spc ac 210738
day 3 convective outlook corr 1
nws storm prediction center norman ok
0238 am cdt tue apr 21 2026
valid 231200z - 241200z
...there is a slight risk of severe thunderstorms across portions of
southern minnesota southward into northern oklahoma...
corrected for textual clarity
...summary...
severe thunderstorms will be possible from northern oklahoma into
southern minnesota on thursday. large hail to very large hail,
damaging winds, and a few tornadoes are possible.
... synopsis ...
a complex convective setup is expected on thursday as a broad,
long-wave trough remains anchored across the western us. while the
initial/primary shortwave will be lifting northward into canada, a
secondary, lower amplitude wave is progged to approach the southern
plains overnight thursday into friday.
at the surface, a pacific cold front and dryline will push eastward
into the central plains. a secondary surface low is forecast to
develop near the triple point in northwest oklahoma/southwest
kansas. this feature will serve as the southern extent of the more
organized severe threat, with a conditional threat southward along
the dryline.
... central plains ...
thunderstorms are expected to develop along an advancing cold
front/dryline across portions eastern nebraska into western iowa,
before expanding both north and south. mucape values between
1500-2500 j/kg and deep-layer shear on the order of 30-40 knots will
support supercellular structures initially, but unidirectional
profiles may favor splitting supercells, storm interference, and a
tendency for upscale growth into one or more linear segments. while
a few tornadoes may be possible early in the convective evolution,
large hail and damaging winds should become the dominant threat with
time.
... southern plains ...
convective initiation along the dryline south of the triple point
remains uncertain due to stronger capping. however, any discrete
cells that manage to initiate will do so in an environment
characterized by mucape values between 2000-3000 j/kg with 30-40
knots of vertical shear. this will be more than sufficient to
support supercells capable of very large hail.
a secondary wave should approach the region overnight thursday into
friday morning. forecast soundings indicate a steepening of midlevel
lapse rates associated with this feature and a subsequent increase
in cape values. depending on the evolution of thunderstorms during
the afternoon and evening, additional thunderstorms may develop
overnight. the severe potential of these storms is uncertain given
the overnight timing of the wave and increasing convective
inhibition. however, strong vertical shear would seem to support a
hail and wind threat.
... northern plains/upper mississippi valley ...
as the cold front pushes into the more modest moisture and buoyancy
profiles across northern minnesota, the severe threat should be
lesser than areas to the south. still, proximity to the upper wave
should support at least some threat for wind with any thunderstorms
that develop.
..marsh.. 04/21/2026
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note: the next day 3 outlook is scheduled by 1930z