289
FXUS63 KFSD 231811
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
111 PM CDT Thu May 23 2024

.KEY MESSAGES...

- Strong to severe storms move across the northern Plains
  tonight. Timing of initiation and stronger storms has been
  delayed 1-3 hours from previous forecast. Main hazards will be
  large hail/isolated damaging wind with initial storms west of
  the James River early evening. This transitions to increasing
  threat of damaging winds through portions of the Missouri
  River to Hwy 20 corridor late evening to overnight, with
  isolated embedded tornadoes also possible.

- Breezy conditions with south winds gusting as high as 30-35
  mph this afternoon. Gusty winds continue for Friday as winds
  become northwesterly gusting to 30-40 mph.

- Periodic showers and thunderstorms for Memorial Day weekend.
  While we are not currently expecting strong storms, lightning
  is still possible. When thunder roars, go indoors!

&&

.UPDATE...
Issued at 111 PM CDT Thu May 23 2024

Isolated showers/storms have been developing in portions of the
James River Valley this midday, along the eastern edge of the
EML characterized by steeper 8-9C/km mid-level lapse rates.
Expect this activity to weaken/lift north through mid-afternoon.

Attention then turns to surface low currently over northwest SD,
with attendant dryline/cold front extending south into the
Nebraska panhandle, as well as associated mid-upper level trough
swinging through the northern Rockies. Seeing good consensus
among various models in farther westward storm initiation as
the surface boundaries move into central SD late this afternoon,
which delays timing of convection reaching our far western CWA
until closer to 7-8pm CDT. This initial activity should be more
discrete cells in central SD, but given the delayed timing,
storms may be somewhat weaker as they approach our western
areas. Still looking at a threat of large hail up to golf ball
size, along with isolated damaging winds up to 70 mph with this
activity.

As storms progress east through the evening and deeper forcing
associated with the upper level trough arrives, expect storms to
evolve into a linear system, complete with bowing segments that
will translate east along and south of the Missouri River corridor
and portions of northwest Iowa into early Friday morning. Still
some uncertainty regarding how far north the better instability
will extend, but seeing modest consensus in the CAMs that support
a threat of damaging winds up to 70+ mph moving east through at
least our southern tiers of counties, roughly along/south of a
line from Yankton to Le Mars to Storm Lake. 0-3km shear vectors
are forecast to be WSW around 30kt at the leading edge of the
line, which could support circulations and perhaps isolated
embedded tornadoes in portions of the QLCS oriented NW to SE,
which appears to more likely be the portion of the line near and
north of Hwy 20. The main question regarding the tornado threat
would be nocturnal timing of the line, as it would be fighting
a somewhat more stable layer below about 2kft AGL.

Storms look to exit our northwest Iowa counties by around 3-4am,
though some trailing showers/scattered storms may linger through
shortly after daybreak.

&&

.DISCUSSION...
Issued at 308 AM CDT Thu May 23 2024

THURSDAY: Southerly winds will increase throughout the morning
as a low pressure system approaches from the west, tightening
the SPG. Look for sustained winds to peak around 15-20 mph with
gusts 25-30 mph by mid-afternoon, highest through the James
River Valley. Plentiful WAA will warm our highs into the upper
70s and 80s for today. Our southerly winds will also transport
modest moisture northward, helping dewpoints to climb into the
mid-upper 50s west of the James River Valley and low 50s east.
These will support some moderate instability of around 1500-2000
J/kg MUCAPE over central South Dakota. Model soundings continue
to indicate a capping inversion today, that will erode quickly
once we reach peak afternoon heating. At the same time the cold
front will be approaching from the west, forcing convection
initiation. Strong vorticity from the top of the boundary layer
through the mid-levels, coupled with 35-45 kts deep layer shear,
is enough to support a few strong updrafts. This set of model
runs also indicates similar steep mid-level lapse rates
mentioned in the previous discussion which will support large
hail up to 1.5-2 inches in diameter. Dry mid- levels at
convection start and high DCAPE values around 1000 J/kg support
strong down drafts, and so damaging wind gusts up to 70 mph are
also possible.

As the storms progress eastward, 700 mb steering winds are nearly
parallel with the forcing, indicating quick upscale growth into a
line. From here, the better instability is along and south of the
Missouri River along the South Dakota and Nebraska border. This
should focus the stronger storms along our southern counties for the
evening/overnight hours. As the storms move into northwestern Iowa
around 05-06Z, they look to become more elevated and hail will
become the more dominant threat. However, at the same time the low
level jet kicks into gear, which increases the chance that a few
strong to severe gusts could make it to the ground. Storms should
move east of the region by early Friday morning.

FRIDAY AND BEYOND: Things dry out for Friday with breezy
northwesterly winds gusting up to 30-35 mph. Strong CAA limits highs
to the 60s. Saturday warms a little more into the upper 60s and low
70s. Meanwhile, another upper trough is digging south along the
Pacific Northwest. Saturday night into Sunday there is another
chance for showers and thunderstorms. Currently, we do not expect
these storms to be strong, as the better dynamics are over southern
Nebraska and Kansas. Light scattered showers continue for most of
Memorial Day and highs remain on the cool side of normal, in the low
70s.

An upper ridge begins to build in for Tuesday, finally allowing us
to dry out and warm up a bit through the end of the week. Look for
highs to warm from the mid 70s Tuesday into the upper 70s and mid
80s by Thursday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 111 PM CDT Thu May 23 2024

Southerly winds gusting 25-30kt this afternoon, but main focus
will be increasing thunderstorm chances moving east across the
forecast area 24/00Z-12Z tonight. Initial storms west of the
James River this evening (00Z-03Z) may produce large hail or
isolated gusts in excess of 40-50kt. Greater threat for damaging
winds, perhaps in excess of 60kt, will come as storms evolve
into a line which is projected to move through the lower
Missouri River Valley and portions of northwest Iowa, including
KYKN-KSUX-KSLB overnight (04Z-09Z). In addition to the potential
for strong winds, MVFR-IFR ceilings and visibility may accompany
the thunderstorms.

Storms are driven by a cold front which will be followed by
increasing west-northwest winds on Friday, with gusts as high as
30-35kt.

&&

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

&&

$$

UPDATE...JH
DISCUSSION...AJP
AVIATION...JH