Northwestwards also means towards the northwest. It is the -s twin of northwestward. In British English this is the usual adverb: the clouds moved northwestwards. As an adjective, writers more often use northwestward without the s, though you will see both. The s does not make it farther, faster, or more than one — it is the same direction, a slightly different spelling habit. If a dictionary lists both, treat them as the same word in two coats. You use it the way you use northwards: with moving, facing, and pointing.