This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
烑 combines 火 (fire) with 兆, which likely contributes the sound. The exact historical development is uncertain.
The left side is fire 火, and the right side is 兆 (a sign or omen). Picture a flickering flame that seems to signal something, though its exact message is unclear.
For ヨウ, imagine a glowing fire that makes you say 'yo!' in surprise: yo -> ヨウ, and the fire 火 flickers beside the omen 兆.