152
FXUS63 KFSD 010830
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
330 AM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Rivers across the area have crested, with a slow downward
  trend in river levels as dry conditions continue.

- Below normal temperatures continue today and through most of
  the week with high temperatures on the order of near
  seasonable to 5 to 10 degrees below average.

- Strong to severe storms along with the potential for heavy
  rain remain on track for this evening and night. Locations
  along and southeast of a Slayton, MN to Sioux Falls, SD to
  Tyndall, SD line are expected to see 1-2 inches of rain.
  Isolated higher amounts up to 3-4 inches is possible under the
  heaviest storms. Large hail up to the size of ping pong balls
  and damaging winds to 60 mph are the primary severe hazards.

- An active pattern aloft will continue rain chances for the
  rest of the week and potentially into the weekend.

&&

.DISCUSSION...
Issued at 329 AM CDT Mon Jul 1 2024

Quiet conditions begin the day early this morning but this will
change as a mid level wave currently over western Nebraska pushes
east and encounters an advancing 850 mb boundary. Scattered showers
and thunderstorms look to develop along this boundary as soundings
show instability on the order of a few hundred J/kg lifting from 850
mb. Given the weak instability, severe weather is not expected but
the showers are expected to produce between a few hundredths to up
to a quarter of an inch of rain. Should see the showers track
eastwards into the afternoon hours as the showers and storms follow
the 850 mb boundary. However, the showers will outrun the minor
instability and weaken. Lots of cloud cover will persist through the
day today which will continue the trend of cooler high temperatures
in the upper 60s and 70s. With a stronger upper level wave ejecting
into the Plains this afternoon, the low level jet (LLJ) will
strengthen in response to the approaching wave with speeds up to 40-
50 knots. Strong warm air advection (WAA) will result from the LLJ
but also stronger southeasterly winds with  gusts up to 30-35 mph.
The WAA could lead to some isolated elevated showers throughout the
afternoon hours but little it any rainfall is expected.

The LLJ will also continue to advect in higher moisture aloft which
will contribute to building elevated instability into the area. The
highest surface based instability will reside in Nebraska where dew
points into the 70s will reside. As the previously mentioned upper
level wave finally approaches the forecast area, convection looks to
develop on the western side of the warm sector. Storms look to fire
in central South Dakota late this afternoon but should encounter
drier mid level air and weaken if not dissipate with eastward
extent. However, storms that fire in central and southern Nebraska
will have much better moisture to work with. These storms will push
eastwards along the surface front but an elevated portion of these
storms looks to develop along the 800 mb front which will extend to
locations along and southeast of a Slayton, MN to Sioux Falls, SD to
Tyndall, SD line. As these storms develop, they will encounter much
better elevated instability on the order of 2,000 J/kg of CAPE and
effective shear values up to about 40 knots. With mean wind flow
mostly parallel to the 800 mb front, convection should remain along
the boundary as it moves east. This will result in upscale growth
into a line that may be capable of damaging wind gusts up to 60 mph.
Large hail up to the side of ping pong balls is also possible but is
most likely as convection initially develops before congealing into
a line. The developed thunderstorms will also poses the possibility
of heavy rainfall as PWATS moisten to over 2 inches along with
strong moisture transport taking place between 925-700 mb. The
heaviest rainfall looks to fall along and southeast of the
previously mentioned line between Slayton, MN to Sioux Falls, SD to
Tyndall, SD where 1-2 inches of rain is expected with isolated
higher amounts to 3-4 inches possible. This will result in rises to
the rivers along with the potential for flash flooding given wet
antecedent conditions. However, a few of the hi-res models are
showing an alternate scenario where convection that develops in
Nebraska takes up most of the available moisture which could result
in lower rainfall totals then currently expected. Think that this
possibility is not as likely given the strong moisture transport
from the strong LLJ advecting sufficient moisture northwards.
Thunderstorms look to exit the area around 3 am but light showers
could persist through the night.

A cold front will slide through the area on Tuesday as the
previously mentioned upper level wave continues to push into the
Northern Plains. 850 mb temperatures in the mid teens will lead to
warmer highs, closer to seasonable up to the upper 70s and low 80s.
Uncertainty remains in how fast the cold front pushes through the
area. If the front pushes southeast of the area, then another round
of strong to severe storms will be harder to come by. If the front
slows down and does not push past the forecast area, then better
moisture will pool ahead of the front contributing to sufficient
instability across part of northwest Iowa. The right entrance region
of the upper level wave will be parked over the area contributing to
enhanced lift and shear over the front. Still too early to say which
scenario will play out but do think the possibility of the front
pushing through the area faster may be the most likely scenario as
the direct thermal circulation from the entrance region of the upper
level jet may result in a bit better momentum within the post
frontal northwest flow to push the cold front through at a faster
rate. Any chance for rain will come to an end Tuesday evening
leaving mainly dry conditions for the overnight hours. Low
temperatures will fall to the  60s overnight.

Surface ridging slides through the area on Wednesday which will keep
dry conditions going through most of the day. 850 mb temperatures
will warm to the upper teens which will result in seasonable high
temperatures up to the mid 80s. Southerly return flow will quickly
set up in the wake of the passing surface ridge which will set up
another chance for showers and thunderstorms. Ensembles support this
as they show high probabilities between near 100% for exceeding a
tenth of an inch of rain. However, their probabilities drop to about
30-50% for exceeding a half an inch of rain. The higher
probabilities lie across south central South Dakota. Have stuck with
model blended PoPs as the highest probabilities also lie across
south central South Dakota.

Chances for rain continue through the rest of the work week as an
active pattern continues aloft. Deterministic guidance hints at the
possibility of a closed low developing aloft which could result in a
slightly longer stretch of cooler conditions. High temperatures
look to be on the cooler side regardless if a closed low develops
though with highs only making it up to the mid to upper 70s. Given
uncertainty regarding precipitation potential, have again stuck with
model blended PoPs.

Drier conditions could make a return this weekend as upper level
ridging builds into the western CONUS but too early to say if any
ridge riding waves will bring continued chances for rain to the
area. Highs look to remain near to below average though in the upper
70s and 80s.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1057 PM CDT Sun Jun 30 2024

Southeast winds increase quickly after sunrise by 12Z, with
frequent gusts 25-35kt through the day. Winds will subside
slightly after sunset Monday evening, though occasional gusts
20-25kt will linger through the end of the TAF period.

MVFR-locally IFR ceilings are expected to expand across the area
Monday morning, accompanied by areas of rain/isolated thunder. A
second round of showers and storms develops in the evening, with
heavier rain favored from northeast Nebraska into northwest Iowa.
May also see localized visibility restrictions at times in areas
with moderate to heavy rain.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...JH