670
FXUS63 KFSD 270905
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
405 AM CDT Sat Apr 27 2024

.KEY MESSAGES...

- Mostly dry through the day today, with perhaps spotty
  sprinkles or drizzle. Showers and a few storms return tonight
  and Sunday, ending by early Monday. A few strong to severe
  storms possible in portions of northwest Iowa, though
  confidence in timing/location is low.

- Additional rainfall of 0.50-1.00" for many areas through
  Sunday night, with locally higher amounts possible. Ponding of
  water in low-lying or poor drainage areas is possible, and
  will have to monitor isolated areas which received heavier
  rain on Friday.

- 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 400 AM CDT Sat Apr 27 2024

TODAY: Widespread low stratus still blankets much of the forecast
area this morning, and with weak cold advection in the wake of our
departing storm system, do not expect to see much in the way of
clearing today. Despite the dreary skies, breezy northwest winds,
and below normal temperatures for most areas, today should be the
nicer day of the weekend. Low-level thermal boundary lingers east of
IA/MN Highway 60 this morning, and is expected to slowly drift east
through this afternoon. Locations southeast of the boundary will
have the best chance to see periods of sun that could help push
temps into the lower to perhaps mid 60s, while the stagnant clouds
west of the boundary hold readings in the 50s.

TONIGHT-SUNDAY NIGHT: Our next mid-upper level wave begins to slide
northeast toward the CWA tonight, becoming negatively-tilted and
deepening as it moves across the Dakotas and Minnesota through
Sunday night. Expect rain to expand in coverage through tonight as
a lead mid-level wave lifts north, and the 850mb boundary rotates
back to the west/northwest as an inverted trough, allowing for
broad warm advection ahead of the upper trough. Surface to 925mb
boundary looks to remain near the extreme southeast corner of our
CWA, if not just outside. As such, instability remains extremely
limited across our forecast area through daybreak Sunday, perhaps
100-200 J/kg of MUCAPE in our far southeast late tonight with
mid-level lapse rates near- below 7C/km. Thus think the threat
of strong-severe storms tonight is low, but could see pockets of
heavier rain.

Still some uncertainty for Sunday with respect to severe potential,
especially regarding the track of surface low and timing/location of
the attendant warm front by mid afternoon. Greater consensus appears
to have the low lifting northeast near to just east of the Highway
60 corridor during the late afternoon to early evening. Latest RAP
projections show potential for a ribbon of deep layer shear >40kt in
northwest Iowa Sunday afternoon, as CAPE values near/south of the
aforementioned warm front climb to around 1000J/kg. This, of course,
will be dependent on ability for these areas to recover from morning
rains, with stratus clearing enough to allow for sufficient warming.
However, cannot disagree with the potential for isolated strong to
severe storms in parts of northwest Iowa Sunday afternoon as seen in
the SPC Day 2 Marginal (Level 1 of 5) Risk if this destabilization
is realized.

The trough and associated surface low continue to lift northeast
of the forecast area, which should gradually bring an end to the
rain by late Sunday night. While additional rainfall amounts with
this second system will again be more variable due to convective
influences, looking at the potential for a broad 0.50-1.00" of rain
across the area. While we did see pockets of rainfall in excess
of 2.00" over the past 24-36 hours, broader rain totals averaged
0.50-1.50". The additional rain, which is not expected to fall at
excessive rates given the limited instability in most areas.

MONDAY-FRIDAY: The region remains in a quasi-zonal to broad
troughing pattern through much of the week ahead. Temperatures
slowly recover to near-above normal by Tuesday-Wednesday, but a
stronger trough seen in some models dropping into the northern
Plains by Thursday could bring cooling back for the end of the
week. Latest runs of the deterministic GFS/ECMWF/Canadian show
modest agreement in some sub-zero 850mb temperatures plunging
south across the Dakotas/Minnesota by Friday. However, their
respective ensembles show a much broader spread, with the NBM
10th-90th spread still exceeding 20F across the area. Latest NBM
high temp for Friday in the mid 50s-lower 60s is on the cooler
side the spectrum, near to slightly below its 25th percentile,
but this does not look unreasonable given the cooling seen in
the deterministic model solutions. Will not alter highs given
the high degree of uncertainty, but would expect to see further
flucuations in late week temperatures over the coming days.

As far as precipitation chances, there are multiple shortwaves
sliding through the fast flow aloft, though timing/strength
varies among the solutions. First wave looks to be focused on
the daytime hours Tuesday, but not enough confidence at this
range to make adjustments to the generally low NBM pops beyond
that period.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1020 PM CDT Fri Apr 26 2024

A mix of VFR to LIFR tonight as showers and thunderstorms
continue to move off to the northeast. These storms are not
expected to be severe, but visibility could be reduced to 4
miles under heavier rainfall. Clouds will be persistent through
the period, reducing ceilings to around 500 ft AGL, gradually
increasing through Saturday afternoon to 1500 to 2500 ft AGL,
with the highest ceilings south of I-90. Toward the end of the
period ceilings deteriorate again as a second low pressure
system begins to move into the area.

The current low pressure is situated roughly over eastern South
Dakota. Winds are highly dependent on your location in relation
to the low. KHON has winds out of the northwest, KFSD out of
the west, and KSUX out of the southwest. Winds will be veering
through the period, eventually settling to the northeast by the
end of the period. Winds will be slightly breezy, 10-15 kts
sustained with gusts around 20 kts decreasing through Saturday
afternoon. Winds increase again and shower and thunderstorm
chances return late Saturday night into Sunday.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...AJP