Scientific Forecaster Discussion
fxus63 kfsd 231732
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1232 PM CDT sun Jul 23 2017
Short term...(today and tonight)
issued at 351 am CDT sun Jul 23 2017
Fairly strong upper-level wave pushing into the Great Lakes early
this morning, with jet digging southward through the northern
plains. Synoptic frontal zone marking stronger surge of cooler and
drier air has pushed southward into far northern Iowa and southeast
South Dakota. Ahead of this boundary, moisture lingering near the
surface and light winds have allowed for some areas of fog, some
briefly dense, mainly south of Highway 3 in northwest Iowa. As
front drops southward, this fog is expected to be advected out of
the area by shortly after sunrise.
Other than the fog, weather concerns will be relatively minor over
the next 24 hours as cooler high pressure settles through the
eastern half of the region through tonight. Likely will be some
stratocumulus development by early to mid morning which will advect
through areas east of I-29 through the mid afternoon, perhaps only
briefly more than a partly cloudy sky. Northerly winds also will mix
to a refreshing 10 to 20 mph, as dewpoint readings are as much as 10
to 15 degrees lower than those from Saturday, and temps are near to
a bit below normal for the date.
Tonight, as ridge of high pressure embraces the eastern portion of
the plains, may see some additional fog develop mainly from I-29
eastward late night into early Monday morning. Any fog would likely
be more focused to lower and wind-protected locations. Coolest lows
in nearly a week likely for most, from mid 50s to lower 60s.
Long term...(monday through saturday)
issued at 351 am CDT sun Jul 23 2017
Monday may be a touch breezy, south 10 to 20 mph with gusts around
25 mph in central SD, as high pressure shifts east. Weak upper level
ridging will shift east as well with an upper level jet Max expected
to spread through Montana into ND by Monday night. While the GFS is
producing convection in central South Dakota Monday afternoon/evening none of
the other models are. A look around at the soundings suggests that
the drier solution should pan out so will keep the chance for
thunderstorms less than 15 percent Monday afternoon and evening.
Monday has the potential to be pretty warm in central South Dakota and will
aim for highs in the upper 90s. Farther east across southwest Minnesota and
northwest Iowa surface high pressure will still influence the air mass
so much more pleasant highs of 80 to 85 are expected. At this time
will plan on having all thunderstorm activity north of the area
through Monday night.
Tuesday afternoon and night still appears to be the best chance for
a more widespread chance of thunderstorms. Strong upper level winds
of 95 to 100 knots will move through ND into northern Minnesota during this
time. At the surface a boundary with a good warming signal ahead of
it will be found from about south central South Dakota into northeast South Dakota by
afternoon. Thunderstorms will more than likely develop along this
boundary and shift east and southeast through the night. Surface
based cape values around 2500 j/kg are likely by late afternoon with
solid 40 knots of deep layer shear. For now thinking a decent threat
for severe thunderstorms and will add a mention to the severe weather potential statement.
The threat for thunderstorms should linger into Wednesday afternoon
and evening and depending upon how far south the boundary moves a few
severe storms would be possible.
Thursday into Saturday for now looking dry and seasonal with highs
in the 80s and lows 55 to 65. Some hints of stronger warming on
Saturday but nothing to jump on right now.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1229 PM CDT sun Jul 23 2017
Cumulus clouds will be abundant in southwest Minnesota through mid
afternoon. However any upper end MVFR ceilings will mix out to a
higher level, producing VFR conditions by mid afternoon. The other
item to watch for is fog development late tonight and very early
Monday. As high pressure moves off to the east, fog could be found
at kfsd, but more prominently in northwest Iowa affecting the
ksux taf site. At this time, certainly would not rule out IFR fog
for ksux for a brief time shortly after sunrise.