Flooding in Nebraska, June 24 to 30, 2024

Area Forecast Discussion
National Weather Service Sioux Falls SD
1036 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 /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1036 PM CDT Sun Jun 23 2024

VFR conditions are expected for the TAF period. Light southeasterly
winds will persist through the overnight hours. The low level jet
(LLJ) is on track to strengthen tonight which will bring some low
level wind shear (LLWS) to KSUX. Some showers and thunderstorms look
to develop on the nose of the LLJ but latest guidance keeps any
chance for rain confined to parts of northwest Iowa, east of KSUX.
As such, have kept out any mention of rain in at KSUX but trends
will be monitored. Winds will turn southerly during the morning
hours while gusts strengthen up to around 20-25 knots. A cold front
will sweep through the area during the afternoon and evening hours
tomorrow, turning the winds to out of the northwest. The northwest
winds will finish out the TAF period.

&&

.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...Meyers

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
426 PM CDT Sun Jun 23 2024

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-242130-
Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee-
Buena Vista-Woodbury-Ida-Lincoln-Murray-Cottonwood-Nobles-Jackson-
Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury-Brookings-Gregory-
Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-
McCook-Minnehaha-Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-
Yankton-Union-
426 PM CDT Sun Jun 23 2024

This Hazardous Weather Outlook is for portions of northwest Iowa,
west central Iowa, southwest Minnesota, northeast Nebraska,
central South Dakota, east central South Dakota, south central
South Dakota, and southeast South Dakota.

.DAY ONE...Tonight

Many area streams, creeks and rivers are in moderate to major
flood stage leading to continued road closures. These impacts will
slowly recede over the next couple of days.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

Ongoing flooding concerns continue.

Hot and humid conditions on Monday will lead to heat index values
near to above 100 degrees. Areas south of I-90 may see heat index
values as high as 105 to 110 degrees.

Showers and storms are possible Monday afternoon and evening and
again Tuesday. If storms can develop, strong to severe storms with
locally heavy rainfall are possible. More unsettled weather returns
later this week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.