South Sioux City, Nebraska 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

470
FXUS63 KFSD 160525
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1225 AM CDT Sat May 16 2026

.KEY MESSAGES...

- Thunderstorms increase in coverage Saturday night into Sunday.
  Severe weather risks including 1.5 to 2" hail and 60 mph wind
  gusts will be possible.

- A significant severe weather episode will be possible late
  Sunday afternoon into Sunday night. Very large hail, straight
  line winds, and tornadoes will be possible. This risk may
  extend into Monday.

- If you have outdoor plans Sunday afternoon through Monday, the
  time is now to prepare and have alternative plans in place.

&&

.UPDATE...
Issued at 917 PM CDT Fri May 15 2026

Thunderstorms have moved east of the area. Dry and quiet conditions
are expected for the remainder of the evening. Overnight lows are
expected to fall to upper 40s to mid 50s.

Saturday will be very dry with relative humidity values falling to
16-22%. Highs will be warm, in the 80s. Winds in the early morning
will be light and variable, becoming northerly, and then easterly in
the afternoon. Winds will increase as the SPG tightens ahead of an
approaching surface low pressure system from the southwest. Gusts of
20-30 mph are expected, with the highest gusts over south central
South Dakota. The combination of the warm, dry, and windy conditions
will result in another day of elevated fire concerns. Currently no
headlines are needed, however we will monitor conditions for any
changes. Please continue to be cautious with sparks and report any
fires immediately.

As mentioned in the discussion below, moisture does not begin to
move into the area until Saturday evening. The earlier mentioned low
pressure system will begin to track north-northeast pushing a warm
front into the region. As it does so higher dewpoints begin to
advect in through the late evening and overnight hours. With the
moisture MUCAPE increases to 1000-1500 J/kg and mid-level lapse
rates increase to 7 to 7.5 deg C/km. There are some discrepancies in
guidance on deep layer shear, but in general above 35 kts and as
high as 45-50 kts are possible. All this sets the stage for strong
to severe thunderstorms as a mid-level shortwave moves through the
region. Elevated storms are expected to initiate over south central
South Dakota around midnight and track northeast through the
overnight. Hail of 1.5 to 2 inches is possible, as well as damaging
wind gusts to 60 mph. The current SPC Day 2 Slight Risk (level 2 of
5) looks to be in good shape.

Additional chances for severe weather return on Sunday. See the
discussion below for more details.

&&

.DISCUSSION...
Issued at 129 PM CDT Fri May 15 2026

THIS AFTERNOON:  Poorly defined "cool" front continues to slowly
sink southeast this afternoon now entering southern Minnesota
and far northwestern Iowa as of 1pm. Southerly winds are slowly
advecting 50 dewpoints into the area ahead of the boundary, but
generally models have been too quick to pull higher dew point
air northward. By late this afternoon a weak impulse in the
700:500 mb layer will arrive into the Tri-State. This increased
lift combined with slight pressure rises to the northwest of the
front should create a bit more convergence sufficient for
convection development in a portion of NW Iowa (along and
southeast of a line from Sioux City to Spencer). Soundings
indicate MLCAPE values may approach 1000- 1500 J/KG, with an
increase in effective bulk shear towards 40+ knots. In any storm
that develops, large hail (potentially 2" or greater) will be
possible as mid-lvl lapse rates break the 8C/KM mark. Inverted V
soundings along the periphery of the CWA boundary in NW Iowa
would suggest downburst potential increases as well. With the
mean wind increasing from the west northwest, storms should
begin to congeal into one or more linear clusters as they head
into central Iowa. The peak timing for development and impact
falls within the 6pm to 9pm timeframe.

TONIGHT: Storms move southeast of the forecast area very early in
the overnight period, leaving quiet and dry conditions overhead.
Soundings show a pronounced low-mid lvl dry layer advecting
southward through the night.

SATURDAY: The aforementioned frontal boundary will be pushed by
convection closer to I-80 by Saturday morning, leaving dry and quiet
conditions in the local area for most of Saturday. A persistent
easterly 850 mb wind fetch is not expected to bring meaningful
moisture northward through the day with the surface boundary
tracking northward towards Highway 20 by mid-evening.  Temperatures
under filtered sunshine peak in the lower 80s, and with
persistent low RH and breezy easterly surface winds fire danger
may be elevated.

