223
FXUS63 KFSD 122333
AFDFSD

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

.KEY MESSAGES...

- Scattered showers and storms continue to drift south ahead of
  the passing front tonight. Severe weather is not expected but
  some storms may produce small hail and gusty winds up to 50
  mph.

- Wildfire smoke will spread south into the area tonight behind
  a passing cold front. Some reductions in visibility are
  possible.

- Moderate chances (30% to 60%) for scattered thunderstorm
  develop for Tuesday night into Wednesday. Severe storm risks
  are low.

- Confidence in rain chances is low for the end of the week with
  large 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 /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 626 PM CDT Sun May 12 2024

Scattered showers and thunderstorms will continue to sag south
with a passing cold front tonight. Outside of this activity, VFR
conditions should prevail through the period. Occasional IFR or
MVFR visibility with rain, gusty erratic winds, and small hail
are possible near tonight`s thunderstorms. Showers and
occasional storms will persist overnight near the Hwy 20
corridor/Sioux City, but should mostly move out by 12z.

Ahead of the front, winds are mainly southwesterly and will
turn north behind the front. Canadian wildfire smoke aloft and
near the surface sinks in behind the front overnight and may
reduce visibility to MVFR at the surface at times, but
confidence in this is too low to include in TAF at this time.

&&

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

&&

$$

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