Since this week is Severe Weather Preparedness Week in Indiana, I figured it’s a good time to have a weather post. The National Weather Service is rolling out some changes to the 159 NEXRAD weather radars sites across the country. These changes affect the Volume Coverage Patterns (VCPs) – how the radars scan the sky.
How weather radar works
To put it in the simplest terms, radars work by sending out pulses of energy and listening for the echos. The radar antenna rotates in a circle in order to get a view all around. But it doesn’t just move in a circle. The antenna also tilts upward. By moving up through increasing tilts, the radar eventually gets a 3D image of precipitation.
The key word here is “eventually”. The slowest VCP takes about 10 minutes to complete a full scan. This is generally used with clear skies or light wintry precipitation. The slow speed allows for more sensitivity and saves wear on the radar’s mechanical parts. But even the fastest scan modes take 4.5-5 minutes. During rapidly-evolving severe weather events, that can be a long time.
This spring, the weather service is rolling out changes that will introduce two new VCPs. Critically, the new software build will also remove four existing VCPs. By reduce the total number of options, forecasters will have to spend less time thinking about which radar mode to select so they can spend more time interpreting the radar data.
One of the new VCPs is focused on general precipitation and is designed to include the best parts of the patterns it replaces. The other is a new clear air pattern that shares common scan elevations with the precipitation modes and can be used for non-convective precipitation. The NWS has a paper describing the new VCPs in greater detail.
The changes will happen via software updates planned to begin later this month or in early April. It may take some time to know what the daily impact of the new patterns is. Still, it’s good to see that over 25 years after the first operational NEXRAD was deployed, the system is continuing to evolve.