How can weather forecasters predict the weather accurately a week in advance?
Back in the 1980s it was hit or miss. They were only correct about 50% of the time. However, today, if the weather forecasters say it is going to be hot, you know there is about a 95% chance that they are right.
If they say there is a 40% chance or higher of rain, most of the time, it rains that day.
But how can they tell in advance what the weather is going to be like?
Well to understand the larger picture is necessary. It starts with climatology and then observation. How does the weather today compare against the long past. Secondly, we project where storm systems may be in one week, that includes highs, lows, troughs, ridges, vorticity, etc.
The computer models are so much better because corrections for things like warm overrunning have mad temperature forecasts more useful and even to some extent, rain chances.
I believe better understanding of atmospheric physics, better radar, satellite and observations have all coalesced into a more clear picture of what is going on at the surface and a loft.
I’ve provided a link to the computer models used by the National Weather Service and any decent forecaster. If you have any questions about these models, please let me know.
http://www.weather.gov/mdl/synop/products.php
I’ve also listed a source where you can see actual weather discussions. This is the thought processes that go into a forecast and why. This for North Carolina but you can get this for any major city. http://forecast.weather.gov/product.php?site=NWS&issuedby=RAH&product=AFD&format=CI&version=1&glossary=0&highlight=off