Trosky, Minnesota Weather Forecast Discussion

434
FXUS63 KFSD 171948
AFDFSD
Area Forecast Discussion
National Weather Service Sioux Falls SD
248 PM CDT Wed Sep 17 2025
.KEY MESSAGES...
- Periodic showers and storms are expected this afternoon and evening.
  Locally heavy rainfall, lightning, and small hail are the
  main threats. An isolated funnel or two can`t be ruled out.
- Rainfall amounts through the rest of today may locally reach
  up to half an inch to an inch depending on where showers and
  storms track.
- Continued rain chances and cloudy skies will keep temperatures
  mainly cooler than normal through the start of the weekend,
  though another warm-up is possible heading into next week.
&&
.DISCUSSION...
Issued at 247 PM CDT Wed Sep 17 2025
Scattered showers have developed across the area this afternoon in
response to an approaching shortwave trough. So far, no lightning is
present in the showers. This is likely due to very little if not
just about no instability being in place thanks to the abundant
cloud cover in place ahead of the wave. That said, could still see
some slightly steeper mid level lapse rates get advected into the
area and generate a little more instability to work with. Weak
vertical shear is in place on the order of 20 to maybe 30 knots of
effective shear. With such weak instability and shear, severe storms
are not expected. However, lightning, brief heavy rain, and small
hail are possible in any weak storms that do develop. Like
yesterday, rainfall amounts could locally reach up to a half an inch
to an inch or more in the heaviest storms. Especially if the showers
and storms track over the same area. Can`t rule out the potential
for an isolated funnel or two as well though this looks to be a low
probability given the abundant cloud cover. Chances for rain will
persist for the rest of the evening and overnight hours, though on a
slowly waning trend. Low temperatures will only fall to the upper
50s and 60s tonight thanks to the cloud cover in place.
The nearly vertically stacked low pressure system will drift
overhead tomorrow. With the low levels remaining saturated, the
cooling from the wave itself will steepen mid level lapse rates just
enough to generate minor instability on the order of 500 J/kg or
less. Still, this is enough to generate another round of afternoon
showers and storms though the coverage should be less since the best
forcing for ascent will not reside over the forecast area. With such
weak instability in place along with effective shear values on the
order of 30 knots or less, severe weather is not expected. That
said, these showers and storms will again be capable of lightning,
brief heavy rain, and small hail. Rainfall amounts look to range
between a tenth or two to up to about in inch. Do want to emphasize
again that these rainfall amounts will be localized and be dependent
on where showers and storms develop and track. The diurnally driven
showers and storms will come to an end during the evening hours.
Aside from rain chances, highs will again be near to below average
in the upper 60s towards the Missouri River Valley up to the upper
70s east of I-29. Lows will fall to the 50s overnight.
No break in chances for rain on Friday as the low pressure system
continues to wobble across the Northern Plains. Highs will remain in
the 70s across the area. Rain coverage looks to continue to decrease
as the best moisture with the system will wrap up to the north and
east of the forecast area. That said, there could be enough moisture
for showers and storms to develop east of I-29. The ensembles support
this as they show a 40-80% chance for rainfall totals to exceed a
tenth of an inch in this area. Chances for rain will again wane
during the evening and overnight hours, allowing for low
temperatures to fall to the 50s.
The upper low finally will pivot off to to the north on Saturday,
ending meaningful rain chances across the area for the weekend. Some
light rain showers are possible but details are a bit too uncertain
this far out. Otherwise highs will begin to moderate back to the mid
to upper 70s and up to 70s and low 80s on Sunday.
The next chance for rain may come on Monday as another upper level
wave pushes into the Northern Plains. However, the strength and
overall evolution of this wave is uncertain at this time due to the
variance in its structure in medium range guidance. The ensembles on
the other hand are in better agreement as they show the wave diving
southeastwards through the Northern Plains but the upper level jet
on the backside of the wave lifts into Canada on the north side of a
building ridge. Quite an interesting progression as typically these
upper level waves amplify when a jet is digging into the backside of
the wave. The ensembles also show very low probabilities (

NWS FSD Office Area Forecast Discussion