566
FXUS63 KFSD 232353
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
653 PM CDT Sun Jun 23 2024

.KEY MESSAGES...

- River crests have already occurred or will occur within the
  next 24 hours. While no significant amounts of rainfall will
  occur, areal flooding caused by the prior excessive rainfall
  will continue across portions of the area.

- Hot and humid conditions Monday, with heat indices near to
  above 100 degrees. A Heat Advisory has been issued for most of
  the area outside of southwestern MN.

- Highly conditional threat for severe thunderstorms Monday
  afternoon/evening. While we could see some weak thunderstorms,
  if thunderstorms become surface-based - that`s a big if -
  they would be capable of large hail and damaging winds.

- The pattern through mid week doesn`t favor any long duration
  rain, with another chance for isolated severe storms on
  Tuesday afternoon. Additional scattered rain chances return
  late this week into next weekend.

&&

.DISCUSSION...
Issued at 414 PM CDT Sun Jun 23 2024

Mostly sunny skies out there this afternoon, with light winds and
temperatures in the lower to mid 80s. May start to see a slight
uptick in cloud coverage as surface-based parcels try to break
through the cap, with otherwise quiet conditions continuing into the
evening hours. As a surface high pressure moves off to our east,
southerly flow develops at the surface which will begin advect warm
and moist air up into the region. Aloft, the low-level jet (LLJ)
strengthens during the evening hours, with warm air advection (WAA)
leading to chances for scattered thunderstorms after midnight until
sunrise. Given a fairly deep dry layer beneath these elevated
thunderstorms, expecting only light rainfall amounts to occur, with
up to a quarter of an inch possible.

Behind the elevated warm front, Monday will see a quick warm-up as
we will mix down some very warm temperatures to the surface.
Ensemble situational awareness tables (ESATs) show that both 850mb
and 700mb temperatures reside above the 99th percentile of
climatology, with 850mb temperatures 24-28 deg C. Thankfully we will
not be able to mix down all that warm air, but afternoon
temperatures in the lower to upper 90s are expected. In addition,
ESATs show that specific humidity values are also near the 90th
percentile of climatology. What that means for us is that higher
than normal dewpoint values are likely, with max afternoon dewpoints
currently expected to be in the lower to mid 70s. Combining the hot
temperatures with muggy dewpoints will result in high heat index
values of 100 to 110 degrees F across portions of the area, with the
heat advisory remaining unchanged at this time.

Attention shifts towards chances for showers and thunderstorms
during the afternoon hours, though the aforementioned
climatologically warm 850 to 700 mb temperatures do create a strong
cap which should keep strong thunderstorms from forming. High
resolution guidance does show some scattered ACCAS-y type
showers/storms forming, but as those can only tap into a portion of
the available CAPE, severe thunderstorms look unlikely. However, in
the unlikely event that convective temperatures end up being
reached, or the cap ends up being weaker, given 4000+ J/kg of MUCAPE
would allow for thunderstorms to create large hail and damaging
winds. So, am thinking more along the lines of high resolution
guidance, which would give chances (20-30%) for weaker scattered
showers and thunderstorms during the late afternoon hours, primarily
for areas in the vicinity of Sioux City, IA. Rainfall amounts look
to remain on the light side at around a quarter of an inch, but if a
stronger storm were to form a quick 1-1.5" would be possible.

As the cold front itself begins to move through the area overnight,
light scattered showers will be possible, though amounts look to
remain light at under a tenth of an inch. By daybreak Tuesday, the
cold front looks to stall somewhere along the I-90 corridor, keeping
the warmest temperatures south of the front for the day. Areas north
of the front are still expected to reach up into the lower to mid
80s, while areas south of the front may reach up into the lower 90s,
especially for those south of highway 20. Aside from a weak mid-
level wave, not a whole lot of upper level forcing for showers and
thunderstorms, so will need to see if the cap is able to erode.
Similar to Monday, given 3000 J/kg of CAPE, if a surface-based storm
were able to develop, large hail would be the primary threat.

Ridging aloft is in store for Wednesday as a quick moving shortwave
dives southwards east of the area, with a surface high pressure
moving down throughout the day. This will moderate temperatures for
Wednesday, with highs in the mid 70s to lower 80s, about 5 degrees
below normal. Chances for rain return overnight into Thursday as
push of weaker shortwaves moves overhead, with additional chances
(30-40%) throughout the day as warm-air advection overspreads the
area throughout the day. Guidance does start to diverge on solutions
as timing of waves and surface features begin to differ, but it
looks like the pattern will remain somewhat active through Friday
before calming down into the weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 653 PM CDT Sun Jun 23 2024

VFR conditions continue throughout the period. Winds continue to
shift towards the southeasterly direction, turning further to become
out of the south Monday morning. Wind speeds increase overnight
ahead of an incoming cold front, with low-level wind shear possible
across the Missouri River Valley, including the SUX terminal. Could
see an isolated thunderstorm around midnight in the vicinity of
Sioux City, but spatial confidence is not high enough to include in
the TAF at this time. Breezy southerly winds increase during the
morning hours, sustained in the teens with gusts into the 20s and
lower 30s, becoming north-northwesterly behind the front.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM to 8 PM CDT Monday for SDZ038-039-050-
     052>071.
MN...Heat Advisory from 1 PM to 8 PM CDT Monday for MNZ098.
IA...Heat Advisory from 1 PM to 8 PM CDT Monday for IAZ001>003-
     012>014-020>022-031-032.
NE...Heat Advisory from 1 PM to 8 PM CDT Monday for NEZ013-014.

&&

$$

DISCUSSION...APT
AVIATION...APT