Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180355
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1055 PM CDT Sat Jun 17 2017

.SHORT TERM...(This evening through Sunday)
Issued at 308 PM CDT Sat Jun 17 2017

Primary forecast concern through Sunday afternoon will be the
scattered showers and thunderstorms both days.

Models all agree on a strong wave continuing to push eastward across
SD and into MN this evening. At the same time, relatively cool air
aloft has resulted in a marginally unstable boundary layer with
MLCAPEs between 500-1000 J/kg. With little to no convective
inhibition, expect showers and thunderstorms to continue to develop
through the late afternoon and early evening. There is some shear in
the lowest 6 km which may help organize the strongest storms and
produce some small hail with the best chance north of I-90 where
storms will be more numerous. In addition, a dry adiabatic lapse
rates exists in the lowest 1-2 km. Any rain falling in this layer
will evaporatively cool parcels below cloud base and could produce a
few wind gusts to 50 mph into early this evening.  As the wave moves
east and the boundary layer stabilizes after sunset, showers and
storms will gradually diminish across the area. This will allow for
a brief period of mostly clear to partly cloudy skies before stratus
move southward into southwestern Minnesota and far eastern South
Dakota. The clouds and winds around 10 kts will keep temperature in
the 50s for lows.

On Sunday another weak wave will move southeastward during the
afternoon. The models differ on the exact timing of the wave.
However, there will certainly be some heating west of I-29 where
skies will be partly cloudy and even where there is stratus there
should be a few breaks during the late morning into early afternoon.
Because it will again be rather cool aloft, the limited heating and
weak forcing for ascent should be enough to produce a few convective
showers or storms. Instability is less than today, 300-500 J/kg, as
is the shear so that cell will be less organized and also less
numerous. The best chance for storms will be near I29. Temperatures
will likely warm fairly quickly into the upper 60s to mid 70s by
noon - especially where there is some sun. However, once clouds and
showers develop, most places will see temperatures basically
fluctuate much of the afternoon. This will result in the coldest day
of June with highs from around 70 in southwest Minnesota to the mid
70s in south central South Dakota.

.LONG TERM...(Sunday night through Saturday)
Issued at 308 PM CDT Sat Jun 17 2017

Afternoon showers/thunderstorms should wane pretty quickly with loss
of heating Sunday evening, but another subtle wave dropping into the
area in northwest flow will keep skies partly cloudy throughout the
night. Building upper ridge over the Rockies will nudge the better
moisture and upper support for daytime showers east into Minnesota
for Monday, though could still see isolated activity scrape areas
near/east of the Buffalo Ridge. With the upper trough shifting east,
readings Monday should climb about 5-7 degrees higher than Sunday in
most areas.

Northwest flow continues to dominate into Tuesday, with a backdoor
cool front and subtle wave dropping through the are Monday night.
Limited moisture will result in a dry frontal passage, followed by
mostly sunny skies on Tuesday. Temperatures similar to Monday, with
highs in the mid 70s to mid 80s.

A northern stream wave still on track to cross the Canadian Rockies
midweek, flattening the ridge and bringing a transition to quasi-
zonal flow across the northern CONUS through the latter half of next
week. Models which were stronger with digging this wave into the
Upper Midwest yesterday have since weakened, leading to a greater
consensus for building warmth and instability into the area on
Wednesday. Although still some timing differences, generally seeing
a cold front push across the region late Wednesday into Wednesday
night. With upper jet just north of the forecast area, and moderate
flow in the mid levels, looks to be decent deep layer shear as the
instability increases ahead of the front Wednesday afternoon. May
ultimately depend on frontal timing, but could be looking at the
potential for strong to severe storms late Wednesday.

Differences abound again late next week, with large variances in
temperature/instability depending on strength/timing of northern
stream waves over the Canadian Prairies. Biggest question mark is
currently for Thursday, with GFS/Canadian cooler after pushing the
front east Wednesday night, while ECMWF remains slower and thus is
much warmer for Thursday. A little better agreement in the current
model runs cooling temperatures again Friday/Saturday, with limited
precipitation chances through the latter part of the week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1052 PM CDT Sat Jun 17 2017

The primary aviation concern is the potential for some upper end
broken MVFR ceilings at KHON and KFSD Sunday morning moving down
from the northwest. It likely will be very close between upper
end MVFR and lower end VFR. At this time, model soundings suggest
just inside the VFR category. In addition, some skittish TSRA may
be around Sunday afternoon mainly at the KHON and KFSD TAF sites.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...JH
AVIATION...MJ



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