To judge whether a shower may affect your plans soon, check the radar’s timestamp and legend, find your location, and animate several reflectivity scans. Follow the echo’s recent direction and speed relative to you, then check the local forecast and alerts. Radar shows sampled precipitation and its recent movement—not a guaranteed path or exact arrival time.
Read the radar in six steps
- Open a view for the relevant location. The National Weather Service (NWS) radar viewer offers a national mosaic and individual radar products, along with location, timestamp, and legend controls. It can also show a local forecast and alerts. Confirm which product is selected before interpreting its colors; the viewer’s FAQ describes its version 2.0 update as of September 6, 2023, and the operational interface may change. NWS radar FAQ
- Check the image time and legend. A still image represents one scan time. Verify that it is current, and use the legend for that display rather than assuming colors mean the same thing in every app or product.
- Play the loop or step through scans. Look for the same echo in multiple time-stamped frames. NWS precipitation volume scans are completed on a several-minute cadence that varies with the radar’s scanning strategy, so the frames are snapshots—not continuous tracking. NWS radar explainer
- Track the precipitation echo. Follow its movement across the map. A Doppler velocity product shows motion toward or away from the radar beam; it is not the same as a reflectivity loop showing precipitation echoes. NWS radar explainer
- Compare its path with your location. If the echo appears to be moving toward you, its recent movement can provide a first approximation of timing. Treat that as a rough estimate, not an arrival forecast.
- Check the local forecast and active alerts, then recheck the timestamp before acting. Radar helps show what has been observed and where it has moved; the forecast and alerts add official context about expected conditions and hazards.
What radar colors and dBZ tell you
Reflectivity is the radar energy returned by targets in the beam. It helps show where precipitation is occurring and compare stronger and weaker echoes, but it is not a direct measurement of rain reaching the ground. The relationship between the return and surface rainfall depends on the size and type of precipitation particles; hail and other targets can complicate a simple color-to-rain interpretation. NOAA JetStream puts it plainly: “The reflectivity product will show where precipitation is occurring.” NOAA JetStream: Reflectivity
dBZ is a logarithmic measure of reflectivity. NOAA JetStream gives these approximate hourly rainfall-rate reference points; they are not totals for a location:
| Reflectivity | Approximate rate in NOAA JetStream’s chart |
|---|---|
| 20 dBZ | 0.01 in/hr (trace) |
| 30 dBZ | 0.10 in/hr (3 mm/hr) |
| 40 dBZ | 0.45 in/hr (12 mm/hr) |
| 50 dBZ | 1.90 in/hr (47 mm/hr) |
These are NOAA’s approximate chart values; the retrieved page does not state a publication year. A rate is not the amount that will accumulate at your location: accumulation depends on how intensity changes and how long precipitation remains over it. Use the selected display’s legend, and do not treat a particular color as proof of a specific surface rain rate. NOAA JetStream: Reflectivity
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Estimate whether a shower may reach you
For a rough near-term estimate, compare an echo’s position in successive scans, note how far it moved and how much time passed, then compare that motion with the remaining distance to your location. This is a simple extrapolation: it assumes the echo continues at roughly the same speed and in roughly the same direction. It can be wrong if a shower grows, weakens, changes course, or a new one develops. NWS training describes extrapolating observed patterns for rainfall nowcasts while noting that forecasting storm growth or decay requires additional information. NWS radar training appendix
Choose the reflectivity view that suits the question
Do not assume one reflectivity product is best in every situation. The useful view depends on the coverage and storm detail you need, and on what the selected display makes available.
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| View | What it helps show | Important limitation |
|---|---|---|
| Base reflectivity | Precipitation detection over longer ranges and storm-scale features. | The strongest part of a storm may not be sampled at the lowest elevation; developing storms close to the radar may go undetected in this product. |
| Composite reflectivity | The highest reflectivity through the atmospheric column; useful for assessing storm intensity and showing developing storms near the radar. | It is produced after a volume scan and can mask features such as hook echoes and outflow boundaries. |
For ordinary shower planning, animate a straightforward reflectivity product, check its legend, and use the product that is available in your viewer. NOAA JetStream explains these strengths and limitations. NOAA JetStream: Reflectivity
Why a shower can be missing or misleading on radar
- Small or distant showers may be difficult to detect. A precipitation target must fill enough of the radar beam for a reliable return. NWS Glossary: beam filling
- The beam samples higher above the ground farther from the radar. It can pass over precipitation near the surface or sample only part of it, leading to low rainfall estimates. Beam overshooting is an important error source, especially at greater ranges. NWS Morristown: Radar Rainfall Estimates
- Reflectivity is an estimate, not a ground-level rain gauge. The conversion from returned energy to rainfall depends on assumptions about drop size and precipitation type. NOAA JetStream: Reflectivity NWS Morristown: Radar Rainfall Estimates
- Recent motion can change. A loop shows separate observations, not a promise that an echo will keep its course or intensity. New showers, weakening cells, or changing movement can defeat a simple estimate. NWS radar training appendix
Use radar alongside the forecast and alerts
Radar is most useful for seeing observed precipitation and its recent movement. Check the location forecast and active alerts before you decide whether to delay, reroute, or continue with a plan. The NWS viewer brings radar, local forecast information, and alerts together for a location. NWS radar FAQ
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