Yep you can create a ‘value template’ trigger in an automation and use something like this:
{{ (state_attr('sensor.legacy_forecasts')[1].temperature < 16) }}
In this example, the trigger passes when the forecasted temperature is less than 16. You can do some other comparisons if you want, more info in the docs here.
You might want to consider having this as an automation condition and the trigger being something else, like the time of day. It depends on what the resulting action is as you might keep triggering the automation repeatedly.