500
FXUS63 KFSD 300857
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
357 AM CDT Tue Apr 30 2024

.KEY MESSAGES...

- First round of thunderstorms this morning may become strong
  to severe, with quarter sized hail being the main risk.

- Strong to severe thunderstorms are expected this afternoon
  through the early evening hours with a Level 2 of 5 risk.
  Large 2 inch hail and damaging wind gusts to 70 MPH are the
  main threats, with a tornado and/or minor flooding from heavy
  downpours being secondary threats. Main period of concern is
  from 1 PM through 7 PM.

- Cooler than or near normal temperatures are favored for
  Wednesday into the beginning of the weekend. Still have a
  couple periods where overnight lows dip into the upper 30s,
  but widespread frost/freeze conditions not expected.

- The cooler temperatures on Wednesday/Thursday will be
  accompanied by additional showers and thunderstorms by late
  Wednesday continuing into Thursday. While severe weather is
  not expected, minor flooding issues are possible.

&&

.DISCUSSION...
Issued at 354 AM CDT Tue Apr 30 2024

Main focus for this forecast package revolves around strong to
severe thunderstorms expected today. Early morning satellite imagery
shows cloud coverage increasing west of I-29 as mid-level warm air
advection (WAA) takes place, with some returns starting to be seen
on radar. Soundings and surface observations show cloud bases are
currently above 10 kft, with dry air below keeping any rain from
reaching the ground early this morning. As the WAA continues while
expanding eastwards across the rest of the area throughout the
morning hours, expect the virga to continue as it increases the
moisture in the middle and eventually lower levels. Thus, aside from
any patchy fog, the pre-sunrise hours will remain quite.

As the upper-level trough with embedded mid-level shortwaves
continues to push towards the area from the west, WAA and moisture
advection will bring temperatures into the 60s and dewpoints into
the 50s out ahead of the cold front throughout the morning hours.
Aloft, the exit region of the upper level jet begins to reach into
the area, with positive vorticity advection (PVA) along with low/mid
level frontogenesis resulting in the first round of scattered
showers and thunderstorms increasing in coverage across south-
central and southeastern South Dakota. While instability will be
increasing throughout the morning hours, most guidance keeps MUCAPE
below 1000 J/kg, leading to a non-zero threat for quarter sized hail
with this initial push of elevated showers and thunderstorms.

The second round of thunderstorms, which are expected to be strong
to severe, will fire off of the cold front as it sweeps through
south-central and southeastern South Dakota by the late morning and
early afternoon hours. While environmental conditions vary slightly
amongst the models, temperatures in the upper 60s to mid 70s with
dewpoints in the upper 50s will lead to SBCAPE values of 1000-2500
J/kg. Bulk shear in our area looks to be around 30 to 50 knots,
though wind speeds in the mid-levels are on the weaker side, leading
to some strange looking hodographs. While confidence is high in
storms occurring there is moderate confidence on exact
location/timing, but current thinking is for storms to fire
somewhere in the vicinity of the I-29 corridor between 2-4 PM. Storm
mode is expected to be discrete at first, possibly with supercells,
and will bring chances for large 2 inch hail along with gusts to 60
mph.

As the storms interact and collide by the late afternoon hours,
storm mode is expected to become multi-cellular before congealing
into a line, though that might occur after the storms exit our
area. This will have switched the primary threat to be strong
damaging winds with gusts to 70 mph possible, though storms
initiating on the leading edge of the line may still produce large 2
inch hail. Given 0-1km helicity of 100-200 m2/s2 out of the south-
southwest across northwestern Iowa and southwestern Minnesota, can`t
rule out isolated tornadoes for any portion of the line that is
moving more northeastwards, but in general the tornado threat looks
to stay on the lower end. Severe weather threat looks to end in the
early evening hours for our area as the cold front clears our
eastern counties by around 7 PM. In addition to the severe threat,
can`t rule out some flooding concerns given the already high soil
moisture content and rising rivers. 1 hour flash flood guidance is
sitting in the 1-1.5" range, so storms that are producing rainfall
rates in excess of 1.5"/hr will need to be monitored, though the
speed of the thunderstorms should help to negate any widespread
flooding issues.

Wednesday and Thursday will see cooler conditions as the surface
thermal gradient resides off to our southeast, keeping the warmer
temperatures out of reach for the area. Wednesday itself looks to
remain dry for majority of the day, but as shortwaves from a cut-off
upper level low along the international border move across the area
we`ll see precipitation chances increase throughout the late
afternoon hours, with the best rainfall chances occurring overnight
into Thursday. With rainfall amounts in excess of an inch
possible, given the high soil moisture content minor flooding
concerns are possible and will need to be monitored. Severe
weather looks to remain low as we remain on the wrong side of
the thermal gradient.

The main wave pushes through late Thursday which ends that round of
rainfall, though the dry period looks to remain short as an
additional shortwave takes aim at the region on Friday. Ensemble
clustering analysis shows there is decent agreement amongst the
various ensemble members, so have left NBM pops as is, which
increase rainfall chances during the evening hours on Friday and
continuing into Saturday. Upper level ridging on Sunday leads to
warming temperatures across the region, though model solutions
diverge as they disagree on how to handle the cut-off upper level
low that moves onto the west coast on Saturday.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1039 PM CDT Mon Apr 29 2024

Low VFR stratus across southwestern MN continues to move east,
as mid/high clouds continue to move into the MO River Valley.
Although some hi-res guidance has backed off, continuing to
monitor for potential fog development overnight from roughly
KBKX to KLYV to south of KSPW, which could lead to MVFR/IFR
visibility. Guidance continues to keep fog to the east of KFSD,
but will continue to monitor.

Light and variable winds through daybreak. Winds begin the day
southeasterly with gusts 20-25 knots, but switch to westerly
then northwesterly as a cold front swings through during the mid
day hours into the evening.

Chances for showers and thunderstorms return late morning ahead
of the cold front, then during the afternoon and evening with
the front. Showers and storms will be mostly discrete in the
early part of the day, forming a line along the front after
roughly 30.20z. Some isolated showers and storms may linger into
the late evening hours, but generally expect them to be out of
the forecast area by 30.23z to 01.01z. MVFR/IFR conditions are
possible with thunderstorms as well as locally heavy rainfall.
Some storms could be strong to severe with large hail and
damaging wind gusts.

&&

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

&&

$$

DISCUSSION...APT
AVIATION...SG