Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 220348
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1048 PM CDT SUN AUG 21 2016

.SHORT TERM...(This evening through Monday)
Issued at 336 PM CDT Sun Aug 21 2016

Surface ridge axis situated east of Interstate 90 this afternoon
will push off into the Mississippi Valley tonight. With this, will
see light west/southwesterly winds back to the south/southeast
overnight on the backside of the high. Resulting warm air advection
and slightly increasing winds in a tightening gradient will bring
about a much milder night, with lows in the upper 50s to mid 60s.
Will most likely have temperatures steadying out or rising slightly
later in the night as winds pick up.

On Monday, an upper level ridge begins to build into the Northern
Plains with a large surface ridge holding over the eastern CONUS. At
the same time, a surface trough deepens over the western High
Plains, strengthening the southerly flow across our area. With the
tightening gradient, it will become windy in the afternoon, and with
850 mb temperatures climbing into the range of 20C to 25C it will be
much warmer. Highs will range from the mid 80s east of Interstate 90
to mid 90s through south central SD. While there will be an enhanced
fire danger in our south central SD and lower MO River Valley zones
with the very warm temperatures and strong winds on Monday
afternoon, with recent rains and afternoon RH values staying above
30 percent the fire danger should remain in check.

.LONG TERM...(Monday night through Sunday)
Issued at 336 PM CDT Sun Aug 21 2016

A large area of upper low pressure moves closer to the plains Monday
night, but is still sprawled across the intermountain west in two
phases. The stronger of the two upper troughs is situated over
Montana, while the weaker southern stream extends over the four
corners area. Ahead of these systems, surface low pressure over the
central Dakotas is helping to maintain a very tight pressure
gradient in our area with strong high pressure over the eastern
seaboard. Off the surface, the low level jet increases Monday night
as the inversion collapses, with 30 to 40 knot winds at 925mb, and
the strongest speeds over southwest MN. Needless to say it will
remain quite breezy Tuesday night over the entire CWA, with wind
speeds possibly pressing wind advisory criteria in a narrow stripe
on the Buffalo Ridge in southwest MN. With the winds and decent
moisture return, going with the warmest guidance readings such as
raw model values are a good idea which give mid 60s to around 70.

On Tuesday as the upper trough gets closer, Tuesday will be another
windy and warm day, with 850mb temperatures strongly suggesting
highs ranging from the mid 80s in our far eastern zones, to the
lower 90s in central SD. Warm air advection and a strong low level
jet will likely begin to spark scattered storms in the mid and late
afternoon hours, especially along and south of I 90. This trend will
continue into Tuesday evening and overnight with high pops still
warranted east of the James River valley ahead of the advancing cold
front. Wind profiles suggest pockets of severe storms, noting the
very strong low level jet east of the James and a strongly veering
warm air advection profile. But thermal wise, there is a strong
inversion between 850mb and 700mb which lead up to fairly weak wind
speeds in the mid levels. Therefore the effective wind shear may not
be all that strong. So at this time, expecting isolated severe
storms, and they could produce some pretty strong rotating updrafts
if they tap into the strong boundary layer winds late Tuesday
afternoon and evening. But as said, many storms may not be able to
do that if the inversion aloft pans out.

Concerning lows Tuesday night, they are a bit challenging. The GFS
is a quick outlier with the cold frontal passage Tuesday night.
Preferred the slower solutions given by the other models, with
temperatures probably dipping down into the 60 to 65 degree range.
Any showers or storms leftover on Wednesday will be early in the
morning in our far eastern zones, with cooler highs prevailing when
compared to Tuesday and Monday. Highs will likely be a couple of
degrees either side of 80 on Wednesday.

Temperatures will be quite moderate Thursday through Saturday with
some more warming by next Sunday. There is a lot of model
discrepancy with regards to the next system Friday Night and early
Saturday, therefore superblend is the way to go with that
disturbance.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1045 PM CDT Sun Aug 21 2016

VFR conditions expected through Monday. The primary aviation
concerns will be with low level wind shear through 8am as well as
very gusty winds in the mid-day through early evening hours.

Latest guidance suggests potential to mix towards 850 mb, and the
ability to tap into and mix down increased channel of air aloft.
Soundings suggest wind gusts up to 27-30 knots may be possible.

Upper level clouds currently over western Kansas will move into
the region in the evening.

 &&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...JM
LONG TERM...MJ
AVIATION...Dux



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