541
FXUS63 KFSD 252350
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
650 PM CDT Wed Jun 25 2025

.KEY MESSAGES...

- Heavy rain remains a focus over the upcoming 24 hours, with
  already observed totals of more than 3 inches in pockets
  today. Rainfall will continue through mid-day Thursday.

- The greatest potential for widespread heavy rain will fall
  along a line from Sioux City to Spencer and southeast this
  evening and tonight. Rainfall totals may exceed 3" in some
  areas.

- A risk of strong to severe storms persists late this afternoon
  and evening, though convective risks seem contained to
  microburst winds and a very low tornadic risk mostly in NW
  Iowa.

- With a more narrowed focus of heavy rain, primary flood risks
  are focused more towards urban and small stream flooding as
  opposed to widespread river flooding.

- Temperatures rise into the weekend bringing uncertain severe
  weather risks late Friday through Sunday. Begin monitoring
  this time period if you have outdoor plans.

&&

.DISCUSSION...
Issued at 242 PM CDT Wed Jun 25 2025

THIS AFTERNOON:  Low-lvl flow remains strongly veered this afternoon
traveling underneath a stream of mid-lvl vorticity.  A continued
stretching and narrowing of narrow convective bands over eastern
SODAK and western Minnesota continues, with potential for highly
isolated stripes of moderate to heavy rain focused mainly in SW
Minnesota.   Further southeast, we`re beginning to see convective
development stretching from eastern Nebraska into portions of
northwestern Iowa. Soundings within this uncapped airmass of
nearly 1500-2000 J/KG show a fairly tall but thin CAPE profile,
with poor mid-lvl lapse rates, but rather low LCLs. DCAPE itself
approaches 500 J/K into areas of far northern Iowa, with the
highest DCAPE values along or south of Highway 20. We`re still
yet to see any strong frontal development over northeastern
NE/IA, but potential does exist for surface flow to veer with a
differential heating boundary developing later this afternoon.
Should this happen, 0-1km CAPE and some boundary enhancement
could develop to produce a limited tornado risk over a very
narrow corridor of NW Iowa into early evening. Given the very
moist soundings, rapid updraft development, water loading could
be an increased potential with updraft collapses producing wet
microbursts perhaps the primary severe weather risk in NW Iowa,
especially along and south of Highway 20. In fact, latest RAP
mesoanalysis shows the highest microbursts composite either
side of Highway 20 and greater to the south. However, the
greatest potential from any of this activity may be locally
heavy rainfall.

THIS EVENING AND TONIGHT: Several rounds of convection are likely
into the overnight hours, with most of the focus along and southeast
of a Yankton to Windom line, and greatest heavy rain potential from
Sioux City to Spencer. Initial storms late this afternoon move east,
while additional convection forming along a differential heating
boundary in south central Nebraska stream northeast later in the
evening. Some potential bowing segments could form on this
convection given the parallel flow. A third and final round of
convection moves northeast after midnight and persists into mid to
late morning Thursday. Given flow generally parallel to the
surface boundaries, a deep warm layer in soundings, and PWAT
values exceeding 200%, heavy rain is expected with 1-3" per
hour rainfall rates possible. Have issued a flood watch for
areas of NE Nebraska and NW Iowa into the Thursday. One other
area to monitor is along a secondary surface front just north
of I-90. Some signals that as vorticity crosses the Dakotas,
this area could produce convection capable of heavy rainfall.

THURSDAY: Morning convection slowly moves off to the east through
Thursday morning as we remain within the broad mid-lvl trough axis.
Drier air should filter eastward in the afternoon, with thunderstorm
risks shifting east. Temperatures will warm back into the upper
70s to 80s.

FRIDAY-SUNDAY: Quasi-zonal flow develops into the upcoming weekend,
allowing temperatures to warm back towards the 90s.  In fact, with
recent rainfall and rising dew points, heat index values Saturday
may peak near or above advisory levels through much of the Tri-State
area. Convection chances will be focused on a narrow area through
central South Dakota late Friday along a surface front and along the
edge of the thermal ridge.  While uncertainty continues into the
weekend, this front edges eastward on Saturday, and given the high
degree of instability that develops, strong to severe storms may
develop into Saturday night west of I-29 and then move east
overnight. We`ll have to watch what happens to convection late
Saturday before any clarity develops on Sunday`s risks.

MONDAY-TUESDAY: At this time, northwesterly flow aloft would favor
slightly cooler temperatures and lower precipitation chances.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 645 PM CDT Wed Jun 25 2025

Showers and thunderstorms remain most likely across parts of
northwest IA through the evening, with more scattered activity
overnight into Thursday morning. To the north the evening will
be quieter with much more isolated activity with much less
thunder potential. Some scattered activity will develop in
central SD and drift eastward overnight and continue into
Thursday, gradually ending from west to east. The other concern
will be MVFR and IFR conditions becoming more prominent later
tonight into Thursday.

&&

.HYDROLOGY...
Issued at 242 PM CDT Wed Jun 25 2025

MRMS 48 hour rainfall estimates, complimented with measured rainfall
reports indicate a swath of 1-3 inches of observed rainfall in an
area south of Yankton to Spencer, Iowa line and a second area
roughly a county either side of I-29 through eastern South Dakota.
This rainfall has wetted soils enough to allow for increased runoff
potential with the additional expected rain over the next 24 hours.
The best overlap between observed rainfall and heavier expected
additional rain looks to be within/near the Flood Watch area.

The short range National Water Model Rapid Onset Flooding (HRRR
forced) output shows a fairly robust signal from northeast Nebraska
into areas south of Hwy 18 in northwest Iowa. This same area also
has the strongest signal in the NBM forced High Flow Magnitude
exceeding high water thresholds. While it appears the most robust
rises will be confined to smaller creeks and streams, HEFS guidance
supports some low (<20%) chance of Minor to Moderate stage on the
Little Sioux and Rock as well as lower portions of the Vermillion,
Big Sioux, Floyd Rivers. The more likely impact from additional
rainfall over the next 24 hours will be flash flooding, with urban
and poor drainage areas the most likely locations.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...Flood Watch through Thursday afternoon for IAZ013-014-020>022-
     031-032.
NE...Flood Watch through Thursday afternoon for NEZ013-014.

&&

$$

DISCUSSION...Dux
AVIATION...08
HYDROLOGY...Kalin