892
FXUS63 KFSD 182005
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
305 PM CDT Tue Jun 18 2024

.KEY MESSAGES...

- A passing cold front will bring a risk for strong to severe
  convection across the eastern half of the forecast area into
  the early evening. Primary risks will be 1.5" hail and brief
  60 mph wind gusts, though isolated tornadoes and flash
  flooding are possible.

- Additional rainfall expected through the Little Sioux River
  basis into this evening may push current flooding higher.

- A mostly dry day Wednesday will give way to yet more rain into
  Thursday-Thursday night. Non-severe thunderstorms will again
  bring widespread rain with higher amounts hopefully remaining
  near/north of the I-90 corridor (Little Sioux River basin).
  Probabilities of additional >0.50" totals are at 80% north of
  I-90, with 1" probabilities above 50%. Several high resolution
  models do suggest some potential for greater than 2" totals.

- Thunderstorms risks continue into Friday, with a low
  potential for strong to severe storms Friday afternoon and
  evening.

- Cooler temperatures and likely dry conditions this weekend
  into early next week, though rain chances return by Tuesday.

&&

.DISCUSSION...
Issued at 305 PM CDT Tue Jun 18 2024

As of 19Z, well-advertised and well-defined cold front is
located near the MN-SD state line, then arcs slightly to the
southwest into northeast Nebraska. As anticipated, have begun to
see a line of showers and scattered thunderstorms develop along
the front over the past hour or two, and these storms should
intensify over the next hour or two as the front pushes east
into a warm and very moist air mass characterized by surface dew
points in the lower-mid 70s, fairly impressive for this early in
the season. This is contributing to SBCAPE values near to above
2000J/kg ahead of the front, with pockets of modest deep layer
shear of 25-35kt (stronger shear is displaced west of the
front). With low level shear largely oriented parallel to the
boundary, expect pulsy multicell activity to dominate most of
our area, though could see more of an isolated tornado threat
north of I-90 where the the line arcs slightly more to the
northwest and becomes closer to normal with the low level shear
vectors.

Precipitable water values remain near/above 1.75+ inches ahead
of the front into this evening, or above the 95th percentile of
climatology per the NAEFS and EC ensembles. This should allow
for efficient rainfall producing storms bringing a potential for
heavy downpours and isolated flash flooding as they move across
areas with already saturated soil conditions and high river
levels.

The front and associated stronger storms are on track to push
east of our forecast area by 00Z-01Z this evening, though could
see some showers/isolated thunderstorms along the trailing
elevated boundary linger a bit longer into the evening.

With focus on the current severe threat, did not spend too much
time evaluating the outer periods of the forecast. Surface high
pressure slides east across the Dakotas/Minnesota on Wednesday
which should keep the area mostly dry, though could see some
light elevated post-frontal showers or isolated storms as a
wave slides northeast along the mid-level boundary. Return flow
and the 850mb warm front will lift back to the north later
Wednesday night, bringing renewed chances for rain and storms.
Most significant change noted from previous forecast is a shift
to the north with the potential for heavier rains from late
Wednesday night into Thursday night. The higher probabilities
for amounts > 1.00" are now focused mostly north of I-90, which
if it pans out, would bring some relief to the water-logged
Little Sioux River basin and other areas across northwest Iowa.

Although finer details may vary, we are still looking at a
modest shortwave sliding northeast across the northern Plains
later Friday-Friday night. Could see temperatures push well into
the 80s or even lower 90s ahead of this system, and with dew
points again approaching 70F, will be looking at heat indices at
least into the lower to perhaps mid 90s. Temperatures should
ease a little lower through the weekend with mostly dry weather
expected.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1255 PM CDT Tue Jun 18 2024

Surface cold front nearing I-29 as of 1730Z will push east
across southwest MN and northwest IA through early this evening.
As the front moves into a more unstable airmass, scattered to
numerous showers and thunderstorms are expected to develop.
Better chance for thunder looks to be east of KFSD but will
likely impact KSUX at times through 18/23Z-19/00Z. Variable
gusty winds, IFR or lower visibility in pockets of heavy rain,
and possibly hail may accompany the strongest storms. Threat for
these stronger storms moves east of our forecast area by 19/01Z
with showers and perhaps isolated thunderstorms lingering near
and south of US Highway 18 into Wednesday morning.

Away from any thunderstorms, MVFR ceilings have become more
widespread west of the cold front, but improving conditions are
expected through this evening as deeper dry air builds in behind
the front. Could see additional MVFR stratus sag southward into
areas mainly north of I-90 late in the period, but confidence is
low in areal coverage/timing, so kept KHON at SCT below 3kft AGL
for now.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...JH