336
FXUS63 KFSD 111115
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
615 AM CDT Fri Apr 11 2025

.KEY MESSAGES...

- A foggy start to the day for some and a Dense Fog Advisory is
  in effect for some areas along/north of I-90 until 10 AM. This
  will give way to plenty of sun by midday, with lighter winds
  and highs in the 60s to lower 70s.

- Strong winds return Saturday (south) and Sunday (northwest)
  along with warm temperatures which will lead to elevated to
  potentially critical fire weather conditions.

- A low (20-30%) chance for showers and isolated thunderstorms
  Saturday night into Sunday.

- Windy on Monday with spotty rain or even a stray snow shower
  possible. Overall impacts should be minor.

&&

.DISCUSSION...
Issued at 410 AM CDT Fri Apr 11 2025

TODAY: Near term concern centers of fog which has developed
within the surface ridge sliding east across the region. Light
winds and clear skies allowed for strong radiative cooling,
especially across the Coteau des Prairies north of I-90 where
satellite/area web cams show dense fog becoming more widespread
over the past hour or two. Have issued a Dense Fog Advisory
through mid morning and will monitor for possible expansion in
these last few hours before sunrise.

After the fog dissipates (expected by 9-10 AM CDT), we should
see plentiful sunshine through the afternoon. Winds remain on
the lighter side, though could become slightly more breezy in
areas west of I-29 as the surface ridge slides into MN/IA. Our
humidity values do drop below 25% along and west of the James
River this afternoon, but the lighter winds should keep fire
danger below critical levels.

TONIGHT-SATURDAY: Increasing southerly flow will spread across
the area starting tonight as a 30-40kt low level jet develops
above the surface. This should lead to stronger winds through
the overnight than we see during the day today, with gusts up to
30-35 mph at times, especially in higher elevations from south
central SD into southwest MN. Southerly winds increase further
Saturday ahead of an approaching cold front. While a subtle wave
aloft may produce increasing mid-level clouds, south-southwest
flow ahead of the approaching front should allow us to follow
the trend of over-performing much of the model guidance with
respect to temperatures (higher) and humidity (lower). 850-925MB
temperatures in the deterministic models support highs topping
80F in the thermal ridge across our SD counties on Saturday,
perhaps even as far northeast as Sioux Falls. The latest NBM is
below its own 50th percentile, but given the favorable low level
mixing environment, have nudged highs upward slightly. Still
room to move toward the 50th or even 75th percentile, but given
the cloud cover potential will remain a bit more conservative
for now. Winds ahead of the boundary could gusts as high as 40
MPH in the morning to midday, but may ease a bit as we move
through the afternoon with a slackening gradient ahead of the
boundary. This may keep us just shy of reaching widespread
Extreme Grassland Fire Danger, but the combination of strong
winds and dry air may still result in a need for fire weather
headlines.

SATURDAY NIGHT-SUNDAY: A weak wave will drive a cool front east
across the region Saturday night into early Sunday. Moisture
remains somewhat limited in the low levels, but the wave could
be enough to produce spotty light rain showers, or possibly an
isolated thunderstorm given some weak instability above an
inversion around 850-800MB. After a mild night with lows in the
mid 40s-lower 50s, temperatures will recover back into the 60s
to mid 70s, this time with the warmest readings in northwest IA.
Post-frontal northwest winds will again be stronger with gusts
exceeding 30-35 MPH at times west of I-29, coincident with the
lowest humidity levels in the afternoon, which could result in
another near-critical fire weather day.

LONG TERM: Monday will be a cool and windy day, with gusts up
to 40 mph possible as the SPG tightens between a deepening low
over the northern Great Lakes and a high pressure system sliding
in from the northern Rockies. With this wind, the fire danger
will remain very high Monday despite spotty light rain chances
in the afternoon. The aforementioned high will lead to lighter
winds on Tuesday; however, continued northwest flow will keep
very dry air in place and thus elevated fire danger remains in
some areas.

Temperatures will warm-up into the middle of next week at the same
time the next storm system will be developing out of the Rockies.
This disturbance, in tandem with a trough moving in from Canada,
will deepen over the Midwest. There is decent model agreement this
far out that this system will strengthen just east of the area,
keeping the bulk of the rain away. Still, there will be a slight
chance of rain (<30%) Wednesday night into Thursday mainly east of I-
29. With that said, the ultimate track of the system will play a
role in the fire weather threat during this time frame. Winds will
likely pick up Wednesday and Thursday, but a farther east system
will mean a higher fire danger as there will be less moisture for us
to play with. A farther west system would mean more moisture and
thus a lower fire weather threat. Bottom line, each day in the long
term will need to be monitored for potential fire danger.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 614 AM CDT Fri Apr 11 2025

Areas of dense fog reducing visibility to VLIFR levels are round
across the forecast area early this morning. Though not directly
impacting our TAF sites, fog is evident on satellite surrounding
the Sioux Falls metro area and to a lesser degree just east of
Huron. Cannot rule out a brief period of fog development on the
airfield around sunrise, especially for KFSD given its already
close proximity, so will carry a mention of fog here through 13Z.

After the fog dissipates (14Z-15Z), expect VFR conditions to
prevail with light/variable winds becoming southeast-south.
Speeds will increase after sunset, with gusts 20-25kt possible
in lower elevations (nearing 30kt in higher elevations) as a low
level jet develops overhead. This could also create a period of
low level wind shear, mainly west of I-29 toward the end of this
TAF period 06Z-12Z.


&&

.FIRE WEATHER...
Issued at 410 AM CDT Fri Apr 11 2025

Light and variable winds early this morning will become
southerly by midday. Speeds remain rather light along and east
of I-29, but a few gusts near 20 mph will be possible west of
I-29 this afternoon. Fire danger will be elevated in areas with
breezier winds, but most locations should see low to moderate
grassland fire danger today.

Increasing winds this weekend will bring a return of critical to
near critical fire weather conditions. Saturday will see south
winds, gusting near 40 MPH at times in the morning to midday
hours before easing slightly as humidity levels drop into the
20 to 30 percent range in the afternoon. A cool front will swing
winds around to the northwest Sunday, with gusts 30-35 MPH again
possible west of I-29. Humidity level are expected to fall as
low as 25-30 percent in the area with stronger winds, while
locations farther east see somewhat lighter winds and higher
humidity levels.

Looking into next week, rainfall will be limited so any days
with stronger winds will bring a threat of elevated fire danger.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ039-040-
     054>056-061-062-067.
MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072-
     080-081-089-090-097-098.
IA...Dense Fog Advisory until 10 AM CDT this morning for IAZ001-002.
NE...None.

&&

$$

DISCUSSION...JH/Samet
AVIATION...JH
FIRE WEATHER...JH