431
FXUS63 KFSD 231933
AFDFSD

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

.KEY MESSAGES...

- Strong to severe storms move across the region tonight. Main
  hazards will be large hail/isolated damaging wind with initial
  storms west of the James River Valley 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!

&&

.DISCUSSION...
Issued at 232 PM CDT Thu May 23 2024

TONIGHT: Focus for tonight will be on potential for strong to severe
thunderstorms moving east across the region. 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 7-9 pm 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 60-70 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.

FRIDAY-SATURDAY: Upper low is a little slower to eject northeast on
Friday, but with strong push of cold advection in the morning, could
see a few gusts to around 40 mph near/west of the James River Valley
through midday. As the day progresses, approaching surface ridge
from the southwest should allow winds to gradually decrease, before
becoming light/variable Friday night. As for precip chances, could
see some lingering light showers or an isolated storm across parts
of southwest MN/northwest IA through mid-morning before the upper
wave exits, but otherwise looking at a brief stretch of dry but
cooler weather as we head into the weekend. Coldest temps aloft
swing through during the day Friday, which should keep highs in the
60s at best, perhaps limited to the mid-upper 50s in some areas.
Slight warming builds in for Saturday, though temperatures look to
remain on the cool side of normal with highs in the upper 60s-lower
70s for the start of the Memorial Day weekend.

SATURDAY NIGHT-MEMORIAL DAY: Cooler than normal temperatures persist
across the area as we head through the latter half of the holiday
weekend. Another deepening trough slides across the region Saturday
night-Sunday with another wave diving southeast later Sunday night
into Monday. These systems could produce scattered showers and
perhaps a few storms, but instability and shear are both on the
weaker side, so not looking at a severe risk at this point.

TUESDAY-THURSDAY: Mid-upper level ridging builds in behind the
departing weekend system, bringing dry weather and warming
temperatures through the middle of the week. The ridge slides
east/flattens with an approaching trough by Thursday, which could
bring scattered precip back to the region by the end of this
forecast period.

&&

.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.

&&

$$

DISCUSSION...JH
AVIATION...JH