682
FXUS63 KFSD 282244
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
544 PM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Intermittent periods of light to moderate showers will
  continue to spread north through late tonight. Additional
  accumulations between 0.25" to 0.75" of an inch are expected
  through Monday morning.

- There are still some low probabilities of an isolated severe
  weather risk across northwestern IA this evening with large
  hail up to half dollar size being the main threat.

- Severe weather threat increases Tuesday afternoon through
  early evening with a Level 2 of 5 risk. Large hail is the main
  threat, but damaging wind gusts, a tornado, and/or ponding of
  water from heavy downpours are secondary threats.

- 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.

&&

.DISCUSSION...
Issued at 237 PM CDT Sun Apr 28 2024

REST OF THE AFTERNOON/TONIGHT: A wet and dreary day continues!
Taking a look across the area, light to moderate showers along
with a few thunderstorms continue to push northward ahead of the
surface low which is currently positioned somewhere over
northeastern Nebraska based on the wind field. With cold front
sitting just west of the NE/IA border and the warm front lifting
across northwestern IA; we`re starting to see more convective
showers develop with some embedded thunderstorm as lift and
instability (300-500 J/kg) increases along the cold front. As
this system continues its slow walk towards the northeast,
expect most of our current activity to either rotate to the
north or northeast into the evening hours. Shifting gears to our
severe weather chances; low confidence continues with a highly
conditional environment this evening.

Between the mostly stable environment brought from this
morning`s rain and the lingering stratus deck across our area of
interest this afternoon, i`m still not particular convinced that
things can get going this evening. With that being said, there is
still a conditional chance for a few stronger storms between the Hwy-
20 corridor and areas east of Hwy-59. Further analysis of RAP
soundings at Spencer (SPW) and Sioux City (SUX) shows long skinny
CAPE profiles (700-1000 J/kg) along with 30-40 kts of bulk
shear. This combined with lapse rates varying between 6-7 degree
C/km could lead to a few storms with some large hail up to half
dollar size this evening with the best window being between 23z
to 04z. However, these chances will be conditionally dependent
on if the showery activity can clear out across northwestern IA
in time for the atmosphere to destabilize. If not, a few
moderate to heavy downpours will be possible with any developing
activity at best. Lastly, as cooler air begins to pool into the
area behind the surface low, temperatures will drop into the
upper 30s to low 40s for the night.

MONDAY: Looking into Monday, most of the lingering shower
activity will continue to lift northeastwards during the morning
hours as the surface low slowly rotates into central MN.
Northwesterly surface winds will increase throughout the day as
the SPG tightens as an upper-level ridge approaches. Wind gusts
between 15-25 mph are expected across the area. Otherwise,
mostly quiet conditions should return by Monday afternoon as
increasing warm air advection (WAA) underneath the ridge brings
our 850mb temperatures back towards the mid to upper teens by
the afternoon. This along with decreasing cloud cover during the
day should allow our temperatures to recover slightly with
highs expected to vary between the low 50s to low 60s across the
area with the warmest conditions expected along the Missouri
River Valley. Unfortunately, the quieter condition won`t last
for long as our attention pivots to the Rockies as a negatively
tilted trough ejects into the northern plains during the day on
Tuesday.

TUESDAY: Warm air advection and southerly return flow bring a much
warmer day Tuesday than the previous few days. Expect temperatures
returning to near to above normal values with highs in the mid 60s
to mid 70s. Breezy south gradient winds start the day, shifting
southwest and west behind a passing cold front. Gusts will peak in
the upper 20s and 30s.

Main focus on Tuesday will be the threat for scattered strong to
severe thunderstorms in the afternoon and evening. Despite initial
zonal flow aloft, a jet streak digs the upper trough over the
Northern Rockies and bringing broad upper level support for ascent
between the upper jet and approaching negatively tilted upper
trough. Despite scattered showers and storms in the morning in
central SD, capping should greatly limit severe weather concerns
until after noon. Moderate to high confidence in rain occurring with
GEPS/GEFS and CIPS analogs favoring at least isolated strong to
severe storms Tuesday afternoon and early evening due to narrow axis
of 1-2 kJ/kg MUCAPE and 50+ kts of 0-6 km shear. 28.12z model
soundings suggest skinny CAPE profiles and 7-8 C/km mid level lapse
rates, and a not-so-favorable weak cold frontal passage suggests
elevated storms near and west of I-29 with a linear mode. Though
there is favorable low level curvature in hodographs, stronger mid
level winds hold off until late in the day as do dew points
exceeding 60F. Can`t rule out a tornado in northwest Iowa if the
right conditions develop Tuesday evening, but the tornado threat is
certainly greater further southeast in central Iowa. Additionally
precipitable water values exceeding 1" (90th percentile of
NAEFS/EFS) favor heavy downpours with some locations receiving over
1 inch of rain. Expect the main threats with the strongest storms to
be large hail and to a lesser extent, damaging wind gusts.

WEDNESDAY AND THURSDAY: Drier air aloft settles in with sfc high
pressure overnight. With the upper low well to our north and the
cold front sagging south to between the I-80 and I-70 corridors,
ensemble clusters aside from 40% of GEFS members suggest Wednesday
should be dry for many sites. Increased cloudiness with peeks of
sunshine Wednesday should make for a fairly nice day with highs in
the 60s to mid 70s.

Another round of convection should fire in the Central Plains
Wednesday afternoon and/or night, forced by an ejecting CO Low and
associated low level jet. While chances for showers and storms
increase Wednesday night and Thursday both from another mid level
disturbance sliding in from the west and convective remnants in
northwest Iowa, at this stage it seems unlikely that we will
destabilize enough (MUCAPE < 500 J/kg) to see another round of
severe weather threat.

FRIDAY AND NEXT WEEKEND: Low confidence in the forecast for Thursday
night and beyond as models struggle to handle the interaction
between an upper low stalled over the Canadian Prairie and an upper
trough approaching from the West. Near to below normal temperatures
are favored for the end of the week and weekend. With lows dipping
into the 30s and 40s, frost or a freeze still remains possible into
early May. CPC highlights a slight (20%) chance of heavy rainfall
over much of the Central U.S. for next Saturday and Sunday, so will
want to monitor that period if you have outdoor activities
planned.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 540 PM CDT Sun Apr 28 2024

Scattered showers will continue tonight, with an isolated
thunderstorm possible over northwestern IA during the evening.
Ceilings will generally be in the IFR range, though may
occasionally jump into the MVFR range. Showers begin to
diminish from south to north later tonight into Monday morning.
By Monday afternoon, ceilings will lift, then scatter out
toward the end of the TAF period.

&&

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

&&

$$

DISCUSSION...Gumbs/BP
AVIATION...JM