132
FXUS63 KFSD 020906
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
406 AM CDT Thu May 2 2024

.KEY MESSAGES...

- With up to 1-1.5" of rainfall fallen across portions of
  northwestern Iowa overnight by sunrise, there may be ponding
  of water on roadways and low-lying area, with some rises on
  the local rivers/streams.

- Main area of showers and thunderstorms will move east this
  morning, with additional pop-up showers possible throughout
  the day.

- Additional chances for scattered showers and thunderstorms
  return Friday night into Saturday, and again Monday night into
  Tuesday.

&&

.DISCUSSION...
Issued at 406 AM CDT Thu May 2 2024

Early morning radar shows showers and thunderstorms shifting
eastwards across the region, and as of 3 AM majority of the rainfall
is occurring east of the James River Valley. This activity is
occurring as an inverted trough is extending northwards into the
region, further aided by shortwaves moving around the cut-off upper
level low parked on the International Border. As the inverted trough
continues to shift eastwards, majority of the currently observed
rainfall will be pushing east of US 71 across SW Minnesota and NW
Iowa by the mid-morning hours. Soundings from across the region show
that the low-levels remain saturated throughout the day, and as
additional, albeit weaker, shortwaves move across the region,
another round or two of showers are possible throughout the day.
Given the saturated low-levels, not impossible to see some drizzle
falling out of the stratus deck during the morning hours.

Otherwise, a surface high pressure will be developing on the
backside of the exiting surface low pressure, which result in a
tightened surface pressure gradient leading to breezy winds
developing throughout the afternoon hours. Highest wind gusts are
expected along the central NE/SD border, where gusts into the mid
30s are possible, whereas elsewhere wind gusts into the mid 20s are
expected. Afternoon temperatures are expected to rebound up into the
mid 50s to lower 60s, possible mid 60s near the SD/IA/NE border.

We`ll be watching a new surface low pressure develop under the
aforementioned cut-off upper level low, sending a cold front towards
the region. After early morning lows in the mid 30s to lower 40s,
with some patchy frost in central South Dakota, southwesterly winds
will advect warm air into the region out. Areas west of I-29 will be
the first to see the cold frontal passage, thus will see the coolest
high temperatures in the upper 50s to lower 60s. Areas well ahead of
the front are expected to reach back up into the mid 60s to lower
70s. The trough axis moves in quickly behind the cold front as it
pushes eastwards across the area throughout the afternoon hours, and
will lead to additional chances for scattered showers and
thunderstorms overnight into Saturday. Instability looks to remain
quite limited, with models showing a modest 200-500 J/kg of CAPE
available, so no severe weather is expected. Given little time for
moisture return since the last event, rainfall amounts are expected
to remain light, resulting in a 20 to 40% chance for a quarter inch,
lowest north of I-90 and highest south of I-90.

As a strong cut-off upper level low moves ashore onto the west
coast, upper level ridging will lead to dry conditions expected for
the rest of Saturday and throughout Sunday. Highs on Saturday will
fall into the lower to mid 60s, increasing on Sunday into the mid
60s to lower 70s.

The upper level ridging breaks down into Monday as the big trough
slides eastwards. The similarly strong surface low pressure will
continue warm air advection as we remain in the developing warm
sector, which will bring surface temperatures into the lower to mid
70s. Chances for showers and thunderstorms return Monday afternoon
as the surface low continues to move eastwards, with a strong cold
front moving across the region overnight into Tuesday. SPC has added
a 15% chance for severe thunderstorms south of the area, with
uncertainties on how north the instability is able to reach. Model
solutions continue to show that the cut-off upper level low will
stall out somewhere over the northern plains, which would result in
continued chances for scattered showers and thunderstorms.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1032 PM CDT Wed May 1 2024

Although ceilings are remaining at VFR levels longer than
anticipated, expect MVFR/IFR conditions to advect in with
showers and thunderstorms from NE through the overnight hours.
Reduced visibility has been observed this evening with locally
brief and heavy rainfall. This looks most likely along and east
of I-29, especially over northwestern IA. Stratus with MVFR/IFR
ceilings continue through the early afternoon Thursday, with
widespread rainfall ending through the morning hours. However,
latest guidance this evening shows the potential for isolated
showers through Thursday afternoon, as well as patchy drizzle
with the low cloud deck. Have not included any afternoon showers
in the TAFs, as confidence in coverage is low at this time.

Winds increase overnight with gusts around 20-25 knots. Evening
model runs have backed off with the potential for LLWS across
northwestern IA as the LLJ increases; however, have continued to
include at KSUX through 02.09z as gusts have currently dropped
off. Wind direction varies through this forecast period, becoming
northwesterly to westerly by late Thursday morning/early Thursday
afternoon.

VFR conditions are currently expected to return by the end of
the period.

&&

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

&&

$$

DISCUSSION...APT
AVIATION...SG