117
FXUS63 KFSD 131039
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
539 AM CDT Sun Jul 13 2025

.KEY MESSAGES...

- Heat and humidity begin to build back into the area for today.

- A low (~20% chance) thunderstorm risk for this afternoon and
  evening, and an isolated stronger to severe storm could
  produce large hail and damaging winds.

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

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

&&

.DISCUSSION...
Issued at 310 AM CDT Sun Jul 13 2025

Warming temperatures for today as an upper level trough oriented
through the Mississippi Valley shifts to the east - resulting in
rising upper level heights over our area. Related to that, an 850 mb
thermal ridge to our west begins to nudge into out area, and with
850 mb temperatures in the mid teens to lower 20s C by afternoon,
highs will climb into the mid 80s (east) to mid 90s (west). In
addition to the warm temperatures, it will become more humid in an
increasing southerly flow out ahead of a surface trough/frontal
boundary to our west. This will push dew points into the mid 60s to
near 70 - which will add to the discomfort. The surface
trough/frontal boundary will push eastward into our area during the
day, and hi-res models continue to develop isolated convection with
this feature in the later afternoon and evening. The main forcing
with this system will be low level convergence along the boundary,
with only weak mid/upper level forcing. As a result, anything that
develops should be pretty isolated. MLCAPE in the vicinity of the
front will run around 2000 J/KG by afternoon, this collocated with
bulk shear around 30 kts. Model soundings indicate tall skinny CAPE
with mid level lapse rates between 5 to 6 C/KM, so would expect only
isolated stronger storms. That being said, with the above mentioned
CAPE values and downdraft CAPE around 1500 J/KG, cannot rule out an
isolated storm with large hail and gusty winds - which aligns well
with the SPC Day 1 Marginal (level 1 out of 5) convective outlook.

The upper level pattern begins to transition to a more zonal flow on
Monday, and with high pressure strengthening over the Mississippi
Valley and low pressure deepening over the western High Plains, the
southerly low level flow will continue to increase over our area.
This will bring temperatures and dew points up another notch, with
highs into the upper 80s to upper 90s - with warmest readings over
central SD. Guidance continues to indicate that mainly dry
conditions will prevail for Monday into early Tuesday, with any
convection remaining to our north and west in the vicinity of a
frontal boundary draped across that area.

By later on Tuesday the boundary finally tracks into our area -
bringing an increasing threat of showers and thunderstorms into
Wednesday as a couple shortwaves track through the upper level flow.
The severe threat for Tuesday afternoon and evening remains somewhat
in question. Although there will be plenty of instability to work
with, models continue to show a rather weak mid level flow and
relatively low bulk shear. A perhaps bigger threat could be heavy
rain as the system works across the area, with ensembles showing
PWAT values in the 90th percentile or greater with respect to
climatology, and as a result the WPC Day 3 ERO slight risk (level 2
out of 4) for portions of our area seems appropriate.

On the backside of the system, temperatures drop back to below
seasonal averages for the middle and end of the week, with highs 70s
to lower 80s. As alluded to earlier, showers may continue into
Wednesday and Wednesday evening (though seeing model differences in
the specifics of this scenario). The overall trend should be for
drier conditions as we move into the end of the week into next
weekend, though periodic rain chances may remain as a series of
upper level shortwaves track across the region through the
period.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 535 AM CDT Sun Jul 13 2025

Any patchy fog and lingering smoke is expected to decrease by
mid morning. Otherwise, isolated thunderstorms may develop this
afternoon and evening along a boundary, though low coverage and
confidence precludes including in TAFs. Southerly winds will
transition to west/northwesterly behind the boundary.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...JM