NWS Radar Images


Note: By their nature, radar images use color as a means of communicating information. This can be a problem for people with color vision deficiency. Visolve is a software application (free for personal use) that transforms colors of the computer display into the discriminable colors for various people including people with color vision deficiency, commonly called color blindness. Download from Visolve web site - http://www.ryobi-sol.co.jp/visolve/en.

Whether looking at the standard or enhanced version of the NWS Doppler radar display the following examples of the different images applies to both varieties. In this section, the enhanced view will be used.

The National Weather Service provides several different images from the network of Doppler radars. These images include reflectivity, velocity and rainfall information.

Reflectivity Images

Base reflectivity image in precipitation mode - click to enlargeThese images are just as they sound as they paint a picture of the weather from the energy reflected back to the radar. There are two types available on the web; Base (or ½° elevation) reflectivity and Composite reflectivity. 

Base Reflectivity is the default image. Taken from the lowest (½° elevation) slice, it is the primary image used to "see what's out there". There are two versions of Base Reflectivity image; the short range version which extends out to 124 nm (about 143 miles) and the long range version which extends out to 248 nm (about 286 miles). Composite reflectivity image in precipitation mode - click to enlargeThis image is available upon completion of the ½° elevation scan during each volume scan. View a sample base reflectivity image. 

Composite Reflectivity images utilize all elevation scans during each volume scan to create the image. It is composed of the greatest echo intensity (reflectivity) from any elevation angle seen from the radar. It is used to reveal the highest reflectivity in all echoes.

Another advantage of Composite Reflectivity is in mountainous regions. Often, the Base Reflectivity ½° elevation scan is not high enough to see over mountains. With the addition of higher elevations scans, weather information over mountain peaks can be seen. View a sample composite reflectivity image.

Velocity Images

Base velocity image of Hurricane Rita September 23, 2005 - click to enlargeOne of the best features on the 88d Doppler radar is its ability to detect motion. However, the only motion it can "see" is either directly toward or away from the radar. This is called radial velocity as it is the component of the target's motion that is along the direction of the radar beam.

Take it to the MAX! Radial Velocity

In all velocity images, red colors indicate wind moving away from the radar with green colors representing wind moving toward the radar. It is very important to know where the radar is located as that is your reference point for proper interpolation of the wind's motion.

Base Velocity images provides a picture of the basic wind field from the ½° elevation scan. It is useful for determining areas of strong wind from downbursts or detecting the speed of cold fronts. However, since the radar only measures radial velocity, the strength of the wind will always be less than what is actually occurring unless the wind is moving directly toward or away from the radar.

Storm relative motion image - click to enlarge.Also, the surface winds are only for areas near the radar. As distance increases from the radar, the reported value will be for increasing heights above the earth's surface. View sample base velocity image. 

Storm Relative Motion images are very useful images to look for small scale circulations (called mesocyclones) in thunderstorms. Often, these small scale circulations are areas where tornadoes form.

What separates storm relative motion from base velocity is the motion of storms are "subtracted" from the overall flow of the wind. As storms move, their own motion can mask circulations within themselves. This motion is removed to make the view of the wind relative to the storm. In effect, what is seen is the wind's motion as if the storms were stationary. View a sample Storm Relative Motion image.

Precipitation Images

One hour precipitation image - click to enlargeThere are two precipitation images made available via the web: One-hour Precipitation and Storm Total Precipitation. The maximum range of these two images is 124 nm (about 143 miles) from the radar location. They will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances. To determine accumulated precipitation at greater distances you should link to an adjacent radar.

Storm total precipitation image - click to enlargeOne-hour Precipitation is an image of estimated one-hour precipitation accumulation. It is used to assess rainfall intensities for flash flood warnings, urban flood statements and special weather statements. View a sample One-hour Precipitation image.

Storm Total Precipitation image is of estimated accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event. View a sample Storm Total Precipitation image.

Always check the time frame from which this image is created. There must be one hour without precipitation anywhere on the radar before the accumulation period begins again and, depending upon the weather patterns, that may be up to several days.

Weather Warnings

Typical polygon warningIf any portion of a county is affected by severe weather, the NWS issues a weather warning for the entire county. However, we actually refine the region affected by drawing the warnings in polygons to indicate the exact region we believe severe weather may occur.

Included with the radar images are graphics of severe weather warnings. (These images can be hidden on the enhanced views of the Doppler radar by toggling off the warnings.) The colors, red, yellow, green and blue represent the four types of warnings that will appear on NWS Doppler radar images.

  • Red - Tornado Warning. Issued when a tornado is imminent or occurring. A Tornado Warning implies an immediate threat to life and property.
  • Yellow - Severe Thunderstorm Warning. Issued when a severe thunderstorm is imminent or occurring. A severe thunderstorm is defined as hail 1" or greater and/or a wind speed of 58 mph (50 kts / 93 km/h) or greater.
  • Green - Flash Flood Warning. Issued with flash flooding is imminent or occurring.
  • Marine - Special Marine Warning. Issued for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings.

Next: Enhanced RIDGE Radar View