.

Winds slacken as weak surface ridge moves across the area tonight, 
and some concern for fog development. Cannot discount the potential 
as dew points today likely climb into the upper teens-lower 20s, and 
temperatures tonight fall through these values into the mid or even 
lower teens. Threat is largely confined to high-res NAM-based models 
though, with other guidance indicating much drier boundary layer. 
Given uncertainty, opted to not include in the forecast at this 
time, but will reassess through the day as we see how dew points 
respond to the expected warming, and as additional guidance comes 
in.

.LONG TERM...(Thursday through Tuesday)
Issued at 347 AM CST Wed Dec 18 2019

Dry pattern continues through the weekend and into the first part of 
Christmas week, with temperatures remaining the primary focus. As 
discussed for today, we continue to see a large spread between the 
raw models and MOS guidance for temperatures, particularly with 
respect to daytime highs from Friday through Sunday, with a spread 
of 5-10 degrees common between the two. Low level winds are still 
favorable for at least modest mixing, an expectation which is not 
reflected in model soundings, so have bumped temperatures up from 
broad blended guidance during this period. However, will remain shy 
of warmest guidance, as nighttime surface winds remain on the light 
side, so have to consider potential for fog development to impact 
temperatures the following day. Sunday expected to be the warmest 
day, with areas toward the Missouri River Valley likely pushing the 
50 degree mark.

Upper wave sliding through southern Canada will drag a cool front 
through the northern Plains early next week. This should knock the 
warmest air south of our region, but still looking at temperatures 
on the warm side of normal to continue as we approach Christmas Day. 
Monday-Tuesday look to remain dry at this time, but long-wave trough 
begins to work east across the Rockies, and models show very little 
agreement in handling of various waves ejecting out of this trough 
into the Plains mid-late week. Confidence in any one solution is 
quite low, but with holiday travel, trends certainly bear watching.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1055 AM CST Wed Dec 18 2019

VFR conditions expected thru the period with a bit of enhancement
in the winds expected through the afternoon hours today before
becoming very light overnight. Still some indication in model
guidance that there could be some fog or low clouds overnight,
likely favoring low lying areas, but various forecast soundings
are less than convincing so did not include any restrictions with
this TAF package. 

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
&&

$$

SHORT TERM...JH
LONG TERM...JH
AVIATION...Kalin