The map projection used in the mentioned webpage is a cylindrical map projection, with the degrees latitude and longitude marked off on the left and bottom edges. Each 10 degrees latitude is about 24 pixels, and each degree longitude is about 27 pixels. It shouldn't be too hard to take each pixel and convert it to a latitude and longitude from this data. Then you'd use the google API to change the latitude and longitude back to the google's projection, which is a variant of the mercator projection. The projections are different, so you can't simply overlay this image on a google map and expect it to match. You'll have to collect the data points in terms of lat/lon pairs, then place them on the google projection using google's tools.
It's reasonably obvious that they are extrapolating this map from limited data, though - the curves and contours suggest that there are only a handful of data points which they then process to generate the map. They themselves suggest this is the case in the text on that page explaining where they get the data from. It appears they don't release their aggregated data in any other form than the map, though. You have two options - contact them and see if they will publish the hourly data in a way you can easily gather it, or go to the sites they list as their sources of data and collect the data yourself.
Neither option is particularly easy, but one or the other should work.