788
FXUS63 KFSD 030839
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
339 AM CDT Wed Jul 3 2024

.KEY MESSAGES...

- Showers and thunderstorms are expected to develop this afternoon
and move into locations west of the James River Valley. Some of
these storms could be strong to severe with large hail to the size
of quarters and damaging winds to 60 mph. Heavy rainfall is also
possible west of the James.

- A second round of showers and thunderstorms is expected on
Thursday/Independence Day. Heavy rainfall is possible, mainly east
of I-29 but looks to be more localized then widespread. Severe
weather chances are uncertain but funnels to perhaps a brief tornado
is possible.

- Below normal temperatures continue tomorrow and into early next
week with high temperatures on the order of near seasonable to 5 to
10 degrees below average.

&&

.DISCUSSION...
Issued at 337 AM CDT Wed Jul 3 2024

Quiet conditions begin the day today as surface ridging continues to
slide through the area. Given clear skies and light winds, patchy
fog may be possible, most likely along river valleys. The surface
ridge will slide past the forecast area through the morning hours.
With the center of the surface high sliding across the Nebraska,
Iowa, Missouri, and Kansas state borders, southerly return flow will
set up, setting the stage for the next chance for showers and
storms. The return flow will not be impressive, especially for this
time of year as dew points will only moisten to the upper 50s to low
60s across north central Nebraska and south central South Dakota.
This will result in lesser CAPE values only up to around 1,000 J/kg.
Some uncertainty regarding the magnitude of instability is in
question for the afternoon hours as soundings show diurnal mixing
reducing buoyancy. In terms of shear, an upper level jet streak will
have the overall entrance region parked over the forecast area,
providing strong speed shear via mainly straight hodographs with
deep layer shear values on the order of 40-50 knots. Convection
looks to fire on the warm front parked along the South
Dakota/Nebraska border (west of the CWA) as a mid level wave hits
the front this afternoon. With an overall more stable airmass over
the forecast area, think convection will be on a downward trend as
it moves into the Missouri River and James River Valley`s. However,
with some instability and sufficient shear to work with, think large
hail to the size of quarters and damaging winds to 60 mph are
possible.

The storms will push into the area during the late afternoon and
evening hours before a second round of showers and storms will come
in on its heels for the evening and overnight hours. This round will
be forced by an upper level wave pushing into the Northern Plains.
This second round of storms will encounter the environment that the
first round of storms developed in. Given the modified environment,
do not expect this second round to be severe but will contribute to
additional rainfall. Rainfall totals for Wednesday are expected to
be between 0.25" to 0.75" with isolated higher amounts up to around
an inch, with the highest amounts falling along and west of the
James River. Aside from rain and storm chances, high temperatures
will be seasonal in the 80s with low temperatures falling to the 60s.

The previously mentioned upper level wave will evolve into a closed
low on Thursday/Independence Day. Abundant cloud cover and cooler
mid level temperatures will result in cooler high temperatures with
highs only warming to the 70s. The cooler mid level temperatures
will also steepen mid level lapse rates a bit to generate some
buoyancy. However, the amount of CAPE remains somewhat uncertain as
it will depend on how the low levels can destabilize. If clouds are
able to break up enough to allow adequate surface heating, then
could see CAPE values up to around 1,000 J/kg during the afternoon
hours. If not, then instability will be on the lower side, below
1,000 J/kg. Shear is also questionable as small pivots in where the
upper low tracks could result in increased or decreased shear. If
the low wobbles further north, then better jet energy will be
brought northward and contribute to sufficient vertical shear. If
the low wobbles further south, then weaker shear would be expected.
With at least some overlap of shear and instability, scattered
showers and thunderstorms are expected throughout the day. Ensembles
point to locations north of I-90, mainly in southwest Minnesota,
having the highest chances for additional heavy rainfall. The GFS,
Euro, and Canadian ensembles all show a near 100% chance for
exceeding a half an inch of rain and a 30-50% chance for exceeding
an inch of rain. Hi-res guidance does depict scattered showers and
thunderstorms throughout the day. This could result in more isolated
heavy rainfall depending on where storms track rather then
widespread rainfall. The questionable environment also keeps severe
weather chances questionable. If better shear and instability can be
realized, then severe weather may come to fruition. However, a cold
front will be pushing through the area which could severe as a focal
point for funnels or brief tornado. 03z RAP shoes sufficient surface
vorticity along the cold front as it moves through the area. As of
now, locations east of I-29 have the highest potential to see funnel
clouds and a brief tornado but could see funnels potentially across
a more widespread part of the forecast area given the upper level
low overhead. Any chance for rain will come to an end during the
evening and overnight hours as the low levels stabilize. Low
temperatures will fall to the upper 50s and 60s overnight.

The upper level low will begin to push eastwards of the area on
Friday, resulting in cooler northwest flow for the day. High
temperatures will remain on the cool side only warming to the 70s.
Some wrap around moisture will contribute to another chance for
scattered showers throughout the day. Low temperatures will fall to
the 50s overnight.

Medium range guidance continues to be in agreement in an amplified
upper level pattern developing for the weekend. An upper level ridge
will be be parked across the western CONUS with troughing across the
eastern half of the US. Latest guidance continues to pull the ridge
a little bit further west, placing more troughing across the
Northern Plains. This looks to keep cooler conditions going through
the weekend and into early next week. Ensembles support this as they
show near seasonable temperatures to temperatures 10 degrees below
average during this period of time. With troughing over the area for
a large part of this time frame, chances for rain will develop with
any shortwave that passes through the upper level flow. Tough to say
when any period may have higher chances for rain as of now. Although
Saturday may have the best chance as of now as medium range guidance
is in decent agreement in a shortwave passing through the upper
level flow. Ensembles support this as they show a broad 40-80%
chance for exceeding a tenth of an inch of rain.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1038 PM CDT Tue Jul 2 2024

Patchy fog remains the biggest aviation concern as we reach our
cross over temperatures, with models continuing to indicate best
chances (20-30%) for patchy fog should occur along the southern Big
Sioux and other low-lying areas south of I-90. Would not be
surprised to see chances for fog expand, but for now have kept
chances for fog mention only for the KSUX terminal. Dry conditions
continue into the morning hours, before chances for scattered
showers and thunderstorms return throughout the afternoon hours well
west of I-29.

&&

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

&&

$$

DISCUSSION...Meyers
AVIATION...APT