In case you were wondering.
http://uk.weather.com/weather/10day-Shepton-Mallet-BA4
The image grabs the latest weather data every 30 mins.
Most, if not all online weather forecasts update several times a day. The data is in a constant state of flux, so it's only right that their weather forecasts reflect the latest data they have.
Two or three days in advance is about the best they can do with any reliability though.