202
FXUS63 KFSD 201746
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1246 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Showers and isolated weak storms spread through southeast SD
  and southwest MN this morning through early this afternoon.

- High confidence in moderate to heavy rain tonight into early
  Friday morning. Highest rainfall amounts are most likely to
  remain near/north of a line from Yankton to Beresford to
  Worthington in a broad swath of 1-2" with isolated 3+ inch
  amounts. Isolated severe storms are possible this evening in
  south central SD with large hail and damaging winds as the
  main threats.

- Another round of storms comes Friday night with threat of
  severe weather. This is expected to bring another swath of
  0.50-2.00" of rain with isolated 3+ inch amounts possible. All
  hazards of large hail, damaging winds, a few tornadoes, and
  flash flooding are possible so be sure to monitor the
  forecast.

- Cooler temperatures and dry conditions are expected Saturday
  night into early next week, though rain chances return Monday
  night.

&&

.DISCUSSION...
Issued at 330 AM CDT Thu Jun 20 2024

TODAY: Early this morning, scattered showers and isolated storms
continue to develop and stream northeast into areas west of I-29,
but much of this rainfall is not reaching the surface yet. As
expected, dry low level air has greatly limited rainfall (to <0.10")
so far. Broad ascent from 850-700mb warm air advection coupled with
an upper jet streak and weaker low level jet nosing in will
gradually bring more expansive showers with occasional rumbles of
thunder across south central and southeast SD this morning. By mid
day, much of the region west of Highway 18 in eastern SD should see
0.25-0.75" of rain. This initial wave of rain will shift further
north through the afternoon, bringing the most rain (0.25-0.75") to
areas near and north of Hwy 14. Further south, expect breaks in the
rain for much of the region near and south of I-90 late this
afternoon through early this evening.

This evening, attention turns to developing storms in the NE
Panhandle and WY. CAMs suggest this activity will merge into an MCS
tracking east down the instability/moisture gradient into central
and eastern SD/NE overnight. Cannot rule out isolated strong to
severe storms tonight with this activity when MUCAPE of 1000-1500
J/kg will nose into the MO River Valley, especially 8 PM to
midnight. Large hail to half dollar size and wind gusts up to 65 mph
will be threats with the strongest storms, most likely in south
central SD.

The greatest impact will be locally heavy downpours overnight
into early Friday morning, prompting issuance of a Flood Watch.
More details are available in the Hydrology section. Showers
and storms training in overnight will bring a broad swath of
1-2" of rain north of a Yankton to Beresford to Worthington line
with pockets of 3-4" of rain possible for a few locations per
HREF PMM guidance. The further south the heaviest rain occurs,
the more likely flash flooding is to occur given recently
saturated soil and heavy rains.

FRIDAY: After Thursday night`s round of storms moves out
Friday morning, a drier period is favored for at least the
morning hours. Low confidence in current moderate to high
(40-70%) rain chances for Friday given poor agreement in CAMs.
As the sfc front continues to lift north and puts us deeper into
the warm sector Friday, we`re in for a hot muggy day with dew
points in the 70s and highs in the 80s to near 90 degrees and a
south or southeast breeze.

On Friday night, attention turns to another round of threats
for severe weather and flash flooding. Low to moderate
confidence in where and when storms will initiate
(central/eastern SD vs NE) at this stage, but 2-3 kJ/kg MUCAPE
coupled with 25-35 kts 0-6km deep layer shear, an attendant
inverted sfc trough and encroaching upper level trough are
likely to bring scattered strong to severe Tstorms Friday night.
With even more anomalous deep layer moisture (top 1% of
ensemble guidance) and a model QPF signal for another swath of
0.5-2.0 inches of rain and isolated 3+ inch amounts, encourage
those with outdoor plans Friday night to monitor this period for
details regarding all hazards of severe weather (hail, damaging
winds, and tornadoes) and flash flooding.

SATURDAY AND BEYOND: Deterministic models mostly put us on the
north/dry side of the sfc front by Saturday afternoon, favoring
drier conditions and near seasonal conditions Saturday night through
Sunday. An early look into next week favors a warmer and wetter than
normal pattern continuing.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1245 PM CDT Thu Jun 20 2024

Scattered showers and thunderstorms continue across the area early
this afternoon. Ceilings are a mix of VFR all the way down to IFR
with one or two locations showing LIFR ceilings. Visibilities are
also erratic as visibilities down to IFR levels is reported in
falling rain while VFR in rain free areas. The showers and
thunderstorms are moving northeastwards which will push the heaviest
rain away from most TAF sites over the next several hours. However,
isolated development is possible through the rest of the afternoon
hours.

Additional thunderstorms are expected to move in from north central
Nebraska and central South Dakota during the evening hours. These
storms may produce strong winds gusts with them. The storms look to
stay mainly along and north of I-90 through the evening and
overnight hours. However, MVFR to IFR ceilings will continue through
the overnight hours and into tomorrow morning. Light showers, low
ceilings, and southeasterly flow turning southerly will finish out
the TAF period.

&&

.HYDROLOGY...
Issued at 330 AM CDT Thu Jun 20 2024

A Flood Watch for localized flash flooding has been issued for a
broad swath of the region tonight through early Friday morning.
Anomalous moisture content develops tonight via precipitable water
at the top 10th percentile of GEFS guidance for this time of year
thanks to return flow from the Southwest. Additionally an
atmospheric pattern with a coupled low level and upper level jet
aided by a mid level wave primes the atmosphere for flash flood
threat as thunderstorms track in overnight.

Low to moderate confidence in where the heaviest rainfall axis
will occur tonight. The sfc front positioned near the I-70
corridor early this morning has already begun a northward trek,
bringing low level moisture already further north than expected
by most deterministic models aside from the RAP. This may shift
the axis of heaviest rainfall further northwest, centered closer
to a De Smet to Chamberlain to Mission SD line. If this occurs,
flash flooding would be less likely because locations such as
Huron to Mitchell have seen below to near normal rainfall for
June thus far (respectively). However a shift a county or two
further south would greatly increase flash flood threat given
recent heavy rain and saturated soil, and this could occur as
far south as Yankton to Sioux Falls to Slayton.

With another round of storms expected to track through the region
Friday night, flash flood threat increases considerably Friday night
into Saturday so will need to monitor this period closely as well.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Flood Watch from 8 PM CDT this evening through Friday morning
     for SDZ039-040-050-053>070.
MN...Flood Watch from 8 PM CDT this evening through Friday morning
     for MNZ071-072-080-089-097-098.
IA...Flood Watch from 8 PM CDT this evening through Friday morning
     for IAZ001.
NE...None.

&&

$$

DISCUSSION...BP
AVIATION...Meyers
HYDROLOGY...BP