Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202046

Area Forecast Discussion
National Weather Service Sioux Falls SD
346 PM CDT Wed Sep 20 2017

.SHORT TERM...(This evening through Thursday afternoon)
Issued at 346 PM CDT Wed Sep 20 2017

The large scale pattern tonight and Thursday shows a positively
tilted flow, with upper riding extending from Texas northeastward
into the eastern Great Lakes, and an upper trough moving into
California and the Alberta/Saskatchewan border. In between
throughout the northern plains, our CWA weather looks pretty quiet
through Thursday. There are a couple of items worth mentioning
however. Very Late tonight through Thursday morning, both the NAM
and GFS are in excellent agreement in showing stratus moving
northward. This stands to reason in that there are currently plenty
of strato cumulus clouds which did not mix out across all of
southern and eastern Iowa. As high pressure shifts off to the east
and shifts our low level winds around to an easterly and
southeasterly direction, the models develop the stratus near 900mb
and usher that moisture first into Sioux City toward day break
Thursday, then expand it north to northwestward deeper into our
forecast area. By midday and afternoon, the stratus will then mix
out into higher based cumulus leaving a mostly sunny or partly
cloudy afternoon. The low level jet becomes quite strong on Thursday
coupled with a tightening surface pressure gradient. Therefore south
to southeast winds will become quite brisk helping to mix the lower
atmosphere through about 1 km, where winds of 35 knots reside.
Therefore surface wind speeds of 20 to 30 mph will be common, albeit
in our far western zones they will likely subside in the afternoon

Concerning temperatures, with a light easterly flow, dropped lows in
northwest Iowa below consensus guidance values, and conversely
raised them a category generally along and west of the James River
valley where southeast winds will steady readings out the second
half of the night. The going forecast highs on Thursday still look
good given 850mb temperatures, and will be much above normal.

.LONG TERM...(Thursday evening through Wednesday)
Issued at 346 PM CDT Wed Sep 20 2017

Stout southerly flow will keep temperatures quite mild Thursday
night.  GFS is not as aggressive with low level moisture on Thursday
night, and have now trended towards a dry but windy forecast.  With
strong south winds, temperatures will remain in the 70s throughout
the night. Have raised forecast highs on Friday with strong
southerly flow driving 850 hpa temperatures into the 20s.

With blocking pattern across the east, have trended the forecast
towards a more westerly solution with pops/precipitation.  Very
slowly evolving pattern with mid level latitudinal trough remaining
in the intermountain west through the weekend. With slow moving low
level boundary and PWAT values of 1.5-2", will have to keep an eye
on flooding potential especially with training storms Saturday night
through Monday. As for severe weather Friday night, there is good
shear but very limited amounts of instability east of the James
River Valley.

Jet stream briefly dips south of the area early next week keeping
temperatures below normal.  Expect high in the 60s, with lows in the
40s.  Temperatures gradually rebound throughout the week, keeping
frost concerns least for now.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1239 PM CDT Wed Sep 20 2017

VFR will prevail for this afternoon and much of the night. However
late tonight, there are indications that MVFR stratus will wrap
back in toward Sioux City IA toward day break from the southeast,
and eventually work into the KFSD TAF site just before 18Z
Thursday. This scenario is certainly plausible, given that there
is plenty of lower end VFR strato cumulus currently in southeast
Nebraska and southern Iowa ready to come back northward once our
light winds shift directions.




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