Explanation:
Although both are used, forecast is the preferred form. Forecast is an irregular verb, meaning that its past forms don’t follow the general rule of adding ed to the base.

There are lots of other verbs that are unchanged in past tense forms. You’d never say, for example, “I putted all my money into the healthcare IT tech stocks.” To a stickler, forecasted sounds just as wrong.

That said, you can probably get away with forecasted in informal writing — but why would you want to? And if you follow AP style, stick to forecast.

UsingEnglish.com explains irregular verbs and provides a handy list.

Follow us on Twitter @tao_of_grammar