Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 210856

Area Forecast Discussion
National Weather Service Sioux Falls SD
356 AM CDT Fri Sep 21 2018

.SHORT TERM...(Today and Tonight)
Issued at 355 AM CDT Fri Sep 21 2018

Today, powerful low pressure system at the surface will move
eastward through southeast Canada. This low is the product of the
upper trough which affected our weather yesterday, but has
strengthened further. On its heels is a 1028mb high with a fairly
tight gradient through midday in this area. Therefore the
northwesterly flow will be quite brisk for the first half of the day
with the mixed layer averaging 30 to 35 knots. But the gradient and
winds aloft slacken off this afternoon as the center of the high
moves into this CWA. Stratus is the next concern. In fact very early
this morning there will likely be areas of drizzle or very light
rainfall in our east central SD zones. That light precip will abate
after sunrise but stratus will linger this morning. Humidity time
sections and soundings show the clouds rapidly drying up this
afternoon. Overall, the moisture is based just below 1km near
900mb. Highs today will be much cooler than what we have experienced
lately with readings in the mid 50s to around 60. Any bias corrected
value had to be thrown out as they were way too warm, influenced by
our much above normal temperatures recently.

The same holds true for tonight in ignoring warm bias corrected
values. With the high nudging off to the east, the flow will turn to
a light southerly direction. But temperatures will be quite chilly
in our eastern zones with some readings dipping down into the upper
30s east of I 29. I 29 itself should be close to 40, gradually
warming to the mid 40s in central SD. 925mb temperatures range from
near +7C east of I 29, to +10C near Chamberlain SD.

.LONG TERM...(Saturday through Thursday)
Issued at 355 AM CDT Fri Sep 21 2018

The upper flow turns somewhat zonal for much of the weekend, until
Sunday afternoon when the next upper trough moves into the
intermountain west. Southerly winds will be rather breezy on
Saturday, and downright windy on Sunday, especially along and east
of I 29 where the pressure gradient is tight. These locations should
see 20 to 35 mph winds with relatively lighter speeds est of I 29.
Temperatures will modify through the weekend, and should easily get
into the lower 80s in our south central SD zones on Sunday.

The aforementioned upper trough then moves eastward into the plains
on Monday and Tuesday, with both the ECMWF and GFS showing a
dramatic broadening of the trough from south central Canada,
southward into the southern plains. There may be a few warm air
advection showers late Sunday night and Monday morning primarily in
our SD zones and the northern sections of southwest MN. Then a short
wave will swing rapidly through Monday afternoon ushering a cold
front through Monday afternoon and evening. Using elevated MU cape
off of the GFS and the wind profiles, TSRA is a threat along the
advancing cold front mainly along and east of I 29, with a chance
for showers west of I 29. With the transient nature of the system
and lack of rich deep moisture, heavy rain is not a threat with this
feature. Behind the cold front, highs will be back into the 60s on
Tuesday with the crux of the main part of the upper trough right in
this area.

Further out, the models are pretty consistent in bringing another
upper trough into this area next Thursday, however the GFS is closed
with the upper low and quite a bit stronger when compared to the
ECMWF. They are both in pretty good agreement however in the timing
of the next cold front which is next Wednesday night. The cold
frontal passage may spawn a few showers and storms with
progressively colder weather following the front. Lowered highs
below superblend readings a bit for next Thursday and Friday. With
the GFS stronger than the ECMWF with the upper trough late next
week, it is colder. For instance on Friday the GFS only has -2C to
+2C at 850mb across our forecast area whereas the ECMWF is about +2C
to +6C. That said, the 12Z ECMWF develops a strong 850mb baroclinic
zone from north to south across this area which produces rainfall on
Friday. The 00Z ECMWF holds this off until Friday night, but still
worth some pops for the Friday day period. The GFS very late next
week stay`s mired more in a northwest flow aloft regime with a long
wave trough carved out over much of Canada and the eastern United

This progressive pattern next week will keep the winds stirred up
off and on. Wednesday and Thursday both look windy, first from the
southwest ahead of the cold front on Wednesday, then from the
northwest on Thursday. On both days raised winds above superblend as
the bias correction is keeping wind speeds too low.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1036 PM CDT Thu Sep 20 2018

With the passage of a frontal boundary, northwesterly winds will
gust 20 to 25 kts tonight into Friday. MVFR stratus will wrap in
behind the front, covering much of the area. Skies will begin to
clear by late morning on Friday.




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