While the storm forecast for the next 3 months is showing relatively strong storm activity in Europe during Summer 2021 (and a possibility of ex-hurricanes and heavy rains in September 2021), we should look at a more reliable predictor - the temperature in 850 hPa in 5-day intervals, to try to find, what is waiting for us during the next 3 months.
After surprising June 2021 heatwaves, which are persisting long 20 days between 10. and 30. June 2021 with only short breaks in eastern and southern Europe (but longer break in NW parts of the continent), we should speculate about the next peaks of Summer 2021.
According to current CFS outputs it appears, that the next peaks of Summer 2021 in Europe with supertropical character (above +35°C in continental Europe), should come around 10. July 2021, then after a longer break around 30. July 2021 and after the second break around 15. and 25. August 2021.
Relatively colder should be mainly periods between 15.-25. July and around 10. August (+/- 5 days).
September appears with stronger cyclonic activity, ex-hurricanes, and ex-tropical storms above Europe, but with a shorter, tropical heatwave in the middle of the month. The month should start with a powerful cooldown.
Maybe it will be good to plan vacations for the first decade of July 2021, the transition period between July and August, and the second half of August 2021, because it appears, that colder summer days with storms and rain are waiting for us, too, already in the second and third July 2021 decade, such as during the first half of August 2021.
And maybe not and forecasts will change, yet.
In every case, a short-term outlook should be relatively reliable and heatwave around (and before) 10. July is really confirming, therefore it will be good days for summer activities and vacation. However, be prepared for the risk of severe storms. After this heatwave, a longer colder period is possible for now, but we will see what will produce GFS during the first July 2021 days for the second July decade, soon.
In September 2021, be prepared for colder weather such will be maybe desired. Forecasted one of the 3 strongest hurricane seasons in history should really produce a lot of remnants of tropical systems, which are often linked with powerful cooldowns in Europe. It should bring the next worsening of COVID crisis, too (virus diseases are the worst in winter, while in summer they are reduced).