Harrisburg, South Dakota 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

324
FXUS63 KFSD 171733
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1233 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- Decreasing clouds after noon, dry, and cool conditions on
  today with highs in the low to mid 70s.

- Shower and thunderstorm chances return Friday afternoon and
  night. Precipitation probabilities of 30 to 60 percent.

- Precipitation chances of 20-30 percent daily this weekend into
  early next week. Details remain uncertain at this time.

- Temperatures will rise to near or above 90 degrees in many
  locations by next Tuesday.

&&

.UPDATE...
Issued at 615 AM CDT Thu Jul 17 2025

Some patchy dense fog has developed this morning, mostly in
areas north of I-90 and west of I-29. Please use caution as you
make your way this morning. Fog should begin to burn off as the
surface temperatures begin to rise this morning.

&&

.DISCUSSION...
Issued at 327 AM CDT Thu Jul 17 2025

TODAY: Zonal flow aloft as a surface high pressure builds in from
the northwest this morning. This will push the clouds out of the
area and by afternoon we should have mostly sunny skies. As the high
moves off to the east winds will gradually change from northerly to
southerly, remaining light at around 5-10 mph. Weak WAA will warm
temperatures slightly higher than yesterday, but still 10 degrees
below average for mid July (Average is mid 80s). Highs will be in
the 70s with lows in the 50s and 60s.

FRIDAY: Southerly winds increase slightly to around 10-15 mph with
gusts up to 20 mph. The highest winds are expected west of the James
River Valley. A stronger push of WAA comes with the winds as we
begin to warm closer to the average with highs in the upper 70s to
80s. Lows will be in the 60s. In addition, the southerly winds will
bring north higher dew points in the upper 60s to mid 70s. Humid
conditions can be expected for Friday as a result. At the surface, a
weak inverted trough moves in from the west in concert with a subtle
short wave trough embedded in the quasi-zonal mid-levels. This with
the WAA will support showers and thunderstorms. The GFS indicates
showers beginning as early as Friday morning along and north of
Highway 14, then expanding in coverage and intensity Friday
afternoon and evening. The EC and CAN have more broad showers
beginning Friday morning and increasing in intensity for the
afternoon and evening. The NAM doesn`t have much for showers until
Friday afternoon. Due to the inconsistencies, confidence in shower
initiation is low.

With the increasing dew points comes increasing instability. CAPE
values of 2000-3000 J/kg are possible, with good 0-6km shear of 35-
45 kts, mid-level lapse rates of 7-8 C/km, DCAPE of 800-1500 J/kg
and PW values of 1-1.5 inches (90th percentile for climatology).
These numbers lend confidence to the storm environment being
moderately favorable. However, warming above 850 mb will result in a
strong cap on the environment. There is some disagreement on
whether or not the cap can be overcome with the generally weak
forcing. Considering this I think that the most likely scenario
will be elevated showers and thunderstorms. Threats would
include heavy rainfall, large hail (1-1.5 inches), and damaging
wind gusts (60-70 mph). Timing of strong storms is uncertain due
to discrepancies in models, however Friday late afternoon into
early Saturday morning looks to be the most active.

SATURDAY-SUNDAY: For Saturday WAA and another week embedded mid-
level wave keeps light isolated to scattered showers and
thunderstorms going for most of the day. PW water values remain at
1.5-2 inches, and though the storm environment is less favorable for
severe, locally heavy downpours remain possible. Highs for Saturday
will be warm despite the clouds and rain thanks to the warm
southerly winds, in the upper 70s to 80s. Lows will be in the 60s.
Sunday will be very similar; isolated to scattered showers and
thunderstorms that may include locally heavy rainfall. Highs will be
in the upper 70s to 80s with lows in the 60s.

NEXT WEEK: Guidance indicates the active pattern will continue
through at least the middle of next week. Nearly daily a weak wave
works through the shallow ridge pattern bringing chances for showers
and thunderstorms. Highs warm into the 80s and 90s for Monday
and Tuesday. Then, a cooling trend for the middle part of the
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1231 PM CDT Thu Jul 17 2025

A fairly large scattered to broken CU field is noted across the
region to start the period. Any lingering MVFR ceilings should lift
to VFR through the rest of the early afternoon as daytime heating
continues. A high pressure system is keeping things dry with fairly
light east-southeasterly winds across the area this afternoon. As
this shifts east through tonight, expect southerly flow to increase
and thus moisture will increase as well. Thicker clouds will move in
from west to east through the night as this moisture returns, but
should remain VFR through tonight. With these clouds moving into
areas along and east of I-29 later in the night, can`t rule out some
patchy light fog in these areas as temperatures there have a better
chance of cooling to saturation. However, any fog is expected to
remain fairly thin, and chances were not high enough to include in
any of the TAF sites at this time.

Heading into the daytime hours on Friday, an additional CU field
will develop and lead to a brief period of MVFR conditions during
the mid-morning hours, though these clouds will likely lift back to
VFR just after the period ends. As far as storm chances go, the
better chances will be after the period into Friday evening;
however, can`t rule out a few thunderstorms mainly along the Highway-
14 corridor in South Dakota. For this reason, put a couple of PROB30
groups in for KHON from 13Z to 18Z Friday. Southeasterly winds
will strengthen through the morning on Friday, gusting between
15-20 kts at times.

&&

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

&&

$$

UPDATE...AJP
DISCUSSION...AJP
AVIATION...Samet