397
FXUS63 KFSD 082251
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
551 PM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Isolated thunderstorms possible along and west of the James
  River valley into Wednesday morning. While not likely to be
  severe, some small hail could be possible.

- Mesoscale convective system likely to develop Wednesday night
  over central and north central South Dakota, but a high deal
  of uncertainty on where this cluster of storms keeps forecast
  confidence into Thursday lower.

- Greatest storm chances into the weekend focus around Thursday
  into Friday. Mixed storm modes could bring a variety of
  severe weather risks, but slow storm motion may also bring
  locally heavy rain risks.

- Trends favor drier and slightly cooler weather into the
  weekend, but rain risks may return as early as Sunday into
  Monday.

&&

.DISCUSSION...
Issued at 216 PM CDT Tue Jul 8 2025

THIS AFTERNOON: Weak cold front slipping southward across the CWA
this afternoon. Soundings suggest the atmosphere is relatively
uncapped by mid-afternoon especially over southwest MN and
northern Iowa, and with a passing mid-lvl impulse in central MN,
an isolated sprinkle/shower or weak thunderstorm could develop
into early evening.

TONIGHT: Fairly consistent signals for isolated to scattered
elevated convection forming on the nose of the 30 knot LLJ into
south central South Dakota and areas west of the James river after
midnight tonight. The veering LLJ could push this risk into the
Sioux City area Wednesday morning.  Point soundings would suggest
upwards of 1200-1500 J/KG MUCAPE, but lower effective shear and
modest mid-lvl lapse rates should preclude severe storms.

WEDNESDAY:  Lingering convection may continue into mid-late morning,
but should weaken as the LLJ dissipate and backs towards western
South Dakota. Temperatures then begin to warm into the 80s.
Again, soundings show a nearly uncapped environment in the
afternoon and evening hours, but the absence of forcing
mechanism should preclude convective development. If a piece of
mid-lvl vorticity does move southeast earlier, then we`ll have
to monitor for isolated to scattered convection in western MN
and far eastern South Dakota in the afternoon/evening.

WEDNESDAY NIGHT:  A more consistent signals for convective
development will be stronger over western South Dakota Wednesday
evening as a mid-lvl shortwave enters the Plains.  Discrete activity
should begin to merge into one or more clusters as it begins to
track east and southeast into Thursday morning.  Uncertainty
continues on the exact track of these MCS, which theoretically
should follow the instability gradient towards the CWA after
midnight. Weakening mid-lvl shear and the overall stabilization of
the boundary layer should weaken this convection as it moves into
the CWA, but trends will need to be monitored.

THURSDAY: Another mid-lvl shortwave enters the Central Plains on
Thursday.  Depending on the track of overnight MCS activity, one or
more outflow boundaries may develop and stall near the CWA during
the day. It`s also completely possible that a more eastward tracking
and slower MCS in the morning could stabilize the area though
the afternoon. Nevertheless, these features, along with a poorly
defined front stretching across central South Dakota into
Nebraska will serve as foci for convection later in the
afternoon or evening. Instability in the region should be
moderate to significant in most areas, but true effective shear
may be a bit more marginal. Should convection form locally, then
large hail up to 2" may be possible along with 70 mph winds.
We`ll also watch one or more MCS developing further west and
track towards the CWA into Friday morning. The increase in the
nocturnal LLJ and increasing low-lvl shear could increase the
severe risk overnight, as well as the localized heavy rain
threat (PWAT AOA 130-150% of normal). The difficulty is
pinpointing the corridor of heavy rain as most probabilistic
data is washed out given the large variety in convective
scenarios. Current ensemble approach would place the greatest
risks of excessive rainfall over portions of Northwestern Iowa
and adjacent areas of MN/SD/ND.

FRIDAY:  There has been a slight slowdown in the passage of a mid-
lvl trough on Friday, owing to higher rain chances continuing into
the early afternoon hours. While not likely to be severe, this
continued rain risk could impact any outdoor events in the area
before northerly low-lvl flow clear precipitation by mid-afternoon/

SATURDAY-MONDAY:  Saturday into Sunday continues to remain dry
through the area as weak mid-lvl ridging moves into the Plains.
However, the overall synoptic pattern continues to flatten in
consecutive model runs, already showing a return to a more active
zonal flow by late Sunday into Monday.  Saturday will feature cooler
temperatures, with temperatures rising on Sunday back towards
the 90s. The return of meaningful low-lvl moisture and
instability suggests the increased severe weather probabilities
early next week in machine learning models have some validity
and thus should be monitored.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 550 PM CDT Tue Jul 8 2025

Isolated showers/thunderstorms will be possible through central
SD into the James River Valley late tonight into Wednesday
morning. At this time, guidance would suggest that this activity
will remain west of the TAF sites. Otherwise, VFR conditions
expected with light winds through the period.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...JM