435
FXUS63 KFSD 121948
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
248 PM CDT Sun May 12 2024

.KEY MESSAGES...

- Scattered showers and storms are possible tomorrow afternoon. Severe
  weather is not expected but some storms could produce gusty
  winds up to 50 mph.

- Wildfire smoke will move into the area this evening and night
  behind a passing cold front. Some reductions in visibility are
  possible.

-Scattered thunderstorm chances increase for Tuesday night into Wednesday.
 Severe storm risks are low.

-Confidence in rain chances is low for the end of the week with
 model differences. Temperatures will be at or above normal
 through the period.

&&

.DISCUSSION...
Issued at 247 PM CDT Sun May 12 2024

A cold front continues to push into the area this afternoon,
igniting showers and thunderstorms along and ahead of it. Surface
winds ahead of the front are southwesterly which is allowing for
minor convergence along the front as winds behind the front turn
northerly. High temperatures have warmed to the upper 70s and 80s
resulting in strong mixing across the area. With a deep mixed layer
in place, drier air is being mixed to the surface which has lowered
dew points a bit, now down to mid 40s to low 50s. Instability
remains meager at best with only about 500 CAPE but is good enough
to sustain the storms. Wind shear is weak with 0-6km shear on the
order of 5-10 knots which will keep severe storms at bay. However,
given the large subcloud dry layer, gusty winds are possible in any
storm that develops. Wind gusts may reach up to 50 mph in the
strongest storms.

Storms will slowly wane this evening as the boundary layer
stabilizes. However, smoke from wildfires in Canada will filter into
the area behind the front. The smoke looks to overspread most of the
area this evening and night but some uncertainty remains in the
extent and visibility in the smoke as it may be disrupted by the
remaining showers and storms. As of now, reduced visibilities are
expected in the smoke down to around 6 miles with isolated locations
seeing further reductions in visibility down to 3 miles. Low
temperatures will fall to the upper 40s to mid 50s.

Vertically stacked low pressure system will slowly move through the
Central Plains on Monday. While the best lift with the system will
be south of the forecast area, some light wrap around moisture may
hang onto locations along and south of a Sioux City, IA to Storm
Lake, IA line. Minimal rainfall accumulations are expected with the
light rain. Otherwise, Monday will be a more quiet day with high
temperatures warning into the 70s along with a light northeast
wind. Low temperatures will fall to the 40s overnight.

A weak surface ridge axis situated across our area on Tuesday
morning will shift eastward during the day - this as low pressure
deepens over western and central SD during the same time frame. Most
models keep it dry through much of the day, before thunderstorms
develop over western/central SD as an upper level shortwave drifts
out of the Northern Rockies into the Northern Plains in the late
afternoon/evening. Shower and thunderstorm chances then increase on
Tuesday night into Wednesday as the aforementioned surface low
pushes into our area. The chances for severe storms remains low,
with deterministic models (with the exception of the NAM) indicating
little instability over our area during this time frame - this
supported by ensembles which have a 20% or less probability of CAPE
greater than 500 J/KG in our area - with the greater instability
confined to central SD. Highs on Tuesday will remain above normal -
in the lower to mid 70s.

There are indications that there will be a lull in shower
thunderstorm activity by Wednesday morning, before possible
redevelopment in the afternoon/evening as the surface low pulls a
frontal boundary through our area and a secondary upper level wave
slides into the Dakotas. Again, severe chances look to be low with
the lack of significant instability. While rainfall doesn`t look to
be overly heavy with the system for Tuesday night through Wednesday,
ensembles do indicate a 60-70% of our area receiving at least a
quarter of inch during this period. Latest models indicate lessening
rain chances on Thursday as the upper level trough finally slides
off to the east during the day.

Uncertainty grows toward the end of the week into the weekend with
model variations on the upper level pattern. The 12Z GFS and
Canadian trend toward a more zonal upper level pattern by Saturday,
while the ECMWF develops a deep elongated upper trough through the
central CONUS for Friday into early Saturday. As a result,
confidence in rain chances is low. Ensembles would support only low
chances (generally 20-30%) of receiving a tenth of an inch of
precipitation in any one 24 hour period for the end of the week into
the weekend. Temperatures do look to remain at or above normal for
this time of year.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1232 PM CDT Sun May 12 2024

VFR conditions begin the TAF period. A weak boundary is pushing into
the area this afternoon which will set the stage for scattered
showers and thunderstorms. A second area of rain is between I-80 and
highway-20. No lightning is expected with these showers so have only
included VCSH in KSUX`s TAF. The storms though are most likely to
affect locations around and south of I-90. As such, have included
VCTS in both KFSD and KSUX`s TAFs.

Winds behind the front will turn northerly but remain on the lighter
side with speeds between 5-10 knots. The other aspect to the front
coming through is smoke. Smoke will push into the majority of the
area this evening and night. Some uncertainty remains regarding how
far south the smoke may push along with visibilities in the smoke
due to the showers and storms. For now, have included smoke in both
KHON and KFSD with VFR visibilities but visibilities could fall to
MVFR levels. Trends will be monitored. Winds will turn northeasterly
along with the smoke to finish out the TAF period.

&&

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

&&

$$

DISCUSSION...Meyers/JM
AVIATION...Meyers