Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172021

Area Forecast Discussion
National Weather Service Sioux Falls SD
321 PM CDT Sun Sep 17 2017

.SHORT TERM...(This evening through Monday)
Issued at 306 PM CDT Sun Sep 17 2017

The surface high pressure that brought cool, crisp weather and clear
skies this morning will slowly migrate to the NE this afternoon and
evening as mid-level moisture works its way over the tri-state area.
Current satellite imagery shows scattered cloud cover beginning to
move into the area already.

After midnight, a southwest to northeast oriented jet streak slides
east across the northern Plains, placing the region under the right
jet entrance region. This coincides with a 700 hPa theta-e ridge
advecting in from the southwest. The combination of moisture and
lift will be enough for widespread showers during the second half of
the night into Monday morning. Clouds and rain will keep
temperatures much warmer than last night. Overnight instability will
be limited, so thunderstorm potential is low, but areas south of I-
90 could see embedded thunder with the strongest showers. Around a
half an inch of rain is likely for NW Iowa, with amounts decreasing
as one moves to the northwest. Areas closer to central South Dakota,
such as Huron, will see much lighter amounts, generally closer to a
tenth of an inch or less.

These showers will move east of the area by Monday afternoon,
leaving a slightly warmer and notably more humid air mass in their

.LONG TERM...(Monday night through Sunday)
Issued at 306 PM CDT Sun Sep 17 2017

Mid and long range will see a few areas of weather concerns, as a
large closed upper level over the Northwest directs a series of
waves into the northern Plains.

The first area of interest will be on Tuesday evening into Tuesday
night. Trough digs into the western Dakotas on Tuesday, with a
frontal boundary moving into central SD in the afternoon. Gusty
southerly winds ahead of the front and warming aloft will allow high
temperatures to warm into the upper 70s to mid 80s with dewpoints
increasing into the 60s. Should see some good instability around
1500 to 2000 j/kg develop, however we will be in a capped environment
during the afternoon. With the front moving into the forecast area
in the evening, scattered thunderstorms will be possible if the cap
can weaken enough, mainly along and north of I-90 where forcing will
be stronger and the capping is weaker. With strong shear greater
than 40 kt and steep mid level lapse rates, severe storms will be
possible. It is likely that storms would quickly organize into a
line and track east overnight. As they move into the I-29 corridor
and points eastward into a more stable environment, the severe
weather threat should quickly lessen.

Wednesday and Thursday look to be largely mild and dry out ahead of
the western trough taking a more active digging pattern across much
of western US late week. Models remain at odds regarding the timing of the
shortwaves ejecting from this trough and lifting into northern
Plains, therefore confidence remains low for Friday and beyond. It
does look like we will see wet weather return into the weekend, but
the details are uncertain. Did think that the blended temperatures
were a bit too cool due the influence of the CMC, so nudged highs up
a few degrees Wednesday through Friday.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1240 PM CDT Sun Sep 17 2017

Mid-level cloud cover will increase through the afternoon hours,
with light winds at the surface. Later tonight, roughly after
18/0600z, precipitation will begin spreading into the area ahead
of an upper level shortwave trough. Cloud ceilings will lower
during this timeframe, with some visibility restrictions possible
in NW Iowa with the heavier rain. Widespread lightning is not
expected with showers, but the strongest showers could produce a
few strikes. Rain chances quickly decrease by late morning




SHORT TERM...VandenBoogart
AVIATION...VandenBoogart is the U.S. government's official web portal to all federal, state and local government web resources and services.