Franklin, South Dakota Weather Forecast Discussion

608
FXUS63 KFSD 151918
AFDFSD
Area Forecast Discussion
National Weather Service Sioux Falls SD
218 PM CDT Mon Sep 15 2025
.KEY MESSAGES...
- Tuesday will again be warm in the mid to upper 80s with some
  90s possible east of I-29.
- Late Tuesday afternoon and into the overnight isolated showers
  and thunderstorms are possible. Uncertainty remains as far as
  timing of storm initiation and severity. If strong to severe
  storms are able to initiate, the greatest risks will be
  damaging wind gusts and heavy rainfall.
- Showers and thunderstorms continue through the end of the week
  and into the weekend. While overall severe threat will be low,
  pockets of isolated excessive rainfall is possible.
&&
.DISCUSSION...
Issued at 216 PM CDT Mon Sep 15 2025
Mostly sunny skies are expected for this afternoon. Breezy southerly
winds will blow 10-15 mph with gusts 20-25 mph. Strong WAA will
increase temperatures in the mid-levels to the 20-22 degree C range.
Mixing this down to the surface will yield highs in the 80s.
Overnight winds will decrease to around 5-10 mph over central South
Dakota. West of the James River winds may become light and variable,
allowing some shallow fog formation. Especially along rivers and low
laying areas. While confidence in fog formation is low, it could be
dense at times, so please exercise caution on your Tuesday morning
commutes. Lows by Tuesday morning will fall to the low  60s. Tuesday
will be another warm and mostly sunny day with continued WAA on
light southerly winds. Highs will climb into the mid to upper 80s
with a few 90s possible east of I-29. Overnight lows will fall to
the low 60s.
Looking aloft Tuesday morning a broad upper trough will be located
over the western United States. This trough will slowly work its way
east and elongate during the day. By Tuesday late afternoon a short
wave will form along the eastern periphery of the upper trough.
At the surface a low pressure system will be located over
central South Dakota, dragging a cold front along with it. At
roughly the same time, the low level jet kicks in, setting the
stage for thunderstorm development over south central South
Dakota. Confidence in storm initiation timing is low at this
time due to wide variance in mid- range model guidance and the
existence of a weak cap. Thermodynamic factors indicate a
marginally unstable environment with instability ranging from
1000-3000 J/kg of CAPE. Mid-level lapse rates are moderate at
7-7.5 deg C/km. Deep layer shear is sparse, less than 20 kts.
DCAPE values of 1000-2000 J/kg indicate the primary threat in
any severe storms would be damaging wind gusts. Secondary to
that is hail the size of nickles to quarters. This aligns well
with the SPC Day 2 Outlook for a Marginal Risk (level 1 of 5)
for severe weather. As mentioned, timing of initiation is in
question, but should stronger storms develop they will most
likely occur between 6 pm to midnight. Surface stability begins
to increase after midnight. Storms at this point should
transition to elevated storms, with damaging wind gusts
remaining the most likely threat.
The system slows significantly and stalls over the region Wednesday
and Thursday. Weak convergence along the surface boundaries will
keep showers in the forecast. Occasionally a stronger vorticity
wave will work through the pattern, bringing chances for
increased thunderstorm and shower activity. Precipitable Water
(PW) values for Wednesday through Friday are at the 90th
percentile for climatology. Sounding profiles show tall, skinny
CAPE Wednesday onward. This with the high PW values indicates
the potential for pockets of heavy rainfall. The overall severe
threat, however, is low. As far as totals are concerned,
guidance is again widely varied. There remains approximately a
40-70% ensemble probability for 0.5 inches or more from midnight
Tuesday to midnight Wednesday. And a roughly 30-70% probability
for 0.5 inches or more from midnight Wednesday to midnight
Thursday. With this in mind, it will be possible for much of the
area to see a quarter of an inch to an inch of rain with
isolated pockets of higher totals by the time the system clears
the area Sunday. Considering the potential for heavy rainfall,
the Weather Prediction Center has placed us in an Excessive
Rainfall Outlook for Wednesday, Thursday, and Friday.
By late Thursday into early Friday the system begins to move slowly
east. Similar to the previous days, stronger impulses within the
system will occasionally result in isolated increased shower
and thunderstorm activity. This pattern will repeat through
early Sunday morning, when the system finally clears to the
northeast. Temperatures Wednesday through the weekend will
gradually decrease from the 80s to the 70s by the weekend, the
result of persistently cloudy skies. By Sunday afternoon an
upper ridge will settle across the northern Plains. Southerly
winds, WAA, and clearing skies will help temperatures begin to
warm up again for the first part of next week.
&&
.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1218 PM CDT Mon Sep 15 2025
VFR conditions expected to prevail this period. Breezy southerly
winds at 10-15 kts with gusts to 20-23 kts this afternoon will
decrease overnight becoming light to light and variable.
Some slight reductions in visibility are possible near sunrise as
shallow, patchy fog may form in river valleys and low laying areas.
KFSD and KSUX are the two sites most likely to see fog, however
confidence is low. Any fog should burn off by 14 to 15 Z Tuesday
morning.
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&
$$
DISCUSSION...AJP
AVIATION...AJP

NWS FSD Office Area Forecast Discussion