temperature in stockholm in september
1 The average temperature in Stockholm in September is quite mild at 11.9 Â°C (53.42 Â°F). 2  Afternoons can be mild with average high temperatures reaching 15.1 Â°C (59.2 Â°F).  Overnight temperatures are generally a little cool with an average low of 9 Â°C (48.2 Â°F).