490
FXUS63 KFSD 290904
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
404 AM CDT Mon Apr 29 2024

.KEY MESSAGES...

- Remnant rain showers will be ending throughout the morning
  hours, with clouds moving out during the afternoon giving way
  to partly/mostly sunny skies.

- Severe weather threat increases Tuesday afternoon through
  early evening with a Level 2 of 5 risk. Large hail to the size
  of golf balls 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 8 PM.

- Cooler than or near normal temperatures are favored for
  Wednesday and beyond. A few periods of low temperatures in the
  30s and lower 40s suggest frost/freeze concerns will linger
  into early May.

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

&&

.DISCUSSION...
Issued at 404 AM CDT Mon Apr 29 2024

As our surface and upper level lows continue to slowly drift
over/out of the region, early morning satellite imagery shows
abundant cloud coverage remains in place. 2 AM radar imagery shows
remaining rain showers are moving northwards into northeastern SD
and southwestern MN, with surface temperatures in the upper 30s to
lower 40s along and west of I-29, while areas east of I-29 are
reaching up into the upper 40s and lower 50s. Can`t rule out some
light additional rain showers into the mid-morning hours, especially
for those along and north of the highway 14 corridor, given
sufficient moisture and positive vorticity advection from the exiting
upper level low.

By the late morning hours, dry air begins to filter into the region
from west to east, and will be slowly decreasing cloud coverage for
the rest of the day. As a result, highest temperatures today will be
occurring for areas in south-central South Dakota and those along the
Missouri River valley getting up into the lower 60s, while the rest
of the region will reside in the lower to upper 50s. As the surface
pressure gradient (SPG) remains tightened, expect semi-breezy west-
northwesterly to continue before they weaken this evening as they
transition to southeasterly winds overnight into Tuesday.

The change in wind directions is due to our next elongated surface
low pressure moving in from the west, with the affiliated shortwave
moving along with it coming off the Rockies. The warm front will be
pushed northwards across the area throughout the morning hours,
bringing temperatures in the lower to mid 60s for areas west of the
James River Valley before the cold front moves through during the
morning hours, and warmer mid 60s to lower 70s expected elsewhere
well ahead of the eastward progressing cold front. While there are
minor nuances amongst the models, most bring 1-2 kJ/kg MUCAPE and 40
to 50 knots of 0-6km shear straddling the I-29 corridor shortly
after noon, with everything shifting eastwards as the cold front
progresses eastwards into the early evening hours. Moisture return
behind the warm front is one area we`ll need to keep an eye on as a
point of failure, as less moisture would lower the available
instability, but for now convection allowing models would suggest
that isn`t a problem for the time being.

So, we`ll be watching morning showers and thunderstorms west of I-29
moving eastwards ahead of the cold front, with the greater
instability moving into the area throughout the morning hours.
Current thinking is the event will start off between 1 to 3 PM with
supercells given the favorable deep layer shear, with the primary
threat being large hail to the size of golf balls and damaging wind
gusts to 60mph in the vicinity of the I-29 corridor. As storms
collide into a linear/cluster mode with a well established cold
pool, the main threat switches towards strong damaging wind gusts to
70 mph, though can`t rule out storms creating large hail on the
leading edge of the line. 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. Severe weather threat looks
to end in the early evening hours for our area as the cold front
clears our eastern counties by around 8 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 closely.

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
start off dry, 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 day, with the best
rainfall chances occurring overnight into Thursday. Again, given the
high soil moisture content, minor flooding concerns are possible and
will need to be monitored, but severe weather looks unlikely as we
are on the wrong side of the thermal gradient.

The main wave pushes through late Thursday which looks to end most
of our precipitation chances, though the NBM holds onto light rain
chances as some ensemble members show rainfall chances continuing.
Near to below normal temperatures look to continue into Saturday,
with temperatures and precipitation chances becoming less clear as
model solutions diverge into the beginning of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1035 PM CDT Sun Apr 28 2024

Showers and isolated thunderstorms begin to diminish from south
to north later tonight into Monday morning. Ceilings will
generally be in the IFR range, though may occasionally jump into
the MVFR range. By Monday afternoon, ceilings will lift, then
scatter out by late afternoon. Clouds decrease on Monday
evening. Winds will be west/northwesterly tonight into Monday,
gusting around 20 kts at times into mid evening on Monday. Winds
then drop off by late Monday evening.

&&

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

&&

$$

DISCUSSION...APT
AVIATION...JM