I had this vexing issue where the Google Now notification would show the temperature in Fahrenheit on my desktop Chrome browser notifications. It seems that this is a known bug, and is not fixed properly. The workaround that did the trick for me (as highlighted in this comment) was to switch to Fahrenheit in Google Now on the mobile app and then switch back to Celsius again. The desktop notification finally started displaying the temperature in Celsius. A very silly issue, and a reminder that practically all the OSes (both mobile & desktop) are now being developed in a country that does not follow the metric system.