499
FXUS63 KFSD 171954
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
254 PM CDT Sun Aug 17 2025

.KEY MESSAGES...

- Isolated to scattered showers and storms continue through mid this
afternoon. Severe weather is not expected with this activity, but
may see locally heavy rainfall and small hail.

- Next chances for organized convection come by this evening  into
tonight with isolated strong to severe storms with damaging wind
gusts to 65 mph, large hail to quarter size, and heavy rainfall.

- Area of greatest concern for heavy rainfall is across southwestern
Minnesota - a Flood Watch has been issued for portions of the area
this evening into Monday morning.

- Much quieter conditions likely return for much of the week
bringing in lower dew points, near-normal temperatures, and limited
precipitation chances.


&&

.DISCUSSION...
Issued at 249 PM CDT Sun Aug 17 2025

NOW THROUGH TONIGHT: Showers and thunderstorms have been more
persistent today than expected, with CAMs handling the activity
poorly. Complexes of storms have continued to feed into the area,
with one now across US Hwy 14 (closer to the stationary/quasi warm
front) and another moving north out of NE. Expect this to continue
to move through the area this afternoon and into early this evening.
Severe weather risk is low through mid evening as the area from NE
moves into a worked over environment, although could see some heavy
rain and small hail with storms.

Uncertainty for this evening/tonight given today`s continued
convection as the low pressure over west central SD moves east, with
the warm front lifting north of US Hwy 14. Low pressure should move
north as well. Clouds have limited warming over our forecast area,
although areas to the southwest have warmed into the 90s in northern
NE. Think that we`ll be waiting for development ahead of the low
pressure near/west of the James River near a boundary sometime later
this evening after 7 PM supported by an increasing LLJ and ahead of
the next wave moving out of NE. Storms move east through the
overnight. Isolated to scattered severe storms are possible, with
the greatest risk 7 PM to 4 AM. Damaging wind gusts to 65 mph will
be the main threat with ample instability and DCAPE. Shear is not
incredibly supportive of large hail (less than 30 knots), but any
more discrete cell could produce quarter sized hail. With numerous
boundaries in place, tornado threat remains low but not zero.

Concern for tonight will be any convection that develops over
southwestern MN, where some areas picked up 4-6+ inches of rain last
night and early this morning. Thunderstorms areawide could produce
locally heavy rainfall, but any areas which saw heavy rain overnight
into early this morning will be at higher risk for flash flooding.
PWATs remain high - generally between 1.5-2+ inches through the
overnight hours. Given the very low FFG (less than 0.5 inch) for
portions of Lincoln, Lyon, Murray, and Cottonwood counties, have
issued a Flash Flood Watch 7 PM tonight to 7 AM Monday.

Storms may continue through daybreak Monday as the front tracks
through the area.

MONDAY: Messy forecast for Monday as well, as we`ll wait and see
where boundaries end up from today/tonight`s show. Think that the
main warm front will end up near/south of US Hwy 20 (possibly
meandering back toward I-90), which should limit daytime heating and
convective potential somewhat during the afternoon especially with
the better forcing from 700:500mb wave across NE and far southern
SD. Further west and later in the day, may see storms fire across
western SD with another boundary, LLJ, and mid level wave during the
late evening and early overnight hours. Wave may dive to the
southeast into NE fairly quickly. Again, will have to wait and see
how the next 18-24 hours play out, but can`t rule out a stronger
storm with instability around 1500 J/kg and shear around 30-40
knots. Mid level lapse rates around 7 deg C/km would also support
large hail. Locally heavy rainfall again is possible again with
PWATs around 1.5 inches, although this threat is also highly
dependent on where the warm front sets up.

TUESDAY ONWARD: High pressure begins to build aloft across the
Rockies, with mid week surface high pressure overhead. By Friday,
closed low pressure swings through southern Saskatchewan and
Manitoba into the Upper Midwest by next weekend. Temperatures are
expected to return near seasonal averages, although may see some
warming mid week with the ridge axis sliding overhead. Most of the
period should be dry, although expect at least some lower end rain
chances later in the week into the weekend with the trough sliding
through. Cooler high pressure returns by early next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1219 PM CDT Sun Aug 17 2025

A mix of conditions across the area this afternoon and through
the period. Patchy MVFR stratus associated with earlier
convection continues through the early afternoon hours.
Otherwise, watching a few areas of convection at the time of
this discussion - one across southwestern MN, another near the
James River Valley, and a third along the SD/NE border near
KYKN. All of this activity is moving east or northeast, and
continue to do so over the next couple of hours. Guidance is
handling this activity very poorly, leading to lower confidence
in storm chances later tonight. Have maintained some PROB30
groups for KFSD and KSUX at the times when convection currently
look the most likely. A few storms could be strong to severe
later today/tonight and produce locally heavy rainfall.

Guidance hints that there may be some redevelopment of MVFR
stratus overnight and into Monday morning; confidence is too low
for prevailing outside of KHON.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...Flood Watch from 7 PM CDT this evening through Monday morning
     for MNZ071-072-080-081.
IA...None.
NE...None.

&&

$$

DISCUSSION...SG
AVIATION...SG