The whole process starts HERE at the NEXRAD National Mosaic Images site. Click on the "Get/View" menu item to go to the Radar Data Access page.
Click the link to NOAA's Weather and Climate Toolkit, which you will need to download and launch on your computer. It's a Java application and it will check to ensure that you have an adequately recent version of the Java run-time environment available.
Now, back at the Data Access page, click the link for the Nexrad Inventory Search tool. You'll be presented with a map showing the locations of all the NEXRAD sites. Click on the one that interests you. On the next page, select the date of interest (available dates range from about 1992 to "the day before yesterday". It takes about 48 hours for the system to crunch all the data and archive it.) Then in the list box, highlight the product of interest (one of the COMPOSITE REFLECTIVITY images is a good place to start). Then click the "Create Graph" button.
Now you are presented with a graphic that displays the population of available images for the date being examined. Below the graphic you can enter a time range (say, the time of interest plus/minus an hour) and your e-mail address. Then click on the "Order Data" button.
In a few minutes the system will send you an e-mail with a link to a dynamically-generated web page that will list the images available during the time period you've specified. The links will be in a format like this: KLWX_SDUS51_NCRLWX_200905292200. The first part is the station identifier and the last part is the date-time group. Click on the one(s) you want to keep and save them to your local drive.
Now, remember when we downloaded the viewer application a few paragraphs ago? Fire that up. It will present a graphical interface window. Pull down the Data menu and click Load Data. This will bring up a Data Selector dialog. Click the Local tab and browse to the data file you saved a minute ago from the web link in the e-mail.
The app should load the data and, Presto! There's your archival NEXRAD image. You can save it locally as a *.png file and manipulate it with any of the usual graphics software.
A useful tool set.