Foursquare tells people where you are. A new mobile app called Forecast, which is opening to Facebook users on Tuesday, instead tells them where you will be.

Here's how the free app for iPhone and Android works: Users make "Forecasts" that include what they plan to do and what time. Those Forecasts are broadcast to their friends, and can serve as informal invitations to join.

Friends can accept them by clicking a "me too" button, and when they arrive, they can check in the same way that they do on Foursquare. Pinnell says more than 80% of forecasts are followed through to the check-in.

"The special thing about the future is that it hasn’t happened yet," explains CEO René J. Pinnell, "which means you can change it."

This thing is especially special to advertisers.

It's the reason that Google makes so much money off of search ads. When advertisers can reach people at a time they are making a decision, like searching for a dentist or declaring their desire to go out for pizza, they can influence that decision. Targeted deals and suggestions for complementary activities are both business models that work nicely with the future checkin.

For this reason, Forecast isn't alone in its pursuit of what I call the "preemptive checkin". Ditto, Hotlist and Crowdbeacon are just a few others.

None of these, however, dominates the concept in the same way Foursquare dominates the real-time checkin.







Pinnell says that about 100,000 beta users have signed up to use the app, which launched in beta after his previous app, a group messaging app for planning parties called Hurricane Party, failed to translate well outside of South by Southwest, where it launched.

Currently, it's only been available to Foursquare users. In time for South by Southwest 2012, Forecast is opening the app up to Facebook users as well — a much bigger potential userbase.

Will you join? Let us know why or why not in the comments.

Photo courtesy of iStockphoto, mattjeacock