334
FXUS63 KFSD 070013
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
713 PM CDT Mon May 6 2024

.KEY MESSAGES...

- Lines of showers and thunderstorms will move across the
  forecast area this evening. While an isolated stronger storm
  is still possible, the risk of severe weather appears to be
  decreasing.

- Periodic rain chances continue for much of the upcoming week
  into next weekend with the focus on Wednesday. Highs in the
  60s to 70s are expected.

&&

.UPDATE...
Issued at 713 PM CDT Mon May 6 2024

Still seeing isolated gusts over 40 mph as of 7 PM, but overall
expect winds to be on a downward trend through this evening, and
thus have allowed the Wind Advisory to expire.

That said, with a fairly stout low level jet still over the
area, cannot rule out isolated stronger gusts with the showers
and thunderstorms moving across the area. While these isolated
gusts may be marginally severe (60 MPH), instability across much
of our forecast area seems to be lacking given general downtrend
in storm intensity as the line of storms in eastern Nebraska
moves northeast. Area of greatest concern at this point would
seem to be portions of the Highway 20 corridor (Sioux City) and
perhaps into Storm Lake.

&&

.DISCUSSION...
Issued at 250 PM CDT Mon May 6 2024

Rest of Afternoon and Evening: At a large scale, pronounced
mid/upper low is evident on WV imagery across the northern Rockies
with surface low deepening across western SD. A warm front is
lifting through central NE with a narrow warm sector before a
quickly approaching dryline/cold front is crashing eastward. These
features will all lift east or northeast into the evening and
overnight hours and provide for thunderstorm chances.

Severe Storms: Forecast soundings show a ribbon of modest elevated
instability lifting from roughly 850:800 mb around 1000 J/kg with
higher values as you go south. Surface parcels would obtain closer
to 50o J/kg if they can overcome a modest inversion, although
current thinking is this may be tough given persistent cloud cover
thus far. A robust and dynamic wind field through the entire
atmospheric column is helping to drive deep layer shear of 40-50 kts
however effective shear for elevated parcels looses a good chunk of
the lower directional and speed shear. If a surface parcel can be
realized, nearly 30 kts of 0-1 km shear exists with a well curved
elongated low level hodograph.

With the above stated, well forced dynamics should result in
increasing coverage of storms through the 654second half of the
afternoon with likely a rapid transition to linear mode.
Damaging wind gusts up to 60-70 mph seems the most likely threat
with a 50 to 55 kt LLJ at 850 mb. Even less organized
convection may be able to tap into this higher momentum air,
especially while the boundary layer remains mixed during the
daylight hours. Hail would be a secondary risk although again,
elevated parcels lack a better effective shear profile. Any
tornado threat would be conditional on realizing surface parcels
and at this time seems low but should be monitored given the
low level wind field. Any more organized bowing segment that can
orient from northwest to southeast would also need to be
monitored with 0-3 km shear vectors out of the SW AOA 30 kts

Heavy Rain: PWATs around 1.25, and near the top of climatology,
should provide for a period of heavy rain with the passage of the
line of thunderstorms but with a warm cloud layer generally less
than 10 kft and the progressive nature of any thunderstorms, any
flash flooding threat should remain isolated and probably confined
to urban areas. QPF amounts near and east of I-29 are a bit either
side of 1-1.5 inches with HREF PMM suggesting pockets of up to 2-2.5
inches, more likely across NW IA. Given saturated soils as of late,
renewed issues with ponding of water and river rises will be the
more likely impact.

Synoptic Wind: Aforementioned deepening surface low to the west will
continue to provide for Advisory level wind gusts (45 mph or higher)
into the early evening before decreasing BL mixing should result in
a decreasing trend closer to sunset.

Tonight: After storms exit to our east toward the midnight hour, the
remainder of the overnight looks fairly benign.

Tuesday and Wednesday:  Vorticity ribbons pivot off the filling
upper low into midweek. This will bring occasional rain chances with
Wednesday currently appearing to be the focus as the main lobe
shifts east. With the main upper low overhead, Wednesday would be a
timeframe to monitor for slow moving storms capable of locally heavy
rain as well at least some potential for low topped funnels.

Thursday through Sunday: Transition to upper level northwest flow on
the backside of the upper level trough will bring seasonal
temperatures for the second half of the week with perhaps a few low
chances for a light shower.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 713 PM CDT Mon May 6 2024

Lines of showers and thunderstorms will rotate across the forecast
area through this evening. The thunderstorms are expected to push
east of the I-29 corridor by 03Z-05Z, and east of the Highway 71
corridor by 05Z-07Z, though an area of showers may linger across
southwest Minnesota into the pre-dawn hours of Tuesday. MVFR
ceilings and MVFR-locally IFR visibility are expected with the
showers and thunderstorms, becoming predominantly VFR behind the
lingering showers. Additional stratocu will spread over the region
from the northwest on Tuesday. While this is currently expected to
be VFR, a brief period of MVFR ceilings is possible. Spotty light
showers or an isolated thunderstorm is possible Tuesday afternoon,
but chances are too low to include in the TAF at this time.

The strongest of the southeasterly gusty winds should diminish with
sunset, though occasional gusts over 30kt are still possible this
evening. Speeds diminish further as a trough axis slides east behind
the showers tonight. Southwest to west winds behind the trough will
again increase after daybreak Tuesday with gusts 25-30kt common in
most areas, perhaps up to 35kt near/west of the James River
Valley.

&&

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

&&

$$

UPDATE...
DISCUSSION...Kalin
AVIATION...JH