It's a good idea to check the Jamaica weather forecast before visiting the island. You'll be better prepared for what to expect, i.e. what activities you'll be able to participate in and also what type of clothing to pack.
But bear in mind that the weather in Jamaica doesn't change much and doesn't change often.
There are a number of websites you can check to get the current weather conditions in the island. Most will give you the 3 day forecast. In addition you can sometimes get the 10 day forecast and even the 14 day forecast for the island.
You could visit Weather.com which is owned by The Weather Channel. At this website, you'll get the current weather conditions... the forecast for tomorrow.. the forecast for the weekend... a 10-day forecast... and even a month long forecast!
They list weather forecasts for Jamaican resort areas such as Montego Bay, Ocho Rios, Negril and the capital, Kingston.
If you're trying to get the weather forecast for Runaway Bay, please note that it's very similar to Ocho Rios. This is because Runaway Bay is only 10 miles west of Ocho Rios, and both are along the coast.