Start a workflow based on Conditional Branch in another list?

create a site workflow that will search for all the item where FutureDate == Today.

for each item found it will then start respective list workflow.

finally schedule the site workflow to run acc. your needs.

Hello Marian-- thank you. I think that's what I created, but my concern was how will that workflow start? It will not start just because FutureDate==Today is True right? That's not considered a "Start when Item is Modified"? I think I'll just have to have a workflow that runs all the time and Loops each day until FutureDate==Today = True.

how will that workflow start?

which one? site or list workflow?

site workflow would be started by a schedule, list one would be started by site workflow

It will not start just because FutureDate==Today is True right? 

you need some event to trigger a workflow. create item, modify item, schedule,....

Some day becomming equal FutureDate doesn't fire any trigger on its own.

That's not considered a "Start when Item is Modified"?

correct. it doesn't fire modified trigger since no change happened on an item

 I think I'll just have to have a workflow that runs all the time and Loops each day until FutureDate==Today = True.

no, you don't need a workflow running and looping 'all the time'.

you just need a workflow that does it job (find items for which  FutureDate==Today, start a workflow on such items...) and finishes. and you just need to schedule such a workflow to run (eg) daily. 

Thank you so much, Marian! This is a huge help.