Hi Rob - we visited Iceland for a week in April 2015, and have had to scrap our plans for a Norway trip in April 2016, so Norway remains on our wish list.
As for Iceland last April, there are nightly forecasts for whether the Northern Lights will be active or subtle, as well as for the likelihood of cloud cover versus clear skies. So either one could have a factor in viewability, but as noted above, if it's overcast, you won't see anything, no matter how spectacular the light show is above the clouds.
We had overcast skies pretty much every night. We'd signed up for a "Warm Baths and Cool Skies" hot springs soak/Northern Lights viewing package, and it was announced early-on as we boarded the tour bus that the lights would likely not be viewable that night from the interior of the country.
However, we spoke with some folks one night who had gone on the Elding Tours boat (they run whale watching tours during the day, and Northern Lights watch parties at night), and they saw the Lights, while other people who'd gone on a land-based tour didn't. While weather can vary and even the best prediction is only a guesstimate, we surmised that someone out on the water, away from the land mass, might have a better chance of seeing the Northern Lights, and if we were to go again, we'd sign up for Elding's tour. We'd taken their whale watching boat on our first day, and spotted a Minke Whale. On their Northern Lights boat, they include hot chocolate, so passenger warmth and comfort are part of the package. Good luck and happy travels!