858
FXUS63 KFSD 181123
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
623 AM CDT Sat May 18 2024

.KEY MESSAGES...

- Not as warm but still breezy today. Occasional gusts 30-40
  mph this morning, but probability of reaching Wind Advisory
  criteria is low.

- Thunderstorm chances return Sunday into Monday. Level 1 of 5
  risk for severe storms, mainly Sunday night and perhaps late
  Monday afternoon, though still some uncertainty in exact
  timing/location. Pockets of heavy rainfall also possible.

- Another round of thunderstorms arrives late Monday night-
  Tuesday bringing a risk of a few severe storms as well as
  potential for widespread 1-2+ inches of rainfall. This could
  bring renewed flooding along some area rivers.

- Cooler but still unsettled for the latter half of next week.

&&

.DISCUSSION...
Issued at 433 AM CDT Sat May 18 2024

TODAY-TONIGHT: Breezy south winds continue early this morning, ahead
of a cold front which has entered our western CWA, located east of
Huron-Chamberlain as of 3 AM. Strongest winds appear to be downslope
winds east of the Buffalo Ridge into Marshall at this time, though
several areas gusting 25-30 mph near and east of US Highway 81.
Spotty light echoes on radar, but having a more difficult time
getting rain to the surface with deeper sub-cloud dry layer. Despite
that, will continue to carry a mention of sprinkles as the ACCAS
slides east through mid-morning.

Focus for today will be on post-frontal winds, especially this
morning as cold advection enables mixing into some stronger winds
aloft. Forecast soundings show a brief period where winds atop the
mixed layer reach 40kt, mainly in areas west of the James River
within an hour or two of sunrise. Seeing isolated gusts in this
range upstream in western SD as of 08Z, so cannot rule out some
isolated advisory level winds in our far west early this morning.
However, does not look to be widespread or prolonged enough to
warrant a Wind Advisory.

Aside from the wind, today should be a seasonally pleasant day with
highs mainly in the 70s and plenty of sunshine. Winds actually will
decrease through the afternoon in response to approaching surface
high pressure which will provide light/variable winds through much
of tonight, increasing late tonight along/west of the James River
Valley.

SUNDAY-MONDAY: Rain chances on Sunday look to some in two waves.
First will spread northeast across the area through the morning to
early afternoon, in response to broad warm advection and a mid-level
wave sliding through the Tri-State Area. This wave may initially be
fighting some dry air, especially in our east as the surface high is
just departing, but eventually expect scattered-numerous showers and
a few storms move across the area. Instability is rather limited at
this time, so severe weather risk is low and rainfall amounts will
likely be on the lighter side, largely less than a quarter inch.
CAMs are in fairly good agreement that this activity will wane by
mid afternoon as the wave lifts off to the northeast.

Attention then turns to a trailing mid-upper level wave which will
drag a cool front east into the area Sunday night-early Monday. This
boundary should trigger scattered storms in southwest/central SD as
it moves through that area during peak heating Sunday afternoon.
Question is whether storms will maintain organization and strength
as they push east of the Missouri River, most likely after sunset.
CAMs would suggest a threat of at least isolated strong to severe
storms, most likely a potential for hail with elevated storms in the
late evening-overnight given moderately steep mid-level lapse rates.
However, deep layer shear appears lacking due to weak mid-level
winds as storms move east, so may be difficult to remain organized.
SPC Day 2 Marginal Risk (Level 1 of 5) seems reasonable to cover the
isolated threat, though confidence is on the low side given the weak
shear profile.

This activity pushes east of the forecast area in the morning, then
will watch for redevelopment across our southeast Monday afternoon,
depending on the timing of the cold front which is currently progged
to be east of the IA/MN Highway 60 corridor by 18Z. As is often seen
with cold fronts, greater shear is displaced behind the boundary
while greater instability is along/ahead, but could see some overlap
along the boundary to support a few strong to severe storms in our
far southeast later Monday afternoon-early evening.

MONDAY NIGHT-TUESDAY: Should see increasing broad-scale lift across
the area as Monday night progresses, as a deepening trough slides
east into the Plains. Will see increasing shear as this system moves
across the forecast area Tuesday, with potential for destabilization
across our far southeast as mid-level dry slot works into northwest
Iowa. Some uncertainty in the details regarding exactly where a
developing surface low/associated boundaries may track, but current
thinking brings the surface low northeast across northwest Iowa
during peak heating, with differential heating adding to the thermal
gradient across the boundary. Given proximity of the surface low and
boundaries, along with strong shear/moderate instability (1000-1500
J/kg), all severe weather hazards appear possible Tuesday afternoon.

Northwest of the surface low track, widespread/persistent rain and
embedded storms could produce areas of moderate to heavy rainfall.
Ensemble mean rainfall tops 1" across portions of southeast SD and
southwest MN Tuesday into Tuesday evening, with the 90th percentile
greater than 2" across this same area. Given the lack of instability
in these areas, would not expect excessive rainfall rates, though
localized urban flooding or ponding in low-lying areas is possible.
Additional rainfall into already swollen rivers across northwest IA
may also bring a renewed threat of river flooding in these areas.

WEDNESDAY-FRIDAY: Details for this period are low confidence as
there is little agreement among the longer range models. However,
moderate consensus in some cooler air settling into the northern
Plains mid to late week with periodic rain chances.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 623 AM CDT Sat May 18 2024

Cold front moving through the I-29 corridor at the start of this
TAF period. Front will be followed by gusty northwest winds, with
gusts generally 25-30kt, but briefly topping 35kt at times this
morning. Winds will slowly ease through the afternoon, with winds
becoming light and variable after sunset this evening.

VFR conditions will prevail through the period, though could see
FEW-SCT clouds in the 1500-2500ft AGL layer for a few hours after
frontal passage. Mid level clouds expand northward late in the
period in advance of increasing rain chances which are expected
after 12Z Sunday.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...JH