How are model run time and forecast lead time related to valid time?

[blaylockbk]

For a given model initialization time and forecast lead time, what is the valid time? How does this work?

[blaylockbk]

I keep getting this question, so here's a discussion to help clarify. Maybe I (or someone else) can provide an illustration later.

When you request data, the date you request is the "model run time", then you add the forecast lead time hour to get you the "valid time." Here are some examples

The 00z model run with forecast lead time 0 hours is valid at 00:00z
The 00z model run with forecast lead time 1 hour is valid at 01:00z
The 00z model run with forecast lead time 6 hours is valid at 06:00z

The 01z model run with forecast lead time 0 hours is valid at 01:00z
The 01z model run with forecast lead time 1 hour is valid at 02:00z
The 01z model run with forecast lead time 6 hours is valid at 07:00z

The 06z model run with forecast lead time 0 hours is valid at 06:00z
The 06z model run with forecast lead time 1 hour is valid at 07:00z
The 06z model run with forecast lead time 6 hours is valid at 12:00z

Also, if you are using xarray, you can look at the "time" (model run time) and "valid_time" (time data is valid)