Where does Google Now get its weather data?

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.

Answer

Google displays Weather conditions using Weather Underground as per Google Web Search Features page.

All weather conditions and forecasts are provided by Weather Underground, Inc.

So, Google Now also might be picking from the same source.