I've been using this since HTC so kindly alerted us to its existence.
I have some background on the forecast sources shown in the pulldown menu in the upper left corner.
The default for my (U.S.) location is NOAA. The next selection is met.no, the Norwegian Meteorological Institute.
The forecasts are prepared by computer models of weather changes. The Norwegian model currently runs about 4 times faster than NOAA's model and uses about 4 times as many data points as NOAA. The met.no forecasts are more accurate and cover more days in the future than NOAA. NOAA is currently preparing a new version of their forecasting software to use more data points so they can increase their accuracy.
The fellow who wrote the current NOAA model (and is working on new software) was a former student of Jim Witt and they still compare notes on forecasting. Jim Witt gives the forecast every weekday on Stephen King's Rock-and-Roll station, WKIT FM, broadcasting out of Bangor, Maine (with a signal that covers the radio void to the east of there.)
So check out the MET.NO forecast as well as the NOAA forecast on this great site.
"The belief that there is only one truth, and that oneself is in possession of it, is the root of all evil in the world."-- Max Born, Nobel Physicist