Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 230347
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1047 PM CDT Fri Sep 22 2017

.SHORT TERM...(This evening through tonight) Issued at 322 PM CDT
Fri Sep 22 2017

The Northern Plains continues to sit under strong southwesterly flow
aloft, between the upper level trough over the Western U.S., and a
large scale ridge over the eastern states. At the surface, this has
allowed warm moist air to continue streaming northward, bringing
gusty winds and near record heat to the area. Those with outdoor
plans this afternoon will want to stay hydrated and take steps to
avoid heat exhaustion in the summer-like heat.

Despite bountiful CAPE this afternoon, a strong capping inversion
has suppressed convection over the area. In fact, even cumulus
clouds have been scarce due to the strength of the 700 to 800 hPa
warm layer. Models show a shortwave trough moving northward this
evening which will likely be able to erode the cap sufficiently and
provide lift to allow thunderstorm formation over southeast South
Dakota. These storms are expected to then move northeastward,
staying roughly north and west of a Yankton, SD to Sioux Falls to
Marshall, MN line. The time period with greatest likelihood for
thunderstorms should be 6-11pm CDT. The threat for severe storms is
only marginal, with lightning and strong outflow winds being the
more widespread threats. Further to the southeast in NW Iowa, the
air mass will likely remain capped, and storms are not expected
here.

.LONG TERM...(Saturday through Friday) Issued at 322 PM CDT Fri
Sep 22 2017

The weekend continues to look very wet, with many locations likely
seeing one to two inches of rain over a 72-hour period as a quasi-
stationary boundary sets up over the region. An air mass with 1.5
inch plus precipitable water values will continue to fuel showers
and scattered thunderstorms along this boundary as the large scale
upper level trough slowly pushes east over the weekend. West of I-
29, the heaviest rain looks to move through Saturday afternoon and
evening. The severe threat is only marginal Saturday, as the
strongest shear is expected to remain west of the surface boundary
during the afternoon, and lapse rates are only marginal, at about
7 C/km. For locations east of I-29, Sunday afternoon and evening
look most favorable for the heaviest rain. Temperatures Saturday
and Sunday will not be as warm as on Friday, as abundant cloud
cover limits heating and cooler air slowly pushes in from the
northwest. The boundary and focus for rain move east into
southwest MN and northwest IA on Monday, with rain chances
decreasing for eastern SD.

By the PM hours Tuesday, northwest surface flow in the wake of the
front is forecast to dry the boundary layer and bring an end to
precipitation. A large scale pattern change with the shifting east
of the upper level longwave trough will bring quieter weather and
closer to seasonal temperatures for the middle to end of next
week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1045 PM CDT Fri Sep 22 2017

Scattered thunderstorms will be possible through the James River
Valley into east central SD during the overnight hours. There will
be a period of low level wind shear at KSUX later tonight as a low
level jet develops across the region. Shower and thunderstorm
chances will continue on Saturday roughly over the same areas,
with better coverage on Saturday afternoon.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...VandenBoogart
LONG TERM...VandenBoogart
AVIATION...JM



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