Use ngInit: https://docs.angularjs.org/api/ng/directive/ngInit
<div ng-repeat="day in forecast_days" ng-init="f = forecast[day.iso]">
{{$index}} - {{day.iso}} - {{day.name}}
Temperature: {{f.temperature}}<br>
Humidity: {{f.humidity}}<br>
...
</div>
Example: http://jsfiddle.net/coma/UV4qF/