Thundercloud is also awkward now. Is it even good? It feels worse than just casting Fire now. Fire is a GCD for 180*1.8 = 324 potency that also refreshes Astral Fire and has a 40% chance to give you the ability to- at no mana cost- throw out a 240*1.8=432 potency spell at the cost of a GCD that ALSO refreshes Astral Fire. Thundercloud only brings refreshing thunder to the table and Thunder is probably going to last you until you cast it next anyway.

What do you do when you go Fire 3->Fire IV and then see the Thundercloud proc? Using it immediately means You're looking at 4 seconds left on Astral Fire so you have to Fire afterwards, so it just took Fire IV's slot (you're down 108 potency). Any usage of Thundercloud needs to either 1) Take advantage of the extra DoT ticks or 2) Let you get that Fire IV you replaced in. Which means you're going to spend 3+3+2.5 seconds not casting Fire. Cutting it close.

But if you ignore the proc, is using Thunder during Umbral Ice even good?