103
FXUS63 KFSD 171145
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
645 AM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Unsettled weather conditions continue into Wednesday. Isolated
  storms may produce marginal hail/wind gusts along with heavy
  rainfall this afternoon/early evening, but overall severe risks
  are low.

- Nighttime thunderstorms Thursday night may bring an isolated
  severe hail/marginal wind threat, but details on timing and
  location are uncertain.

- Potential heat wave develops late this week through the
  upcoming weekend. Heat risk categories rise into the "Major"
  category Friday into this weekend with triple digit heat
  indices at times. All outdoor events and activities should
  begin preparing now.

&&

.DISCUSSION...
Issued at 400 AM CDT Tue Jun 17 2025

TODAY-WEDNESDAY: Broad mid-upper level trough slides across the
northern Plains, providing less warm but still unsettled weather
for the region this midweek. Surface front remains well to our
south, but some models do hint at a weak surface boundary lifting
into our far southern counties by this afternoon. Main forcing
would come from a lead wave moving northeast along the 850-800mb
boundary, which may provide enough support for a few stronger
storms. Pre-storm environment is unstable with 1000-1500 J/kg
MLCAPE, but profiles are rather tall/skinny with mid-level lapse
rates around or less than 7C/km. Shear is also on the weaker side,
so would expect heavy rain to be the main concern, with perhaps
some pulsy marginal hail/wind from the stronger cores. Uncertainty
resides with location/coverage of storms though broad consensus
focuses the greater chances south of I-90. This threat looks to
diminish by sunset as the wave slides off to the east.

As the trough shifts east, a couple of trailing waves sliding in
from the west/northwest will keep some low precipitation chances
(20-30%) across the area tonight into Wednesday, but severe
weather is not expected. Cooler temperatures associated with the
trough, along with the anticipated clouds/rain chances will keep
high temperatures in the mid 70s to around 80F.

THURSDAY-THURSDAY NIGHT: As the trough moves off to the east, a
mid-upper level ridge begins to build into the region. Will have
to watch a possible weak wave sliding in from the west Thursday
afternoon which could trigger isolated showers/storms. Confidence
is low that moisture will be sufficient, though, so will stick
with a dry forecast for the daytime hours. Thursday night may be
a period of greater focus as a low level jet increases beneath the
building ridge. Strong warm advection on the nose of the 40-50kt
jet may aid in producing nocturnal elevated storms. Though some
uncertainty regarding location/extent of this activity remains,
increasing mid level lapse rates and moderate effective shear of
30-40kt would support an elevated hail threat. Soundings also
indicate a deep sub-cloud dry layer, so cannot rule out isolated
strong gusts if they are able to punch through a stable boundary
layer below the strengthening inversion.

FRIDAY-SUNDAY: Seasonably strong upper ridge will dominate across
our forecast area Friday into Sunday, allowing low to mid level
temperatures to climb above the 90th-95th percentile of ensemble
guidance for this time of year. Record breaking heat may be tough
to come by, as existing records for this week of June are largely
in the triple digits thanks to a very warm stretch of weather in
1988. The exception would be Sunday when KFSD/KSUX may flirt with
their current record highs of 97F/99F, respectively.

However, ample low level moisture in the form of dew points in the
60s to lower 70s will support afternoon heat indices topping 100F
for multiple days, with little nighttime recovery as the humidity
and breezy south winds hold lows in the 70s. This pushes the NWS
Heat Risk into the Major Category for various areas Friday through
Sunday, most expansive on Saturday across the forecast area. This
level of heat would impact anyone with effective cooling and/or
adequate hydration, so anyone with outdoor plans or limited
cooling resources should begin preparing for potential extreme
heat.

A cold front pushes southeast across the area later Sunday into
Sunday night, which will bring chances for rain/storms back to the
region, but also break the heat with highs settling back into the
70s and 80s early next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 645 AM CDT Tue Jun 17 2025

VFR conditions will prevail much of this TAF period with mainly
afternoon to early evening thunderstorms chances. While difficult
to pinpoint exact timing at any given location, tried to focus
mention of TS during the most probable window for KFSD and KSUX,
specifically 19-20Z through 18/00Z. Cannot rule out thunderstorm
activity outside of these windows, but confidence is too low to
include at this time.

Could see areas of MVFR ceilings develop toward the end of the
period, most likely south of I-90 and potentially impacting KSUX.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...JH