202
FXUS63 KFSD 171914
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
214 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Scattered thunderstorms, mostly south of I-90 will continue
  into the evening. Primary risks of smaller hail and 50 mph
  winds, but higher risks of locally heavy rainfall.

- Scattered showers and thunderstorms may continue through mid-
  day Wednesday, though coverage remains highly uncertain and
  most areas may stay dry.

- Isolated to scattered elevated convection possible late
  Thursday evening and overnight mostly in SW Minnesota. 1-1.5"
  hail the primary risks.

- Heat wave potential becoming increasingly likely late this
  week through the upcoming weekend. "Heat Risk" and "WBGT"
  categories rise into the "High" category this weekend. All
  outdoor events and activities should begin preparing now as
  headlines appear likely.

&&

.DISCUSSION...
Issued at 208 PM CDT Tue Jun 17 2025

THIS AFTERNOON: We`ve reached convective temperatures in areas along
and south of I-90 early this afternoon, with weak surface
convergence and weak mid-lvl vorticity  more than enough to spawn
scattered to numerous thunderstorms.  Steering flow remains VERY
weak with movement in any given area less than 3-5 knots.
Instability is only around 1200-1700 J/KG, DCAPE is marginal as
well (AOA 400 J/KG), and shear is nearly non-existant, so hail
should remain small and downdraft potential fairly minimal.
We`ll have to watch for two things however. The NST/VGP remain
AOA critical levels suggesting potential for some updraft
stretching along weak boundaries. Also, the slow storm motion
will favor very localized heavy rain risks. While 1hr FFG
guidance is between 1.5-2", some guidance does indicate
potential for 2-4". These impacted areas will be HIGHLY
localized however, but remain vigilant.

TONIGHT: Convective chances tonight should weaken through the
evening as diurnal support is lost. We`ll watch to see if
another convective cluster can form over the western Dakotas and
Nebraska Panhandle and move east southeast, though CAMS are
highly variable on the track of this system. We still do however
remain in an area of dPVA with-in the broad SE moving trough
which may bring additional scattered showers/thunderstorms to
the Tri-State area through mid-morning Tuesday. Greatest
potential may end up tracking through the MO River valley.

WEDNESDAY: Aforementioned showers/thunderstorms linger into early
afternoon Wednesday before sliding eastward in the afternoon. Winds
remain light from the northwest as highs climb into the 70s.

THURSDAY: Mid-lvl ridging begins to build into the Plains on
Thursday, with guidance indicating a surge of low-lvl warm advection
and stronger theta-e advection arriving late THursday afternoon and
evening. While a low probability, isolated thunderstorms may try to
develop on the leading edge of low-lvl moisture east of I-29 into
Thursday evening, quickly moving northeast overnight.  These storms
could produce a modest and short-lived hail threat into Friday
morning.

FRIDAY-SUNDAY:  Prepare for a heat wave for the end of the week and
through the upcoming weekend.  Broadening mid-lvl ridging and an
expanding heat dome builds through the Central and Northern Plains
early Friday.  Temperatures may be a bit slow to rise Friday
morning, but should accelerate towards the 90s in the afternoon and
evening.  With a stationary boundary focusing west and north of the
CWA by Saturday, broad and deep southwesterly low-lvl flow will push
the thermal ridge overhead during the day.  With surface dew points
rising into the middle 60s to lower 70s, we should see heat index
values rise towards the 100 to 105 degree range in the afternoon.  A
very similar setup for Sunday with even higher temperatures
possible.  One modest change to the forecast was to bump wind gusts
both Friday and Saturday higher towards the 75th percentile of the
NBM.  Deep southwesterly low-lvl flow suggests 35 to 40 knots of
wind at the top of the mixed layer each afternoon.

All this said, the potential for heat related headlines is very high
from Friday through Sunday.  NWS heat risk products are rising
towards the "high" range through most of the area Friday and
especially Saturday.  WBGT values rise towards the high category as
well through Saturday.  Anyone spending time outdoors or hosting an
outdoor event should begin to make preparations now for the heat.

MONDAY-TUESDAY: Medium range models and ensembles push deeper
southwesterly flow eastward to start next week, settling a SW to NE
stationary boundary near the CWA by Sunday night and Monday.  This
synoptic setup would suggest a higher than normal potential for
rainfall early next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1141 AM CDT Tue Jun 17 2025

Scattered showers and thunderstorms are expected to initially
develop along a surface convergence zone over northern Nebraska
into northwestern Iowa early this afternoon. Additional
thunderstorms may develop throughout mostly central and eastern
South Dakota, with lower chances in SW Minnesota. VFR conditions
are anticipated outside of convection, which may briefly produce
visibility and ceiling restrictions.

The loss of daytime heating should allow most of today`s
activity to dissipate later this evening. However, we`ll still
remain in a favorable area for new scattered thunderstorm development
closer to daybreak which could impact TAF locations. Confidence
remains low on this development, so will indicate with a PROB30
attm.

&&

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

&&

$$

DISCUSSION...Dux
AVIATION...Dux