What source/data provider does Google use for its weather data for Google Now (and other services)?
I've checked its forecasts against weather.gov, weather.com, accuweather.com, wunderground.com, and weatherstreet.com and can't find one that matches for the same location. The reason I ask is because there have been multiple times that it's forecast is horribly wrong. Case in point: currently its forecast is 13 degrees F lower than the closest one of the others above.
Though this question is almost 5 years old but I think an updated answer can help users looking for updated info. Nowadays Google gets its weather data from weather.com as mentioned in bottom left corner of Google weather search results when you look for temperature of any location (highlighted in red rectangle).
But as the OP says a lot of times Google's weather data is way off the actual current weather. For example as of this writing it shows current temperature in Shimla, India to be -4 C at 9 pm, but according to official website of Indian weather department or IMD even whole day's minimum temperature was 1.4 C for 6 Jan 2018 and minimum temperature of whole day usually happens in early morning hours. Similarly I have matched data for other cities and locations in India and current weather is not what actual current weather is. I think from wherever Google takes its weather data they just provide an average of previous years and not the actual current temperature. And various weather websites showing wildly different current temperatures for same location.