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| I'm attaching an image of the comparison between the two. Here in Michigan, Jeff Andresen, the state climatologist and I do a growing season daily comparison between the gages of the Michigan Ag Weather Network and the nexrad cells they sit in. Below is the comparison from April 1 - Oct 31, 2009.
This graph has a total of about 10,000 data points - ~50 stations across ~200 days.
For the most part it does pretty well - where you see outliers are on those big storm events. We are all familiar with those big thunderstorms where it rains like cats and dogs on one side of the road and it is dry on the other. What this stage 4 NEXRAD is doing is taking a conglomerate of smaller stage 2 / stage 3 cells and really just taking that average of those cells over that 4 km X 4 km area. So on big convective storms, there can be a lot of variability over a small area and that 4kmx4km sort of averages that out. When you go comparing it to a rain gage value - you are just measuring at a point, a 6" diameter circle where you are catching the rain. So comparing NEXRAD to rain gages is sort of like comparing an apple to a red orange. It is about the best thing you can do for a comparison. But NWS uses their own network of ground rain gages to do real time adjustments of the radar to make that stage 4 product so it comes out pretty close.
Hope this helps explain things.
 (nexradcomp.jpg)
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 nexradcomp.jpg (33KB - 47 downloads)
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NEXRAD rainfall tool
