507
FXUS63 KFSD 132306
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
606 PM CDT Sun Jul 13 2025

.KEY MESSAGES...

- A low (~20% chance) thunderstorm risk continues this evening,
  and an isolated stronger to severe storm could produce large
  hail up to the size of a quarter and damaging winds to 60
  mph.

- Heat and humidity continue for Monday and Tuesday, with
  increasing storm chances on Tuesday and Tuesday night. With
  that comes a chance for severe storms along with heavy
  rainfall.

- Cooler temperatures expected for mid to late week, with
  periodic low precipitation chances continuing through next
  weekend.

&&

.DISCUSSION...
Issued at 238 PM CDT Sun Jul 13 2025

Surface trough and its associated boundary have pushed into the area
this afternoon, currently sitting west of the James River. A weak
shortwave trough at 700 mb is just west of the boundary and looks to
develop convection along it during the later half of the afternoon.
CAPE values on the order of about 1,500 J/kg is present along with
deep layer shear values up to 30-40 knots. With only minimum forcing
in place from the previously mentioned wave, this will support
isolated severe storms along the front with large hail up to the
size of a quarter (1 inch) and damaging winds to 60 mph being the
main threats. The coverage of storms is expected to be mainly
isolated as the main forcing from the wave looks to stay mainly
across and northeast of southwest Minnesota. Storms could still
develop on the boundary towards the southwest across south central
and southeast South Dakota but think that showers are more likely
then actual storms due to the lack of forcing. Should see any chance
for showers and storms persist into the evening hours before coming
to an end before midnight. Lows will fall to the 60s overnight.

Another shortwave trough will begin to push out of the Rockies and
into the Northern Plains on Monday. This wave will spawn a surface
low via lee cyclogenesis and set the stage for rain chances.
However, the best lift will be situated well west of the forecast
area. Thus, Monday will be dry but hot and humid as high
temperatures warm to the upper 80s to 90s along and west of I-29.
Locations towards central South Dakota could see highs top out at
100F. With a tighter surface pressure gradient in place, breezy
southeasterly winds will return to the area. Overnight low
temperatures will remain on the mild side thanks to the
southerly winds with lows only falling to the upper 60s and 70s.

Rain chances return on Tuesday as a cold front pushes into the area
from the northwest. Moisture will pool ahead of the front, pushing
dew points to the 60s and 70s. Highs again will warm to the 8-s and
90s, making for another hot and humid day. With steeper mid level
lapse rates on the order of about 7 C/km, sufficient instability
will be in place. The GFS, Euro, and Canadian ensembles show a 40-
80% chance for CAPE values to exceed 2,000 J/kg. Vertical shear will
be marginal however as the upper level jet will be displaced well
north of the area, towards the international border. In fact, any
marginal shear will be behind the surface front. This makes severe
storm potential a bit tricky as there is not much overlap between
the instability and shear. However, think that the instability may
be able to overcome the weaker shear keeping a lower risk for severe
storms in place. The other aspect to these storms is their heavy
rain risk. Precipitable water (PWAT) values look to rise to the 90th
percentile of climatology per the NAEFS ensemble. The Euro Extreme
Forecast Index (EFI) also shows about a 50% chance for anomalous
rainfall Tuesday evening as well. Mean flow is also parallel to the
front, further increasing confidence in this potential heavy rain
threat. At this time, the ensembles show a broad 40-80% chance for
rainfall amounts to exceed a quarter of an inch. While not a high
amount of rain, this event is looking to be convectively driven
which could make for localized heavier amounts possible.

While the front will push through the entire forecast area during
the overnight hours on Tuesday, the elevated front looks to remain
over the area on Wednesday. This will continue rain chances across
the area though it looks to be dependent on the evolution of a new
surface low developing on the boundary. Outside of rain chances,
Wednesday will be a cooler day, relatively speaking, with highs only
warming to the 70s to mid 80s.

Thursday through the rest of next weekend look to see continued
chances for rain as weak shortwaves pass through the upper level
flow. However, medium range guidance shows large variance in the
upper level pattern and especially mesoscale details. Tough to say
which day has the best potential to see rain at this point in time.
Will continue to monitor this potential over the coming days. Aside
from rain chances, high temperatures will remain near to below
average, coolest in the 70s and 80s on Thursday and Friday before
warming to the 80s for the weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 601 PM CDT Sun Jul 13 2025

Isolated showers and storms (< 20% chance) this evening and
into tonight. Isolated coverage precludes mention at any TAF
site. Storms may produce locally heavy downpours in addition to
lightning. Severe risk remains low. Winds shift behind a
boundary tonight, becoming light and variable overnight. Winds
shift to southerly Monday morning, with gusts in the afternoon
and early evening around 15 to 20 knots.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...SG