FIS-B Weather, the Broadcast Latency Hidden in Every NEXRAD Tile on Your Cockpit Display, and the Five-Minute Rule That Separates Safe Convective Avoidance from a Mistake

The NEXRAD picture on your cockpit display is not a live feed. It is a recent photograph of where the weather was - and in active summer convection, that distinction can close a gap between two cells before your next glance at the screen. Understanding how FIS-B weather data is built, processed, and broadcast is the foundation of using it safely.

What Is FIS-B and How Does It Reach Your Cockpit?

FIS-B, or Flight Information Service Broadcast, is the weather and aeronautical data service that rides on the ADS-B infrastructure. Any pilot with an ADS-B In receiver - a Stratus, SkyGuard, Sentry, or a certified avionics box - is receiving FIS-B. The broadcasts originate from a network of approximately 700 ADS-B ground stations across the National Airspace System, operating on 978 MHz.

The FIS-B product suite includes NEXRAD composite reflectivity, PIREPs, METARs, TAFs, winds aloft, TFRs, NOTAMs, AIRMETs, and SIGMETs. All of it is free, delivered through the ADS-B infrastructure that became mandatory after January 1, 2020. The breadth of that package is genuinely remarkable. The question is how current it actually is.

How Does the NEXRAD System Actually Work?

The NEXRAD network consists of approximately 160 Doppler weather radars operated by the National Weather Service across the United States. Each one is a WSR-88D - Weather Surveillance Radar, 1988, Doppler. These radars sweep the surrounding airspace by rotating their antenna and transmitting pulses of microwave energy at a 10-centimeter wavelength. The return signal measures precipitation intensity, radial velocity, and - with dual-polarization - energy returns in both horizontal and vertical planes simultaneously.

The National Weather Service completed the dual-polarization upgrade of the entire NEXRAD network around 2013. Dual-pol allows the radar to distinguish between rain, snow, hail, freezing rain, and mixed-phase precipitation. It identifies the melting layer and filters out biological targets like bird migrations. The data quality improvement was substantial.

But one constraint cannot be engineered away: the radar must complete a full volume scan to collect its data. The antenna sweeps through multiple elevation angles - from low-altitude tilts out to steep-angle tilts examining storm tops. A full volume scan in precipitation mode takes 4 to 6 minutes. That is the fundamental update rate at the source, regardless of the software running on top of it.

How Much Latency Is Actually Built Into the FIS-B NEXRAD Picture?

The latency accumulates in layers. After a radar completes a scan, the data transmits to National Weather Service processing systems for quality control - filtering anomalous propagation returns and assembling the national mosaic composite products. That processing is automated but not instantaneous.

The processed data then moves to the FAA’s FIS-B broadcast system. The specification calls for NEXRAD composite reflectivity to update in the broadcast cycle approximately every 2.5 minutes. But the data being broadcast at any moment was already assembled several minutes earlier.

The result:

• Best case: Your receiver picks up a fresh broadcast the instant it transmits and the radar just finished a scan. The picture is approximately 5 minutes old.
• Realistic case: You glance at the screen between broadcast cycles and the radar was mid-scan when the last transmission went out. The picture is 8 to 12 minutes old.

At 25 knots - a typical mid-latitude summer storm movement rate - a cell travels approximately 7 miles in 15 minutes. A gap that looks like 7 miles on a 10-minute-old display may be 3 miles in the real world. Or it may already be closed.

Does SiriusXM Weather Solve the Latency Problem?

SiriusXM delivers weather data via satellite broadcast directly to a cockpit receiver, bypassing the ADS-B ground station network. Its NEXRAD update cycle runs approximately 2.5 to 5 minutes, and the data pipeline is somewhat more direct. It has real advantages at low altitude where ADS-B In coverage is incomplete, in remote terrain, and on the ground at airports before departure.

But the fundamental constraint does not change. The WSR-88D radar still takes 4 to 6 minutes to complete a volume scan. Neither FIS-B nor SiriusXM shows you what the weather looks like right now. Both show you a recent picture. The margin between them is meaningful in some situations and minor in others - neither one is a live feed.

Why Is This Problem Worse Near Active Convection?

The latency problem is most dangerous precisely when pilots are most likely to be staring at the screen. Near active convection, cells are moving fastest, intensifying quickest, and generating the biggest surprises. A 10-minute-old picture has the greatest chance of having diverged from reality exactly when the stakes of that divergence are highest.

