southernmost

As adjectives the difference between southmost and southernmost

is that southmost is furthest to the south; most southern while southernmost is farthest south.

As adjectives the difference between southeast and southernmost

is that southeast is southeastern (uk : south-eastern), southeasternmost; occupying a position that is relatively southeast of another while southernmost is farthest south.

As a noun southeast

is the direction of the cardinal compass point halfway between south and east, specifically 135°, abbreviated as se.

As an adjective southernmost is

farthest south.