A day-long autumn heat wave is headed to southern Finland on Tuesday, according to Yle meteorologists and the Finnish Meteorological Institute (FMI). The heat wave threshold in Finland - locally known as "helleraja" - is when the thermometer breaks the 25 degree Celsius barrier.
Such warm temperatures this time of year are rare but not unprecedented. As recently as 9 September, 2002, the heat reached 25.2 degrees in the south-western town of Mietoinen.
On the first day of September of this year, the heat wave threshold was broken in western cities of Vaasa and Rauma, when temperatures reached 25.7 degrees.
The highest autumn temperature ever recorded in Finland during September - 28.8 degrees - was in Rauma, on 6 September, 1968. The latest recorded day over 25 was 17 September, 1947 when the mercury reached 25.2 in Tampere.
Not Indian summer
Yle meteorologist Anniina Valtonen said the warm, sunny weather expected on Tuesday will be caused by two low pressure systems. She said that the heat wave threshold will almost certainly be broken, if for only one day, in south-western areas.
However, the forecasted warmer temperatures will not amount to a so-called Indian summer, as that usually occurs after the thermal summer is over. There are two definitions for summer: thermal or meteorological and calendar. Thermal summer ends when daily temperatures do not exceed 10 degrees Celsius, so it is possible that an Indian summer could still be in store this autumn.
According to the calendar, summer ended in Finland just over a week ago.