FAA Advisory Circular 00-45, the agency’s primary guidance document on aeronautical weather reports and forecasts, is explicit on this point: cockpit NEXRAD displays are intended for strategic planning and situational awareness. They are not appropriate as the primary means of thunderstorm cell avoidance. That language is not boilerplate. The NTSB accident record includes documented cases where pilots continued into deteriorating weather while monitoring cockpit displays showing an apparent clear route - and the weather had moved into the flight path in the interval between the last FIS-B update and the accident sequence.

What Are the Latency Characteristics of Other FIS-B Products?

Each product in the FIS-B suite has its own currency:

• METARs broadcast on approximately a 5-minute cycle. The underlying ASOS report comes out once an hour at most stations, with special observations for significant changes. For an airport you are approaching in rapidly deteriorating conditions, the observation you receive may be approaching an hour old.
• TAFs update every 6 hours, with amendments when warranted.
• AIRMETs update on a 6-hour cycle. SIGMETs are issued as conditions develop and are reasonably current when received.
• PIREPs have a density problem more than a latency problem. The entire system depends on pilots voluntarily reporting to ATC or filing through an app. A single PIREP from an airliner at a different altitude and a different wind profile two hours ago tells you something - but extrapolating it to your aircraft and your moment requires honest judgment.

What Is TAMDAR and How Does It Improve Aviation Weather Data?

The density problem with PIREPs has been addressed incrementally over the past two decades through TAMDAR - Tropospheric Airborne Meteorological Data Reporting. TAMDAR is an automatic sensor package installed on hundreds of regional airline aircraft, including Bombardier Dash 8 Q400s, Embraer regional jets, and Saab 340s. These sensors automatically report temperature, humidity, turbulence encounters, and icing conditions throughout every flight - not when a pilot decides to file, but continuously and automatically on every segment.

The TAMDAR network, now maintained by Vaisala following their acquisition of the original developer, generates tens of thousands of automated observations per day. Those feed directly into National Weather Service forecast models and into products like the Graphical Turbulence Guidance forecast, available through ForeFlight, Garmin Pilot, and similar planning tools. It is not perfect, but it is considerably richer than voluntary PIREPs alone could produce.

What Should Be Your Primary Reference Near Active Convection?

When evaluating cells at close range, the FIS-B NEXRAD display is one input among several - not the primary reference. Real-time sources at close range include:

Out the window. Anvil tops are visible from 70 to 100 miles on a clear day. The anvil spreading downwind at high altitude, scud blowing ahead of an outflow boundary at low level, and a wall cloud forming under a rotating base are current signals happening right now.

Airborne weather radar. The microwave pulse leaves the antenna, hits the precipitation, and comes back. The display updates in seconds. Latency is measured in seconds, not minutes.

Lightning detection. A Stormscope or similar system gives strike bearing and approximate distance in near-real time. A cell generating 3 to 5 strikes per minute is active. A cell producing 20 to 30 strikes per minute is severe and intensifying. That information is current.

The FIS-B picture provides the context layer - the broader regional picture of where systems are and where they are tracking over the next hour. That layer is genuinely valuable for strategic planning. But when you are in proximity to active cells, the window, the radar, and the lightning detector are the primary real-time sources.

What Does FIS-B Coverage Look Like During Departure and Arrival?

FIS-B coverage is tiered by altitude. Below 2,000 feet AGL, particularly in mountainous terrain, coverage can be incomplete or absent. Full reliable coverage generally begins above 4,000 to 5,000 feet AGL.

This means during departure and arrival - the phases of flight where weather most directly affects a go/no-go decision - pilots may have the least complete and most delayed weather picture available. That is worth factoring in before departing under a questionable sky.

Key Takeaways

• The NEXRAD picture in your cockpit is 5 to 12 minutes old in realistic conditions - not live. In fast-moving convection, that represents 5 to 7+ miles of positional uncertainty per cell.
• A full NEXRAD volume scan takes 4 to 6 minutes at the source. No broadcast service - FIS-B or SiriusXM - can compress that fundamental physics constraint.
• FAA Advisory Circular 00-45 explicitly categorizes cockpit NEXRAD as a strategic awareness tool, not a primary means of thunderstorm avoidance.
• The latency problem is most dangerous near active convection, precisely where pilots are most likely to rely heavily on the display.
• At close range to active cells, visual cues, airborne radar, and lightning detection are the real-time references. FIS-B provides the strategic context layer around them.