The company that certifies local stations, WeatheRate, is an independent team of researchers and meteorologists that survey TV weather teams all over the country. They record each local team's daily forecasts including temperature, precipitation, and other weather metrics. Then, they compare those forecasts to what actually happened.
Each team is given a daily score based on how far off their predictions landed. Every February, the team with the lowest score for the year is awarded the WeatheRate seal as certified most accurate forecast. The scoring method is like golf.
The forecast tracking starts over in March for a a new year of measurement.