What is NEXRAD?

The US network of WSR-88D Doppler radars: how the radars scan, what Level II and Level III data are, how the network was built and upgraded, and where it cannot see.

NEXRAD (Next Generation Weather Radar) is the United States network of roughly 160 WSR-88D Doppler weather radars, operated jointly by the National Weather Service, the Federal Aviation Administration and the Department of Defense and supported from a single Radar Operations Center in Norman, Oklahoma.

The how to read weather radar guide explains what the radar measures and how to read it. This page is about the instrument itself: what the hardware is, how it scans, what the data levels mean, how the network has changed since the 1990s, and where its coverage runs out.

The radar: WSR-88D

WSR-88D stands for Weather Surveillance Radar, 1988, Doppler. The Radar Operations Center (ROC) publishes the fleet’s specifications. The ones that shape what you see:

S-band. The transmitter operates between 2,700 and 3,000 MHz, a wavelength of about 10 cm (NOAA National Centers for Environmental Information).
A big dish and a narrow beam. The antenna is a parabolic dish 8.53 m (28 ft) across, producing a beam 0.925° wide at 2,850 MHz, with 650 to 750 kW peak transmitter power.
Six base variables. Reflectivity, radial velocity and spectrum width, plus the dual-polarization variables: differential reflectivity, correlation coefficient and differential phase.

On the count: the ROC’s technical information sheet describes a fleet of 159 radars supporting the NWS, FAA and DoD, its About page says 158 operational systems, and NCEI describes a network of 160. The number depends on the date of the page and on whether non-operational support and training radars are included. Call it about 160.

How the network was built and upgraded

1990s: installation. The radars were installed through the decade, and the National Weather Service’s own timeline records the nationwide WSR-88D network as fully deployed in 1997.
2008: super-resolution. Software Build 10, deployed to operational sites beginning April 2008, cut the reflectivity gate from 1 km to 250 m and the azimuthal spacing from 1° to 0.5° on the lowest “split cut” elevations, generally 1.5° and below, according to the NWS notice announcing the change.
2011–2013: dual-polarization. The first operational radar was upgraded at Vance Air Force Base, Oklahoma, on March 8, 2011. The NWS announced in April 2013 that every NWS-operated radar was done, with a final FAA-operated site in Alaska scheduled for that June.
2015–2024: Service Life Extension Program. Five projects replaced or refurbished the signal processor, transmitter, equipment shelters, pedestal and backup generator. The ROC says the work keeps NEXRAD maintainable beyond 2035.

Level II and Level III

These are the two data streams most people encounter, and the difference is not cosmetic.

Level II is the base data at original resolution: reflectivity, mean radial velocity and spectrum width, and since the dual-pol upgrade the three polarimetric variables, for every elevation scan. NCEI’s archive of it runs from June 1991, though most stations began observing in the mid-1990s.

Level III is what the Radar Product Generator makes from Level II. NCEI describes it as “reduced resolution, low-bandwidth, base products as well as many derived, post-processed products”: storm-relative velocity, echo tops, precipitation estimates, hydrometeor classification and many more. Anything that has been through that processing is no longer the raw sweep.

Both are public. Beyond NCEI’s own archive, the NOAA Open Data Dissemination program has released Level II and Level III on commercial cloud storage; on Amazon Web Services, Unidata maintains buckets for the Level II archive, real-time Level II, and real-time Level III.

Volume coverage patterns

A WSR-88D does not scan one fixed way. It runs a volume coverage pattern (VCP): a defined set of elevation angles, rotation rates and pulse settings, chosen for the weather at hand. Figures below are from the ROC’s technical information sheet.

VCP	Used for	Elevations	Volume time
12	Rapidly evolving convection	14, 0.5–19.5°	~4.3 min
212	Rapidly evolving convection	14, 0.5–19.5°	~4.6 min
112	Large, high-velocity systems	14, 0.5–19.5°	~5.5 min
215	Default precipitation	15, 0.5–19.5°	~6 min
35	Default clear air	9, 0.5–6.4°	~7 min
31	Clear air, long pulse	5, 0.5–4.5°	~10 min

Two adaptive features change these times in practice. AVSET ends a precipitation volume early, at 6.4°, when there is not enough reflectivity higher up to justify the upper tilts. SAILS (Supplemental Adaptive Intra-Volume Low-Level Scan) goes the other way and inserts extra passes at the lowest elevation in the middle of the volume. SAILS arrived with Build 14 and allowed one extra low-level scan; MESO-SAILS extended it to one, two or three, and became available to all sites with Build 16.1 in 2016. With three supplemental scans in VCP 12 or 212, an NWS technical notice put lowest-elevation updates at an average of every 75 to 90 seconds.

That is why the lowest tilt in a severe-weather loop often updates several times while the upper tilts update once. It matters for low-level rotation, the subject of the tornado signatures guide, and it means the time stamp on a “volume” hides slices sampled minutes apart.

Where NEXRAD cannot see

Low altitudes far from a radar. The beam leaves at an angle and the earth curves away beneath it. In a standard 4/3-earth-radius calculation, the center of the 0.5° beam is about 1.5 km above radar level at 100 km and about 4 km at 200 km. NOAA’s 2019 study of gaps in NEXRAD coverage, prepared for Congress, puts the range at which the bottom of the beam reaches 6,000 ft above ground at about 165 km with no obstructions.
Much of the mountain West. The same study notes that most coverage gaps are in the intermountain western US, where terrain blocks the lowest tilts and radars sit far apart. Earlier work it cites also found scattered gaps in the central US at 3 km (about 10,000 ft) above ground.
Detail at range. By simple geometry, a 0.925° beam is about 1.6 km wide at 100 km. Super-resolution tightens the sampling, not the beam, so small features far out are still averaged.
Everything at once. A full volume takes roughly 4 to 10 minutes depending on the VCP. Upper tilts are older than the lowest tilt by the time a volume completes.
Non-weather targets. Birds, insects, smoke and debris all return energy. Dual-pol variables, especially correlation coefficient, help separate them, but the radar itself does not label anything.

What comes after the WSR-88D

The service life extension was designed to buy time, not to replace the radars. NOAA’s Weather Program Office says the options under evaluation include another life extension, a new rotating-dish radar, and phased-array radar, which it is studying with the National Severe Storms Laboratory. It states that the NWS will decide on the future of NEXRAD by 2030, with a target of implementing a follow-on plan by 2040.

Until then, NEXRAD is the network. Its warnings come from the National Weather Service, and those are the warnings to act on.

WeatherStory Radar reads raw Level II from all 159 WSR-88D radars in NOAA’s public archive and gives the beam height of any gate you click, computed through a 4/3-earth model.