Why are 24 hours forecasts not 24 hours into the future?

[n12365]

I have never understood why the 24 hour forecast prog chart is never valid 24 hours from when it is issued. I could understand if it was 30 minutes less than a full 24 hours, but the attached 24-forecast is valid 19 hours and 15 minutes from when it was issued. This is significantly less than 24 hours. It is still a useful tool, but I think it would be more appropriate to call it the 18 hours forecast. Can anyone help me understand this?

 

[Rigged4Flight]

The explanation I got when I went through aerostat training is that the difference between issued and valid is the time it takes to study the data and then determine what the forecast will be and finally publish the forecast.

This may or may not be true, but it's what I was told.

 

[bbchien]

Why it's the same reason a six year AME assisted Special Issuance is only good of rfive and a half years: evaluation time....!

 

[gismo]

I have never understood why the 24 hour forecast prog chart is never valid 24 hours from when it is issued. I could understand if it was 30 minutes less than a full 24 hours, but the attached 24-forecast is valid 19 hours and 15 minutes from when it was issued. This is significantly less than 24 hours. It is still a useful tool, but I think it would be more appropriate to call it the 18 hours forecast. Can anyone help me understand this?

AFaIK, The time quoted in the forecast is relative to the time that the data used to generate the forecast was acquired. For example a 24 hour forecast issued at 1800Z and covering the period from 1800Z to 1500Z the next day was based on measurements taken at 1500Z.

 

[MAKG1]

Analysis and especially data assimilation requires running hours of supercomputer code. This is a substantially more difficult problem than you think it is.