Towards the southwest — the same meaning as southwestward, with an -s on the end. The difference is not meaning but habit and grammar. British writers tend to prefer the -s form (the ship sailed southwestwards), American writers tend to drop it (the ship sailed southwestward); both are correct and both are understood everywhere. The -s form is essentially always an adverb, describing how something moves. Before a noun, English normally goes back to the shorter form: you write "a southwestward course", not "a southwestwards course". Like its twin, this is written-register vocabulary — maps, weather bulletins, travel writing, ships' logs. In speech people just say "southwest": "keep going southwest and you'll hit the coast".