A weather geek's guide to obtaining and viewing advanced radar imagery
On this page are links to the most recent NEXRAD Level III data files available to the public. Generally they are 5 to 10 minutes old. While many sites offer free radar imagery that does not require user interaction, these data files can be downloaded and with the appropriate software, one can magnify a region of interest for clarity, and view several types of data not found on conventional weather websites. In some cases the Level III data files are newer than pre-built images.
The catch is that these files are placed into cryptically named directories that make it difficult to determine what file contains what data and when they were generated. I have created links to the directories that are a bit more clear. Upon clicking a link below the user will be taken to the NOAA FTP site where the radar data is kept. There the user will see a large number of directories with the naming scheme
SI.xxxx
where xxxx is the 4-character FAA radar identifier. A list of radar identifiers can be found here. (You must add a "K" prefix to identifiers in the contiguous U.S., "P" to identifiers in Alaska/Hawaii/Guam, and a "T" to identifiers in the Caribbean.) Clicking on a directory will produce a listing of all the files currently on the server for that radar site. If you scroll down to the bottom of the page, there is a file called sn.last that is the data file that was most recently added to the directory, i.e., the newest radar scan. That's all you'll need to get the latest info. However, there are occasions on which you might want to look at older data; perhaps you wish to see how a particular storm evolved. Obtaining the old stuff is a bit more tricky. You must use the file-update times on the server to determine the ages of the files in relation to the current time (that of the sn.last file). Again, if you look at the list, there is a standard naming scheme:
sn.xxxx
where xxxx is the sequence number. The number of the most recent file increments with each new file, starting at 0000 and continuing to 0250, then starting back at 0000. Thus, the most recent 251 images are available for download. Find the most recent file by noting the jump in update times. Progressing down the list, the files will be in chronological order until you encounter the oldest files, which should be about two days old. That's the point at which the sequence currently stands; for the most recent data you'd want to download the last file with the current date. (Remember, all dates and times are in UTC.) So, for example, if today were March 30, you'd progress down the list until you saw files dated March 28. The last file prior to the shift would be the most recent. (As of this writing, there is something wrong with the dating and the files jump from 3/30/05 to 3/31/04 and back to 3/29/05, but the files dated 3/31/04 are current. It's probably best to ignore the year altogether, being as the archive only goes back two days.) Remember that if you just want the latest image, choose the sn.last file I mentioned above.
In order to view the file you'll need an appropriate NEXRAD viewer. Fortunately one is provided free of charge by NCDC. It is a wonderful cross-platform application that installs itself via Java Web Start and allows easy examination of the imagery. However, it requires a significant amount of memory and processor speed; you certainly want to run it on a good machine. Note that another Java program, Unidata's IDV, has the capability to display Level III data in addition to a wealth of other products. It has a distinct advantage in that it downloads most of its data automatically; all the user has to do to get fresh radar imagery is pick the radar site and data type. However, IDV is harder to install and use, and most notably requires even more memory (the user manual recommends a minimum of 1GB). A word of caution: Don't do anything important in other applications while running either of these programs, as they may run out of memory and cause your computer to crash or freeze without any warning.
The NCDC viewer permits the user to load Level III data files either from an Internet source or from files saved locally. This is another advantage over IDV, because IDV can only use data it downloads from its list of servers. Thus, NCDC's viewer can be used in the study of past events. Archived Level III data as far back as 1992 is available by request via the NCDC HDSS Access System. One note: the files found in the directories given below cannot be accessed directly by the NCDC viewer; the user must save the desired files locally and then load them from disk.
Links to the file directories are presented below; I have omitted some of the directories that contain more obscure data. For more information on the files, including how to access the other data and specific details of the naming scheme, visit NOAA RPCCDS.
I'm not going to get into the details of the individual products and their interpretation, but if you're just looking for something like you typically see on TV weather reports, you'll probably want to choose Composite Reflectivity. Then again, with the exception of the ability to zoom in and out, viewing the composite image probably won't give you any more information than a regular radar image from weather.com.
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