The weather in Sri Lanka is extremely difficult to forecast as there are multiple factors that can cause rainfall. It is very common for a weather website or app to forecast rain but for it not to rain. Weather apps in general are not useful tools to help forecast the weather in Sri Lanka. The limit to any forecast should be two or three days in advance. A 10 day weather forecast is very unlikely to be accurate.
Department of Meteorology
The government experts are probably the most accurate. Here are two links to the most useful forecasts:
In most areas of Sri Lanka during the majority of the year there is always a chance of a rain shower. This is especially true in the Hill Country at higher altitudes. In addition, even outside of the monsoon seasons, one or two days of prolonged rain every month is likely.