217
FXUS63 KFSD 180036
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
736 PM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Showers and thunderstorms will lift through the area starting
  tonight. While severe weather is not expected, locally heavy
  rainfall will be the main threat. Widespread accumulations of
  1-2+ inches are expected along and north of I-90. Locally
  higher amounts are also possible, potentially exceeding 4+
  inches. Minor impacts to rivers and streams, ponding in poor
  drainage areas, and urban street flooding are all possible.

- A low end severe storm threat is possible Friday afternoon and
  evening from about 4 pm to 9 pm, mainly across portions of
  northwest Iowa. Damaging winds and an isolated tornado will be
  the main hazards.

- Near to below average temperatures will persist through this
  weekend and most of next week. Additional rain chances are
  possible but details are uncertain.

&&

.DISCUSSION...
Issued at 303 PM CDT Thu Sep 17 2026

Low level stratus persists across parts of the area this afternoon.
Locations that have broken out of the stratus have warmed to the 70s
while those who remained under it are only seeing temperatures up to
the 60s. The broken up stratus will persist through the rest of the
rest of the afternoon hours. The wave that will be responsible for
tonight`s heavy rain threat is currently located over Wyoming and
will encounter a very moist environment, characterized by
precipitable water values (PWAT) up to the max of climatology for
this time of year. Quasi-geostrophic (QG) forcing for ascent will
increase late this evening and night as this wave pushes into the
right entrance region of an upper level jet. The low level jet will
ramp up with winds as strong as 40 knots, strengthening warm air
advection (WAA) in the low levels and tightening the thermal
gradient. The tighter thermal gradient will result in a
strengthening elevated front and thus frontogenesis (FGEN). This
front will be in less stable environment characterized by moist
neutral lapse rates. This will result in negative equivalent
potential vorticity and thus the potential for a narrower, more
intense band of rain. At the same time, weak CAPE values near to
less than 500 J/kg will be advecting into the area from the south.
Thus, some weak thunderstorms are possible which could result in
locally heavier rainfall. The bulk of the rain looks to fall near
and north of I-90 with the highest totals expected around highway-
14. A cross section analysis shows deep omega (upward motion) over
the front, primarily over areas in the northwest part of the
forecast area, generally northwest of Chamberlain, South Dakota to
Tracy, Minnesota line. This area looks to see the highest rainfall
tonight due to the greatest forcing for ascent residing in this
area. The rain will be on the move out Friday morning, especially
during the second half of the morning hours. While some lighter rain
showers may persist through the rest of the morning and into the
early afternoon timeframe, the bulk of the rain will be done
falling. In terms of rainfall amounts, a broad 1-2 inches is
expected along and north of I-90. Lower amounts down to 0.25 inches
to 0.75 inches is expected south of the interstate. There remains
the potential for a localized maximum of 4+ inches of rain northwest
of the previously mentioned line as supported by the HREF local
probability matched mean (LPMM), the individual hi-res members of
the HREF, and the RRFS and it`s ensemble counter part the REFS.

While the first half of Friday afternoon will be on the quieter
side, some questions remain regarding a low end severe weather
potential late Friday afternoon into the evening. Surface based
instability looks to potentially exceed 2,000 J/kg with sufficient
vertical shear magnitudes of about 30 knots. An inverted trough may
produce enough low level convergence to potentially develop some
storms. The main limiting factor is that most of the better forcing
for ascent will not east/northeast of the forecast area. Think that
if convection were to develop in this surface trough, it would be
initiated by the vort max itself which seems to be displaced a bit
north of this boundary. As of now, think convection only has a 30-
50% chance of developing due to the limited forcing for ascent. If
convection were to develop, it would develop primarily over
northwest Iowa between 4 pm to 9 pm before racing southeast of the
area. 0-7km theta-e difference of about 15-20K along with looping
low level hodographs suggest a low end damaging wind and an
isolated tornado threat IF storms are able to develop.

An active pattern aloft will keep chances for rain in the forecast
through the weekend and most of next week. Details remain uncertain
due to increasing variance amongst medium range guidance.
Temperatures look to remain near to below average for this period of
time with highs in the 60s and 70s.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 733 PM CDT Thu Sep 17 2026

Outside of convection, MVFR conditions currently then
deteriorating to IFR for the rest of the period. Showers to drop
ceilings further across the CWA over the next few hours. With the
first round of showers expect light precipitation, and isolated
lightning.

Temporary improvement may be seen in portions of northwest IA and
around KSUX between 5z-7z. Following the periods of scattered
showers, thunderstorm potential returns to the entirety of the CWA
around 6z, starting in northwest IA/KSUX area and then extending
northwestward. Isolated storms cause confidence in exact timing and
coverage to be low. Conditions to shift back to just a rain impact
after 11z for a majority of the CWA. Precipitation lingers north of
I-90 for the rest of the period.

Conditions improve to lower VFR in northwest IA early Friday
afternoon. Winds increase with gusts 20-25kts through the overnight
and into the afternoon hours Friday. Wind direction may shift late
in the period to be more northerly behind a front.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...SG/Rafalski