Some verbs are regular verbs, which means that they follow a set pattern when forming their tenses (for example, they add –ed when forming the past tense). Other verbs are irregular verbs, which means that they don’t follow those rules; rather, each irregular verb makes its own rules, and we just have to memorize each tense.
Such is the case with broadcast and forecast. They are irregular verbs, and the past tense of broadcast is broadcast, not broadcasted, as in The show was broadcast from Hollywood last week or They broadcast the show from Hollywood last week. The same is true for cast and forecast. My husband forecast the weather for many years. Andy cast his line in the water, and the fish took the bait.
We would never think to say casted, as in Andy casted his line, and the fish took the bait, yet we do say broadcasted and forecasted. SOME dictionaries and references list these –ed forms as what are referred to as alternative forms, which means that so many people mistakenly have used them that even though they’re wrong and we should instead be teaching people the correct forms, we’re giving in and saying, ummmm, sure, you can use either the correct forms (broadcast and forecast) or these incorrect—-ummm, we mean alternative—-forms (broadcasted and forecasted).
The alternative –ed forms should be avoided. Instead, use the primary, or preferred, forms that ALL dictionaries and references agree on: broadcast, forecast, and just plain cast. That goes for telecast and simulcast, too.