035
FXUS63 KFSD 240925
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
425 AM CDT Tue Jun 24 2025

.KEY MESSAGES...

- Multiple rounds of showers and storms are expected today
  through Thursday which will bring heavy rainfall to the
  region. Amounts of to 2-4 inches is expected with locally
  higher amounts possible.

- Small creeks and streams may experience flash flooding, as
  well as urban areas. Rises in rivers are possible, but
  widespread river flooding is not expected at this time.

- Isolated strong to severe storms are possible Tuesday and
  Wednesday during the evening and overnight hours. The main
  threat aside from heavy downpours is damaging wind gusts to
  60 mph.

- While a brief break in rain is expected for Friday, moderate
  (40 - 60%) chances for rain and potential for strong to
  severe storms return for this upcoming weekend.

&&

.DISCUSSION...
Issued at 420 AM CDT Tue Jun 24 2025

TODAY: Looking aloft we find southwesterly flow and the right
entrance of a jet streak. This provides broad synoptic lift that
combined with weak WAA will continue to produce scattered light
thunderstorms across northeastern Nebraska, southeastern South
Dakota, and northwestern Iowa. These storms will continue through
the morning before tapering off through the afternoon and evening as
the WAA and upper level support wane. As far as the rest of today,
mostly cloudy to cloudy skies will limit our highs just a bit, in
the mid to upper 70s.

Looking at the larger picture we see an upper trough and surface low
over the Hudson Bay, with a surface frontal boundary draped through
the Great Lakes, Iowa, and connecting to a surface low in eastern
Utah / western Colorado. Farther west, an upper trough is digging
over the western Rockies that will slowly progress east over the
next 24 hours. As it does so the low pressure system in
Utah/Colorado lifts northeast, pushing the surface boundary to the
north. The mostly west to east orientation of the boundary is known
as a Maddox Frontal Type and is associated with heavy rain events.
Through the day, southerly winds will transport substantial moisture
northward that will pool on the south side of the warm front. Warm
air advection in the 850 mb layer increases greatly after 04-06Z in
concert with the increasing LLJ and the passage of a 500 mb
shortwave, forcing the warm, moist air mass to overrun the surface
boundary. As a result, showers and thunderstorms are expected across
much of northeastern Nebraska, southeastern South Dakota,
northwestern Iowa, and southwestern Minnesota Tuesday night,
continuing into Wednesday. These storms are expected to produce
heavy rainfall through the overnight hours. There is also some risk
for strong to severe thunderstorms with this activity. However, the
better shear will be north of the boundary, and the better
instability to the south. Any stronger storms should remain along
the boundary where surface convergence is highest. Since shear in
this area will be marginal, (<30 kts) and the lapse rates will be
increasing, the most likely threat with stronger storms aside
from heavy rainfall is damaging wind gusts to 60 mph.

WEDNESDAY-THURSDAY: Continued WAA on southerly winds will warm 850
mb temperatures into the upper teens to low 20 degree C range. At
the surface this translates to highs in the mid 70s to 80s north of
I-90, and upper 80s to low 90s south. Meanwhile, the upper trough
over the Rockies and associated surface low eject out onto the
central Plains and begins to lift northeastward Wednesday morning
and into the afternoon. Ahead of the main system a series of short
waves and vorticity impulses coupled with strong WAA keep light
scattered showers going through the day Wednesday. Wednesday evening
and overnight into Thursday will have a similar set up to Tuesday
night, with moisture once again pooling south of the boundary.
Though the main upper wave is weakening, it still provides enough
synoptic support with the WAA and LLJ to force the moist airmass
over the boundary. Resulting in another round of potentially heavy
rainfall over the same areas as Tuesday. The main difference this
round is the best instability and shear line up a little bit better
over northwestern Iowa. And so some stronger elevated thunderstorms
are possible here. Though the main threat remains heavy rain,
marginally severe hail and damaging wind gusts to 60 mph will
also be possible in this area. Showers will begin to taper off
by late Thursday morning and move east of the area by Thursday
night. Despite all the rain showers, highs will still climb into
the upper 70s to mid 80s thanks to the persistent WAA.

RAINFALL TOTALS: The main risk with these storms is heavy rainfall.
Many factors were considered to reach this conclusion, including the
orientation of the boundary, moisture flux, upper flow orientation,
and LLJ orientation. In addition tall, skinny CAPE profiles are also
often indicators of potentials heavy rain. Ensembles indicate
an 80- 100% probability of receiving 2 or more inches of rain.
Though, they differ in the exact location of the higher amounts.
Finally, precipitable water values greater than the 99th
percentile for climatology combine with the other factors to
give high confidence in a multiple day, heavy rainfall event.
Total rainfall by Friday morning looks to range from around a
half an inch to an inch in the Huron area, increasing as you
move southeast to between 2-4 inches in northwestern Iowa and
southwestern Minnesota. Isolated pockets of higher amounts are
possible. With the recent rains in this area, there are some
concerns for possible localized ponding or minor flooding. The
good news is that river levels are currently at or below normal.
This means they are capable of absorbing the majority of the
forecasted rainfall. While some may get close to or slightly
over flow their banks, widespread river flooding is not expected
at this time. However, smaller creeks and streams could rise
above their banks resulting in flash flooding. Urban areas will
be especially prone to flash flooding during heavier downpours
due to their impervious surfaces. We will be monitoring this
closely over the next few days.

FRIDAY-NEXT WEEK: Friday we get a brief break from the rain. Clouds
will clear off and we`ll warm into the 80s and 90s. The upper level
flow becomes more zonal, but a series of embedded short waves will
bring chances for rain again this weekend. Despite more rain
chances, highs for Saturday will climb into the 90s. The active
pattern looks to continue with periodic chances for rain through the
middle of next week. Highs will generally be trending cooler from
Sunday onward.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1051 PM CDT Mon Jun 23 2025

MVFR ceilings are lifting from southern Nebraska and southern Iowa
into our area, and will bring impacts to airports along and south of
a KYKN to KSPW line through the rest of the night into Tuesday
morning. There will also be scattered showers and thundershowers in
these areas through Tuesday morning. These showers will try to make
a push towards I-90, and thus added VCSH to KFSD to account for
this. MVFR CIGS will return to the area from roughly south to north
Tuesday evening as additional showers and storms move into the area.

Winds will be mainly light tonight and into Tuesday morning. Winds
will gust up to 15-20 kts in the afternoon, strongest in and around
K9V9. Winds will gradually turn from the northeast to the
east/southeast gradually throughout the period.

&&

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

&&

$$

DISCUSSION...AJP
AVIATION...Samet