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Forecast Discussion for FSD NWS Office

040
FXUS63 KFSD 160003
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
703 PM CDT Sat Jun 15 2024

.KEY MESSAGES...

- An area of showers and strong to severe storms will spread
  east into far northeast Nebraska and northwest Iowa, and
  perhaps adjacent portions of far southeast SD and southwest MN
  this evening. The main threats will the strongest storms will
  be large hail, damaging wind gusts, heavy downpours, and
  perhaps a few tornadoes. Severe weather threat is greatest
  through 10 PM, though may persist through midnight.

- Heat advisory criteria with heat index readings nearing 100
  degrees will be approached over Nebraska and Iowa Sunday
  afternoon and possibly Monday afternoon.

- Potential for a higher end severe weather risk Sunday night
  into Monday across the region. Very large hail will be the
  primary risk, but damaging wind gusts may develop. Confidence
  remains low to moderate on location and timing.

- Concern for heavy rain and flash flooding develops late Sunday
  night into Monday north of the I-90 corridor. By early Monday
  in this area, there`s a high chance of 1+ inches of rain with
  a few locations potentially receiving 2+ inches.

- An active weather pattern continues through most of next week.
  Ensemble guidance through the week suggests a high (>70%)
  chance for 2+ inches of rain with potential for some locations
  to see 4+ inches by next weekend.

&&

.UPDATE...
Issued at 638 PM CDT Sat Jun 15 2024

Severe storms continue to merge into a slow moving cluster of
storms in northeast Nebraska early this evening. This MCS is
likely to track northeast into northwest and west central Iowa
late this evening. The resulting MCS is likely to remain well
organized, fed by a stout LLJ and moving into a fairly unstable
environment supportive of severe weather despite its slow
motion. The environment as of early this evening has recovered
with 1500-2000 J/kg MLCAPE, STP of 1-3, Pwats near 2.0, and
modest mid level lapse rates across northwest Iowa and far
northeast Nebraska. Though strong to severe storms developing
ahead of the MCS cannot be ruled out, there are major doubts in
this based on what has happened thus far and waning daylight,
leaving the more likely severe threat to be the incoming MCS.
Main threats over the next few hours are large hail up to golf
ball size, localized flash flooding with rates of 1-2"/hr, and
perhaps a few tornadoes.

&&

.DISCUSSION...
Issued at 301 PM CDT Sat Jun 15 2024

Instability slow to recover after a morning MCS in eastern NE has
carved out a large relatively stable air mass. The outflow can be
seen on satellite extending from eastern KS into central NE early
this afternoon. With the wave in place across central NE into
eastern SD a combination of diurnal heating and increased southerly
flow should gradually work through this stable air mass. A decent
pool of instability at 1500-2000 J/kg CAPE exists across south
central NE which should spread northeast. The big question mark is
just when storms may fire. With the delayed response from the
morning MCS the deeper instability may not approach far northeast NE
and northwest IA until closer to 0z, bringing the best chance for
isolated severe storms into northwest IA from about 7 pm to 11 pm.
While the overall shear profile is not very impressive the lowest km
or so does have some decent speed and directional shear along with a
fairly low LFC/LCL which means an isolated tornado cannot be ruled
out. If development is much later than 0z isolated hailers will be
the main threat.

Once this wave passes this evening, weak mid and upper level ridging
will build into the area as a strong low level cap develops. This
cap will server a couple of purposes. The first will be to suppress
any convective potential on Sunday afternoon and possibly into the
early evening, even with an incoming weak boundary. The other
purpose this boundary will serve is to limit mixing and allow
moisture to pool near and ahead of this incoming front, resulting in
surface dew points in the upper 60s to lower 70s. Combined with
highs of 90 to 95, heat indices in mostly northwest IA could climb
to around 100 in the afternoon.

This brings us to Sunday night into Monday. The big battle will be
between capping and forcing/moisture. A significant cap will
continue into Sunday night, and current trends suggest that just
about any parcel that tries to lift below about 850-800mb will not
make it. This suggests we may need to wait for the main wave to
eject northeast late Sunday night which would put the main threat
for any severe weather and heavy rain from about midnight to noon
Sunday night into Monday. Just how much instability will exist if
lifting above 800 mb will be a big question. Overall there will be a
late evening threat for some isolated elevated supercells with
damaging hail the biggest threat, but the main threat will be later
in the night with a likely lesser severe threat but a higher heavy
rain threat. This potential continues to remain highest near and
especially north of I-90.

The models become less agreeable Tuesday into Wednesday with the
strength of the wave to the northwest and the resultant position of
the surface front. For now not enough confidence to worry about a
whole lot during this time, but there will be a continue threat for
showers and thunderstorms.

Once the convective potential lifts north on Monday morning the
precipitation chances should wane, but another system is expected to
move into the area on Monday night into Tuesday which will bring a
renewed threat into the area. The better chances appear to be near
and west of I-29.

The pattern remains active Thursday into Saturday with seasonally
warm and humid conditions. The flow aloft is expected to become a
bit more westerly but still indications that multiple weak waves
will traverse this flow and bring continued chances for showers and
thunderstorms.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 638 PM CDT Sat Jun 15 2024

VFR conditions with southerly winds will prevail across much of
the region near and west of a Yankton to Sioux Falls to
Marshall MN line tonight. Further southeast, a complex of
showers and strong to severe storms will spread east into far
eastern NE and western IA late this evening. IFR to MVFR
conditions as well as damaging and/or gusty erratic winds, large
hail, and perhaps even a tornado or two are possible with this
cluster of storms. This activity is most likely to impact the
Highway 20 corridor near KSUX to KSLB before 06z, but could
impact areas as far north as KSPW and KSHL.

After 06z, VFR conditions will prevail across the region through
Sunday. A passing cold front will turn winds southwesterly and
eventually northwesterly at KHON and KFSD on Sunday, becoming
stationary between KFSD and KSUX by the end of the day.

&&

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

&&

$$

UPDATE...BP
DISCUSSION...08
AVIATION...BP

NWS FSD Area Forecast Discussion