When trading commodities (especially ag-related ones) weather inevitably plays a huge role if you're performing long-term analysis. Even if you are daytrading it can queue you in to the sentiment of the market in conditions which would otherwise be difficult to read.

Specifically to those of you who trade long-term (week-month basis) are there special sources which you get your weather forecast/data from? My father is a farmer and has special services specific to his state, but I'm wondering if there is anything that is more "global" so to speak.

There are lots of commercial weather services that provide this service, but probably not at price that you want to pay. Not only do they provide longer term weather forcasts but they will also interpret them into more meaningful numbers for their audience. For example in the Energy markets there is a lot of focus on ' heating and cooling degree days' as they are extremely highly correlated to energy (electricity and heating oil) demand. So not only will these vendors provide forecasts, but they will interpolate that into 'heating and cooling degree days' and in some cases even provide "population weighted heating and cooling degree days". But as @myrrdin said, a lot of longer term forcasting can be a little 'voodoo' and unreliable. Things like " La Nina" and " El Nino" do have an impact on weather, and can be identified months in advance though - but everybody is looking at that.

I can't find a link to a story now, but it was always said that Enron contracted time on the Cray Super Computers to do their weather analysis!