696
FXUS63 KFSD 310934
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
434 AM CDT Fri May 31 2024

.KEY MESSAGES...

- Scattered showers/storms near and southeast of a Yankton to
  Sioux Falls to Marshall line this morning become more numerous
  through the afternoon and evening. Risk of severe weather is
  low, but locally heavy rainfall is possible.

- Warmer and more humid air build northward Sunday, leading to
  growing instability and a Level 2 of 5 risk for severe storms,
  mainly during the afternoon and evening. All modes of severe
  weather appear possible.

- Active pattern continues into early next week with seasonable
  temperatures through Tuesday, followed by slight cooling
  midweek.

&&

.DISCUSSION...
Issued at 401 AM CDT Fri May 31 2024

TODAY-SATURDAY: Weak surface boundary located near a Yankton to
Sioux Falls to Marshall line early this morning drifts slowly
southeast today toward the IA/MN Highway 60 corridor. Similarly,
925mb-850mb boundary stall in a southwest-northeast orientation
across the CWA by midday, before lifting back to the north through
this afternoon and evening. These boundaries delineate modestly
unstable air over the southeast half of the forecast area from much
more stable air across the northwest. Models in fair agreement
showing a mid-upper level wave lifting northeast along the stalled
boundaries during the afternoon, which should result in an increase
in coverage of showers and scattered thunderstorms along and
southeast of the boundaries. While MUCAPE may top 1000J/kg at times,
instability profiles are tall/skinny with weak mid-level lapse
rates. Additionally, deep layer shear is practically non-existent,
so not anticipating severe storms. However, precipitable water
values approach 1.25-1.5 inches, and weak mid-level flow could lead
to slow moving storms which will likely be efficient rain producers.
While not expecting extreme rainfall rates, HREF indicates moderate
probabilities (40-60%) of total rainfall exceeding 1.00" in many
areas east of a line from Vermillion to Canton SD to Slayton MN,
where rainfall thus far in the Spring has been well above normal.
Will have to monitor for ponding in low-lying/poor drainage areas,
as well as renewed rises on rivers like the Little Sioux which are
still running near to just above flood stage.

This activity should wane through the late evening-overnight hours
as the upper forcing lifts northeast and the low level boundaries
weaken. This should lead into a fairly nice Saturday across the
region, as surface high pressure provides light winds and most areas
see plentiful sunshine. Most models keep greater instability locked
south of our forecast area across Nebraska with our area remaining
precip-free through the day. RAP/HRRR have been somewhat consistent
showing an area of convection developing near/south of the Missouri
River Valley, so will carry some low chances in our far south during
the afternoon.

SUNDAY-SUNDAY NIGHT: This period will be the primary focus as far
as potential for severe storms through this forecast period. As
the aforementioned surface high slides east, increasing southerly
flow will draw warmer and more humid air northward into our area.
Consensus focuses a plume of moisture ahead of an approaching
cold front, through the I-29 corridor and into far western MN/IA
near US Highway 75 (Canby-Pipestone-Sioux City) by mid-afternoon
Sunday. More conservative models indicate MUCAPE approaching
2000-2500J/kg, and indeed ensembles show a moderate (40-70%)
probability of SBCAPE topping 2000J/kg Sunday afternoon, with
high (>80%) probability of exceeding 1000J/kg. While shear is
not excessive, joint probabilities of CAPE > 1000J/kg where
deep layer shear > 30kt are also moderate (50-75%) across the
western/central portions of the forecast area.

These parameters should support organized storm clusters
developing in the mid to late afternoon in central-eastern South
Dakota as a mid-upper level trough moving across the Dakotas
interacts with the unstable airmass. Currently large hail and
damaging winds look to be the primary threats. However, a few
tornadoes appear possible, especially near boundaries where low
level shear/helicity is enhanced. In addition, the influx of
deeper moisture with precipitable water values topping the 90th
percentile of climatology in both the NAEFS and ECMWF ensembles
supports a potential for locally heavy rainfall. This activity
should weaken somewhat as it slides into our eastern counties
during the overnight hours, as it moves into a less favorable
air mass with decreasing instability.

MONDAY-THURSDAY: The first week of June looks to remain a bit
unsettled, at least for the start, as a deepening trough moves
into the northern Plains by Tuesday-Tuesday night time frame.
Temperatures remain near to just above normal as we start the
week ahead of this trough, which will bring another chance for
showers and storms to the forecast area. Details are far from
certain, but may have to monitor this period for additional
strong to severe storms.

Slightly cooler air briefly settles in behind the trough for
Wednesday and Thursday, which look to favor drier conditions
ahead of an upper ridge building over the Rockies.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1027 PM CDT Thu May 30 2024

Scattered showers and thunderstorms are ongoing across the region
this this evening and are expected to continue through the
overnight. A few storms may become strong to severe with the
greatest threats being wind gusts of 60 mph and hail up to the size
of quarters. Ceilings are currently VFR, but expect these to bounce
around a bit between VFR and MVFR, especially under any stronger
storms that develop. Visibility may also be reduced under stronger
storms. Winds have decreased to mostly light and variable. By
daybreak they will become east-northeasterly at 5-10 kts. Daybreak
sees another round of showers over central South Dakota and move
east-northeast across the region through Friday afternoon. Severe
weather is not expected with Friday`s showers.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...AJP