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[edited by: tedster at 5:41 pm (utc) on Feb 1, 2012]
I think the drop in traffic with my site on the 24th was not necessarily due to Panda. Probably just an algorithm change with signals that downgraded various pages on my site for whatever reason.
Google did confirm a Panda refresh (recalculation of all site's quality) about a week ago.
It seems to me that the machine learning part of Panda is used to establish what factors are used in the algorithm. If those factors change (or possibly even change their relative weight) that would be an UPDATE. But in this case, the actual factors didn't change - just the data about websites that is being measured, as rustybrick explained.
We've detected that some of your site's pages may be using techniques that are outside google's Webmaster Guidelines.
Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate pagerank. Examples of unnatural linking could include buying links to pass pagerank or participating in link schemes.
We encourage you to make changes to your site so that it meets our quality guidelines. Once you've made these changes, please submit your site for reconsideration in google's search results.
If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request.
If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support.
google Search Quality Team
[edited by: ohno at 4:38 pm (utc) on Feb 2, 2012]
when the UK virtually shut down in the last severe winter we had(2010)!