843 
FXUS63 KFSD 160322
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1022 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.

- Scattered showers and thunderstorms return from late tuesday
  afternoon onwards. While ping-pong ball sized hail and
  damaging wind gusts up to 65 mph will be the main hazards with
  any stronger activity, pockets of locally heavy rainfall will
  also be possible.

- Additional chances for showers and thunderstorms continue from
  the midweek into the weekend. While the overall severe threat
  will be low, pockets of locally heavy rainfall will be 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1015 PM CDT Mon Sep 15 2025

VFR conditions will persist this TAF period. Taking a look at
satellite imagery, clear conditions persist across the area this
evening. Expect these condition to continue through the first
half of Tuesday before shower and thunderstorm chances increase
into from the late afternoon onwards. Added in a PROB30 group
into KHON to account for the increasing chances. However, with
varying degrees of evolution among high-resolution guidance; 
decided to hold off on anything else for our other TAF sites. 
Otherwise, light southerly to southwesterly surface winds will 
continue.

&&

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

&&

$$

DISCUSSION...AJP
AVIATION...05