246
FXUS63 KFSD 292335
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
635 PM CDT Mon Apr 29 2024

.KEY MESSAGES...

- Strong to severe storms are expected tomorrow afternoon
  between 1 pm through 8 pm. Storms will be capable of large
  hail up to golf ball size initially before growing upscale
  into a line, transitioning the hazards to damaging wind gusts
  up to 70 mph along with an isolated tornado. Brief heavy rain
  is also possible which could result in minor flooding.

- A second round of heavy rainfall is possible Wednesday night
  where rainfall amounts may exceed a half an inch of rain.
  However, there remains uncertainty in where the heaviest rain
  will fall.

- Seasonal temperatures with highs into the 60s and possibly 70s
  and lows down to the 40s will continue through the weekend.

&&

.DISCUSSION...
Issued at 257 PM CDT Mon Apr 29 2024

Upper level low pressure system continues to push off to the
northeast this afternoon. This is resulting in low level stratus
finally lifting off to the northeast with the system and bringing
sunshine back to parts of the area. West/northwest winds have gusted
to around 20-25 mph but will cease as mixing wanes this evening.
However, winds will turn to out of the southeast during the
overnight hours and become a bit elevated. Despite clear skies
tonight, the southeast winds will keep low temperatures just about
seasonal with lows falling to the upper 30s to mid 40s. A few hi-res
guidance members have been pointing to the potential for fog
development across parts of eastern South Dakota, southwest
Minnesota, and northwest Iowa. However, think that this is a low
probability (<20% chance). Will monitor this potential over the
coming hours.

A stronger upper level wave will push into the Northern Plains
tomorrow, setting the stage for strong to severe storms. As the exit
region of the upper jet pushes into the Northern Plains, southerly
flow will commence in response to the jets indirect thermal
circulation which will advect warmer and higher moisture air into
the forecast area. High temperatures are expected to warm to the mid
60s to mid 70s, warmest across northwest Iowa. Dew points will
moisten to the 50s, highest along and east of I-29. A surface low
residing within the left exit region of the jet will drag a cold
front through the forecast area which will serve as the focal point
to initiate convection. Looking at the environment, instability up
to around 1,500 J/kg is expected with sufficient 0-6 km shear values
up to 40-50 knots. Soundings do show weak capping in place due to
the warm air advection (WAA) via the southerly flow. This should
keep convection limited to the afternoon where the lift from the
coupling of the cold front and jet streak circulation overcome the
weakening cap and initiate storms. There remains some uncertainty in
where storms develop though as latest 12z guidance has been coming
in faster with the cold front. With the front passing through at a
faster rate, this may limit severe storm chances to a few hours
during the afternoon across parts of southwest Minnesota and
northwest Iowa. However, there is still the remaining possibility
that storms develop right along I-29 during the early afternoon
hours and race eastwards through the rest of the afternoon hours. As
such, think the timeframe between 1:00 pm to 8:00 pm remains valid.
In terms of storm mode, storms look to develop as individual cells
(potentially supercells) and pose a large hail threat with hail up
to golf ball size. Storms will quickly grow upscale into a line of
storms and transition the hazard from large hail to damaging winds.
0-10 km shear values up to 50-60 knots will support a severe line of
storms but 0-7 km theta-e difference in only marginal, down to -5 k.
Despite the weaker theta-e difference, the strong 0-10 km shear will
support strong wind gusts up to 70 mph. 0-3 km shear will be
sufficient up to 30-40 knots will be oriented more parallel to the
line of storms, but still indicative of a low tornado threat. Brief
heavy rain is also possible with the storms but given fast storm
motions, think any potential flooding threat will also be low and
confined to minor flooding. After storms push out of the area, quiet
conditions are expected for the rest of the evening and overnight
hours. Low temperatures will fall to the upper 30s and 40s
overnight.

Wednesday will be quiet during the daylight hours as large scale
descent resides over the area. Highs will be a bit cooler, only
warming to the 60s during the afternoon hours. An active jet aloft
will continue to provide rain chances Wednesday night. The previous
surface front will stall southeast of the forecast area but the
elevated front may serve as another focus for overnight showers and
storms as WAA strengthens in response to a strengthening low level
jet (LLJ). Instability looks minor at only a few hundred J/kg so
severe weather is not expected. However, heavy rain is possible as
precipitable water (PWAT) values will reside in the 90th percentile
per NAEFS ensemble. The GFS and Euro ensembles keep the highest
probabilities for exceeding a half an inch of rain over central
Iowa. However, the Canadian ensemble is an outlier as it shows a
broad 50-80% chance for exceeding the same amount of liquid QPF over
the forecast area. Cluster analysis also shows similar probabilities
for exceeding a half an inch of rain from the most favored cluster.
Will continue to monitor this potential. Low temperatures will fall
to the 40s overnight.

Chances for rain and storms look to continue for the rest of the
work week and weekend as an active pattern continues aloft. Too far
out to say if any of these chances for rain could coincide with the
potential for strong to severe storms. High temperatures look to
remain in the 60s and possibly 70s for the weekend with low
temperatures falling to the 40s overnight.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 628 PM CDT Mon Apr 29 2024

Lingering MVFR stratus across southwestern MN and northwestern
IA continues to move east through 30.03z. Otherwise, will be
watching for fog developing overnight from roughly KBKX to KLYV
to south of KSPW, which could lead to MVFR/IFR visibility. As of
now, models largely keep this development to the east of KFSD,
but will continue to monitor.

Expect northwest winds to become light and for some, variable
between sunset and daybreak. Winds start the day southeasterly
with gusts around 20 knots, but switch back more westerly behind
a cold front. This cold front will also bring the chances for
showers and thunderstorms. Recent guidance has sped up the
progression slightly, so tried to narrow the window as much as
possible for KFSD and KSUX. Showers/storms start off as single
cells, but will form a line as they track east. Should be west
of the IA/MN Hwy 60 corridor by 30.23z to 01.01z. MVFR/IFR
conditions are possible with thunderstorms.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...SG