Source: SMHI S-HYPE · descriptive facts, not a forecast
Calculated water temperature at Flen on October 12: about 11,6 °C. The last published measurement is 4 days old — the figure firms up as SMHI publishes new measurements.
The temperature at the surface probably swings between +0,1 °C and +0,4 °C compared with the deeper water today — coldest just before first light, warmest just after the afternoon's highest sun. How large the swing is depends on how much sun the water gets, the water's size and depth, cold rain, and wind mixing the surface.
The measurements come from Rosenholmsgölen (~2 km away) because Flen has no measuring point of its own, adjusted for the difference in elevation.
The temperature at the surface swings from about 11,7 °C to about 12,3 °C — warmest around 13:00.
A modelled estimate based on the day's weather, the lake's depth and the wind across the day. The dashed line from 01:00 is a look ahead from the latest measurements.
What does the water temperature mean for the fishing?
The water at Flen is currently around 11,6°. How the temperature plays in is something Huggtabell.se weighs together with weather, air pressure and moon into a huggtabell for Flen. Today's table is free, and if you want to plan further ahead they have forecasts reaching seven days out.