A very friendly Stockholm Outlook

Indeed, the outlooks right now are stable, even across different models from different countries, indicating that a strong high pressure area will reside over Scandinavia, leading to warm autumn weather. In relative terms "warm", which means between 14-17°C during daytime, and relative mild during night time.

However due to the inflow of colder air, we already got frost nights in the inland of Sweden, all the way down to Southern Sweden. This chillier air will now gradually warm up under the influence of the high pressure area. Which a bit later will transport milder air to Scandinavia, while cutting off the inflow of cold air from the north.

This GFS prognosis was made on 22 Sep. There is a newer one below.

This GFS prognosis was made on 23 Sep

Definition of "Indian Summer" vs "Britt summer"
according to SMHI (2012)

Indian summer is essentially the same as what we in Sweden call a British summer. That is, a warm and beautiful period during the fall.

The term Indian summer is only used in Sweden and Swedish-speaking parts of Finland. However, many countries have noticed that some years there is an unusually warm period during the fall, and therefore there are a number of synonyms for Indian summer. In Norway and Denmark, for example, they borrow the Anglo-Saxon term Indian summer.

The difference between Brittsommar and Indian summer is that Indian summer is not linked to a specific day in the calendar. Brittsommar is linked to the period around St. Birgitta's Day on October 7. However, to be called Indian summer, there must have been a noticeable autumn-like period beforehand, preferably with the first night of frost already having occurred.

Uncertain origin
The origin of the expression Indian summer is partly shrouded in mystery. There are two main theories about its origin. The first theory associates the expression with a stable and warm period during the fall that North American Indians used for buffalo hunting and similar activities. According to the second theory, the expression originates from the British colonial period in India and may refer, for example, to the stable and warm period that usually follows the summer monsoon.