Hello,Customerand welcome back to Just Answer.
Thank you for your insightful question.
Yahoo weather gets it's information from 'weather.com', which is the Weather Channel.The Weather Channel gets its information from the National Weather Service (NWS). NOAA is the parent company of the NWS. Both weather.com and NOAA.gov get their information from satellites, and other specialized equipment. Each individual weather forecasting agency has their own meteorologists and unique equipment. Although the 'basic' information may be similar, longterm forecasting is just that--a 'forecast', dependent on how weather components will react in certain predicted situations. Weather is very changeable, as you are aware, so forecasts are not always accurate if there is a different actual situation taking place which will vary from the predicted forecast.
You should find the information at this webpage informative and interesting:http://ask.metafilter.com/144299/Whos-Responsible-for-the-Weather-Report
(read the responses in the dark green boxes)
It is usually found that the most accurate forecasts are those made by local meteorologists for their specific areas, especially when it comes to hazardous weather like tornadoes, hurricanes, etc. However, major TV stations typically get their weather info from the same reporting agencies--the National Weather Service and NOAA.
As you can see at the link above, the most accurate weather information is usually gotten from radar and satellite equipment from local airports and other state/federal municipalities, like schools and the department of highways, etc. for local areas, so they can forewarm of approaching hazardous weather.
The reason the forecast information conflicts when you go to each of these weather forecasting sites is because although meterology is a 'science', it is not an 'exact' science due to everchanging weather conditions. While one agency may say it is 83 degrees in your area, another may say it is 85, it all depends on where their specific information is coming from and where their weather/temperature sensors are located.
This information about the National Weather Service is also informative:http://en.wikipedia.org/wiki/National_Weather_Service
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