The ERA-Interim web interface for pressure level data only shows 6-hourly analysis, but no forecasts.
Forecast data on pressure levels is not available through the web interface, but only through the ECMWF WebAPI. To use the API:
- First install the Web API, following these instructions
- Then run a Python script to extract ERA-Interim forecasts on pressure levels. Please see here for a sample script (at the end of page in comments)