000
FXUS63 KFSD 260944
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
444 AM CDT Fri Apr 26 2024

.KEY MESSAGES...

- Widespread rain/embedded thunderstorms early this morning
  diminish in coverage after daybreak. Strong to severe storms
  develop this afternoon into this evening (greatest risk
  3pm-9pm).

- Rain chances decrease late tonight, with a mostly dry
  Saturday. However, showers/storms return Saturday night and
  Sunday. A few strong to severe storms possible in portions of
  northwest Iowa.

- Through Sunday night, widespread beneficial rainfall of an
  inch or more is expected (>80% chance). Moderate chances
  (40-60%) for more than 2 inches, focused across northwest Iowa
  and adjacent areas.

- Unsettled pattern continues through next week, with periodic
  rain chances, though precipitation chances and temperatures
  are both low confidence forecast due to widely varying model
  solutions.

&&

.DISCUSSION...
Issued at 444 AM CDT Fri Apr 26 2024

THIS MORNING: As anticipated, we`re seeing widespread rain in the
broad warm advection regime ahead of an approaching mid-upper level
trough early this morning. Instability is on the weaker side except
perhaps across our far south, so thunder is largely limited to the
southern portions of the forecast area. Expect this area of rain to
diminish from the south/southwest after daybreak as the lead wave
lifting north ahead of the trough slides north of the forecast area,
and mid-level drying spreads across the CWA. Low level moisture
remains firmly in place, though, and with weak lift continuing,
could see spotty light rain/drizzle persist even after the more
widespread moderate rain lifts north.

THIS AFTERNOON/TONIGHT: Our attention then turns to the main low
pressure system which will slowly lift northeast through central
Nebraska this afternoon, then track across our forecast area
tonight. The track appears to have shifted slightly westward from
projections at this time yesterday, but timing is similar. Still
some questions on ability to see sufficient clearing work into our
far southern CWA by mid-late afternoon to allow for stronger
destabilization. However, the southerly mid-level jet ahead of the
negatively-tilted trough results in very strong deep layer shear
(better than 45-50kt in several models), which could overcome the
weaker instability (high shear/low CAPE environment) and still
support at least elevated storms (possibly supercells) with a threat
of hail up to golf ball size across the southeast quadrant of our
forecast area (generally east of Hwy 81 and near/south of I-90)
where mid-level lapse rates top 7.5-8C/km. This corresponds well
with the SPC Slight (Level 2 of 5) severe weather risk for today.

Back to the uncertainty, still some high-res models which show the
low clouds breaking up/clearing through portions of the Missouri
River Valley by mid-late afternoon as the warm front lifts north,
while others stand firm on the warm front remaining farther south
and the low clouds persisting into this evening. Those that show the
potential clearing would warm enough to break the surface-based
capping inversion, with 0-1km shear of 30-40kt and LCL heights below
2kft supporting a risk of tornadic supercells. Again, this would
largely be focused near the surface boundary/clearing line, with
storms likely becoming elevated with a reduced tornadic threat as
the move north of the boundary.

Potential severe window looks to be relatively short, perhaps
beginning as early as 3-4pm in the Missouri River Valley, and
exiting our eastern CWA by 8-9pm this evening as the wave slides
east. Thereafter, expect coverage of showers/storms to gradually
wane through the overnight hours. Given the expected quick-moving
storms, think any threat of overland flash flooding is low. However,
storms may contain rainfall rates in excess of an inch per hour so
localized ponding in low-lying/poor drainage areas or urban areas
prone to flooding may see brief periods of standing water.

SATURDAY: This could end up being a relatively nice day, certainly
the nicer day of the weekend, as brief mid-upper level ridging keeps
much of the CWA dry through the day. Low-level thermal boundary will
likely be lingering southwest-northeast somewhere in the CWA though,
with consensus pointing to near to southeast of the IA/MN Highway 60
corridor. This will separate widespread cloudiness & cooler temps to
the northwest from at least partial sunshine and near normal temps
southeast.

SATURDAY NIGHT/SUNDAY: A second potent mid-upper level trough
swings northeast across the region during this period, though a
little less stacked than the system today. Greatest uncertainty
in this period is where the low level boundary will be located,
along with the exact track of the mid-upper level trough which
still has some variance among the models. Higher confidence
that much of the forecast area will again see rain at times
later Saturday night-Sunday, with much lower confidence in
potential for any strong-severe storms which may occur near/east
of the aforementioned surface boundary. Timing also differs
from today in that the system reaches our area earlier in the
day, so not working with max diurnal heating. Nevertheless,
still see a period of strong shear ahead of the wave from late
Saturday night into Sunday afternoon, so cannot rule out
isolated strong to severe storms across our eastern counties.

Combining both of these systems together, we have high (>80%)
probabilities of seeing widespread rainfall amounts exceeding an
inch by Monday morning. The greater potential for 2 inches or
more remains focused across northwest Iowa and nearby areas,
where periods of deeper convection are more likely with each of
the systems.

MONDAY-THURSDAY: Mid-upper level flow pattern turns a bit more
zonal after the Sunday wave exits, but indications of one or
more stronger waves sliding across the northern Plains through
early-mid week which could bring at least scattered rain chances
at times. Temperatures return to near or slightly above normal
for the transition to May with highs back into the 60s to at
least lower 70s. May have to monitor temps toward the end of
this week, though, as latest GFS/ECMWF bring a shot of colder
air southward behind one of the stronger waves in the latter
half of the week. Wide variance in solutions on this, though,
with better than a 20F spread between the NBM 10th and 90th
percentiles for high temperatures on Thursday.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1108 PM CDT Thu Apr 25 2024

VFR ceilings have begun to deteriorate as stratus moves in, and will
continue to deteriorate to MVFR with scattered IFR for the remainder
of the period. Scattered showers and thunderstorms have begun across
the region. This first round is expected to remain sub-severe,
however dime to nickle size hail and stronger wind gusts up to 50
mph under thunderstorms are possible.

Winds are out of the southeast at 15-20 kts sustained with gusts 30-
35 kts. The highest gusts will shift from west to east during the
overnight period and will be predominate over northwestern Iowa by
Friday morning. Winds will gradually decrease through the afternoon
to less than 20 kts. There exists some concern for LLWS at KSUX near
the end of the period. Directional and speed shear may be present
between 1500 and 2000 feet AGL at 160 degrees and 45 kts.

A second, stronger round of showers and thunderstorms will develop
over northern Nebraska in the next few hours. Some of these storms
could be strong to severe with hail up to an inch and damaging wind
gusts. Several additional waves of showers and thunderstorms are
expected through Sunday as a low pressure system moves through the
area. Some of these storms could again be strong to severe.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...AJP