Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202312

Area Forecast Discussion
National Weather Service Sioux Falls SD
612 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 00Z TAFS through 00Z Thursday evening)
Issued at 608 PM CDT Wed Sep 20 2017

VFR conditions are expected for the bulk of the forecast period.
Late tonight, there remains indications that MVFR stratus will
wrap back in toward northwest Iowa and Sioux City toward day, but
how far northwest the stratus moves is uncertain. KFSD remains
marginally on the edge of this layer, so will go with scattered
MVFR clouds for now. Skies will begin lifting in the afternoon
hours. There is also the potential for some brief low level wind
shear at the HON taf site late tonight through daybreak Thursday.




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