Weather prediction can be hard. 1 small change in the 1st day can lead to major changes 10-15 days out. Forecasts past a week are unreliable but are useful to see what might pan out. To metrologist (Not Me), seeing the general picture is useful.
Even through ECMWF may show a colder period out 12-15 days, this shows that it might be cold but you can't take anything beyond 7 days. Anything beyond 7 days usually shows the general picture, to quite a few people (like me), I read it and "Okay, we might have high pressure next week so it will be mild".
In the UK, around mid-december I saw a colder period around New Year incoming. All I could see was that the temperatures were around 3-5c lower than usual. Not until around 3 days out I started looking at what might happen during this colder period. ECMWF beyond 7 days is good at see what might pan out but take anything it says with a pinch of salt beyond 7 days. Look at the forecast over 3-4 days and if it is saying the same thing then you can assume that but if it changing, there is usually something going on in the atmosphere that is leading to the "jumpy" models
(Experts - Feel free to jump in and correct me)
I hope this shows why forecasts jump a lot
If you have any follow up question, just reply :)