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000
FXUS63 KFSD 280911
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
411 AM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Periods of light to moderate showers spread north through
  tonight. Moderate to high (60-80%) probability of additional
  rainfall exceeding 0.50" for most through Monday morning,
  with a low (15-30%) probability of exceeding 1.00".

- Severe weather risk is low through tonight, but there is still
  a highly conditional Level 1 of 5 risk for a strong to severe
  storm toward Ida Grove/Storm Lake this afternoon/evening.

- Increased severe weather risk for Tuesday afternoon, with a
  Level 2 of 5 risk in northwest Iowa as a cold front moves into
  a warm and modestly moist air mass. Still some uncertainty in
  timing/location of storm development.

&&

.DISCUSSION...
Issued at 400 AM CDT Sun Apr 28 2024

TODAY-MONDAY: Focus for the next 24-36 hours will be mid-upper
level trough spinning into Nebraska early this morning, as it
slowly lifts northeast across the northern Plains through
Monday. Broad lift ahead of the trough should result in
increasing coverage of showers across the forecast area today,
initially focused to the west of I-29 early this morning, but
expanding east mid-late morning in response to broad warm
advection and the mid level front lifting north ahead of the
trough. As the trough deepens/becomes negatively tilted through
over the Dakotas through this evening, precip distribution
begins to resemble a winter system with a dry slot punching into
Iowa/southern Minnesota this evening, while precipitation wraps
around the north and west sides of the system. Eventually, this
wrap-around precipitation will lift northeast with the departing
trough, but could see light showers linger across our north into
Monday morning. Overall, looking at additional rainfall amounts
exceeding 0.50" over much of the area (60-80% probability of
exceedance in recent ensembles) with lower (15-30%) probability
of exceeding 1.00". Highest of these low probabilities would be
in the wrap-around area across eastern South Dakota, with lower
probabilities in northwest Iowa.

With this strengthening system and aforementioned dry slot,
still have to give some consideration to a low severe weather
risk across the southeast portions of the CWA. Increased mid to
upper level winds ahead of the trough provide ample bulk shear
during the afternoon/evening. However, a couple of questions
remain. First, is whether clouds/rain will decrease in time to
allow for even modest destabilization. Latest HREF maintains
around a 50% probability of surface-based CAPE exceeding 300-500
J/kg in our extreme southeast counties (southeast of Sioux City
to Storm Lake), but soundings reveal instability profiles that
are very thin, with near moist adiabatic mid-level lapse rates
less than 7g/kg. Second question is exactly where the surface
low and attendant warm front will be located by late afternoon.
Models still vary quite a bit, though greater consensus still
tracks the low through northwest Iowa. If greater instability
can develop, juxtaposition with the weak convergence around the
boundary could support an isolated stronger storm. Overall, the
risk of severe weather appears low for this afternoon/evening,
but will need to monitor trends through the day.

As for temperatures, the next couple of days will remain on the
chilly side of normal. Widespread cloud cover/rain across most
of the CWA today should limit temperatures from moving much from
current early morning levels, and expect highs ranging from mid
to upper 40s across our west and north, to perhaps around 60 in
our far southeast. Wedge of colder air wraps around the back
side of the departing low on Monday, which will again limit
highs to the 50s most areas.

TUESDAY: Brief reprieve from this wet pattern later Monday into
Monday night, as surface and brief upper ridging move across the
region. Increasing southerly flow will bring much warmer air
northward Tuesday, ahead of our next trough which moves east
across the northern Plains later Tuesday-Tuesday night. While
some uncertainty in timing of the expected trough/cool front,
thermal ridge pushes 850mb temperatures into the 10+C range by
Tuesday afternoon, with a few solutions showing 925mb temps
nearing 18-20C. This would support highs into the 70s across
parts of the eastern CWA, with some 80s not out of the question,
while moisture advection draws dew points in the 50s northward.
This lends itself to increasing instability by Tuesday afternoon,
1000-1500J/kg as mid-level lapse rates steepen to 7.5-8C/km,
though soundings do show a weak capping inversion. Modest deep
layer shear ahead of the approaching trough as well, but as we
often see with cold fronts in this area, greater instability
seems to be displaced ahead of the stronger shear. That said,
could see a few strong to severe storms with large hail and
damaging winds the primary threats.

WEDNESDAY-SATURDAY: Mild air remains in place mid-week, but
another deepening trough could bring a shot of cooler air back
into the region by the end of this week. Details on the timing
and strength of the late week system are still low confidence,
as are temperatures with growing spreads between the NBM 10th
and 90th percentiles from midweek onward. Periodic rain chances
continue through the latter half of the week ahead. Chances at
this time seem focused around later Wednesday into Thursday,
and again toward Saturday, but similar to temperatures, details
are uncertain and further refinement is likely in the coming
days.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1030 PM CDT Sat Apr 27 2024

Showers will gradually overspread the area late tonight into
Sunday evening, with visibilities falling into the MVFR range.
While MVFR ceilings will be predominate earlier tonight, look
for lowering into the IFR/LIFR range later tonight into Sunday.
Winds will be northeasterly through the period, with gusts 20 to
25 kts late tonight through early Sunday evening, then tapering
downward late in the evening.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...JM