SATURDAY NIGHT: Today`s guidance has slowed the northward advection
of rich theta-e air northward which will likely keep the area
convection free deep into the evening.  We will be watching areas of
central and southern Nebraska tied to surface low pressure and the
existing warm front that remains in place.  Convection is expected
to develop as a stronger wave ejects into the Plains. One or more
clusters of strong to severe storms will attempt to track north or
northeast along the advancing elevated moisture axis and towards the
MO River valley after midnight.  MUCAPE remains somewhat marginal
with the initial surge of moisture, and mid-lvl lapse rates are only
in the 6.5-7 C/KM range throughout the hail growth zone.
Nevertheless, effective shear may just be strong enough to produce a
few elevated supercells capable of larger hail into Sunday morning.
Tend to feel the straight line wind risk may be lower, but should
the surface front lift northward or boundary layer become only
loosely stabilized, then a few stronger gusts could transport down
through the MO River Valley into NW Iowa into daybreak.

SUNDAY-SUNDAY NIGHT: Significant severe weather potential is
possible Sunday afternoon into Sunday night.  After morning
convection lifts northeast of the CWA early in the morning, guidance
is becoming more confident in a period of shortwave subsidence
moving overhead through a large part of the daytime hours of Sunday.
 While we`ll enter a quiet period, moderate to extreme instability
will continue to advect northward into the Tri-State area with some
models hinting at potential for AOA 3000 J/KG by mid-afternoon.
Soundings across the area suggest an EML will hold in place into at
least the mid afternoon hours as we wait for a deeper trough to lift
into the Plains.  This trough should work to push the effective warm
front further north while reinforcing a pre-existing cold front
across central NE/SD. The arrival of deeper synoptic lift after
21Z could lead to explosive convective development by early
evening. With the large reservoir of instability, bulk shear
over 50 knots, and strong low-lvl buoyancy all modes of severe
weather will be likely. Numerous sounding analogs suggest 2-4"
hail in any discrete or initial updraft that forms. Storms
staying discrete is somewhat in question, with guidance hinting
that the potential for quick upscale growth into a linear line
with embedded supercells progressing east northeast instead. The
increase in the low-lvl flow after dark and and slight backing
of surface winds could lead to a much greater potential of QLCS
activity into the late evening and overnight hours as storms
track into Iowa and Minnesota.

As we move past midnight, models suggest the highest severe weather
threat will migrate eastward with the line.  However, there remains
a high degree of uncertainty in the evolution after midnight.
ECMWF/NAM/CMC are holding back the upper trough to the west, and
actually allow deeper instability to lift back northward into Monday
morning. The persistent 40-50 knot LLJ could lead to scattered hail
producing supercells continuing into Monday morning. Still very
uncertain.

MONDAY:  A continued severe weather risks seems plausible on Monday
depending on what happens overnight.  Should the GFS based models
hold true, then both surface based and elevated instability slides
east of the area.  However ECMWF/NAM solutions would pull the
surface front back westward while leaving a large degree of elevated
instability westward.  There`s just too much uncertainty to get into
specifics, but Monday could also feature potential for higher end
severe weather especially in areas along or east of I-90.

TUESDAY-THURSDAY: We`ll see a distinct cooldown in temperatures
as we head towards the middle of the week. Highs on Tuesday
with a cooler northwest wind only rise into the the upper 50s. A
slow recovery towards the lower to middle 60s is possible by
Wednesday. The pattern through the rest of the week remains
quasi-zonal, suggesting one more more weak disturbances moving
through the flow creating a few rain chances by the second half
of the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1224 AM CDT Sat May 16 2026

VFR conditions are expected for the duration of this period. Winds
overnight will be light and variable, becoming northeasterly after
sunrise. Beginning late morning and continuing through the afternoon
winds become easterly and increase, gusting 15-25 kts. Highest gusts
expected over south central South Dakota. Stronger gusts spread
north and east through the overnight, becoming 20-27 kts east of I-
29 by the end of the period. Breezy winds continue into the next
period.

Partly cloudy skies become overcast as a surface low pressure moves
northeast into the region. This also brings chances for strong to
severe thunderstorms. While the highest chances will not occur
until after the end of the period, there is a low chance (<35%)
that a few storms may pop up in the late afternoon. Large hail
and damaging wind gusts are the main threats.

&&

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

&&

$$

UPDATE...AJP
DISCUSSION...Dux
AVIATION...AJP