Dave,
Good historical weather is very tough to find on the Internet. It is like detective work to find the data you need. The AWC or ADDS does not have a "local" archive. In fact, much of the way you see the data on ADDS is gone after it becomes no longer valid. You can get METARs and pilot reports in the recent past, but you won't be able to see a graphical depiction of AIRMETs or SIGMETs in the recent past as you see it on ADDS when it is current. That doesn't mean you can't recover AIRMETs or SIGMETs, but it will likely only be in textual form.
Surface observations (METARs) are very common and easy to find on the Internet form many different sources. On the other hand, pilot reports are very difficult to find in a format that is easy to search. I have spent way too many hours over the last 8 years compiling a set of URLs to pull out historical data. I use it all the time for writing and teaching. The shelf life of some data is 12-24 hours and others drop off the edge after about a week or two. In some cases you can get the data, but you may have to wait for 5 or 6 months when it is "formally" archived and available. ...
