In the Online Weather Project, current radar imagery is available in several formats so we can better visualize where precipitation is falling across the nation. You have become acquainted with one such product, which is an overlay of the composite radar summary that is placed over the surface analysis that you retrieve from the current "Isobars, Fronts, Radar & Data" link on the Online Weather Homepage. This feature that includes various color-coded regions on the surface analysis allows us to see the precipitation areas and intensity for the entire coterminous U.S. Several additional charts are available that provide additional detail. A separate radar summary chart, with a link identified as "Radar", or a radar summary with fronts chart ("Radar & Fronts") can be similarly accessed from the Surface portion of the homepage.
These various composite radar summary charts are prepared hourly at 35 minutes past the hour from the data supplied by National Weather Service radar reports originating from the network of primary weather radar units.
The intent of the national radar summary chart is to graphically depict the large scale distribution of precipitation. Precipitation intensities are indicated on the chart by color codes. Some adjustment of the data may be needed, especially if precipitation echoes are reported by more than one radar site. When applicable, current severe thunderstorm and tornado watch areas ("watch boxes") are plotted on the radar summary chart.
The reflectivity or intensity of the reflected radar signal depends upon several factors to include the size, shape, density (the number of targets per unit volume), and state (rain, snow or mixture) of the hydrometeor target. Usually, the intensity of the reflected echo is assumed proportional to the rain drop density, thus indicating the precipitation rate or intensity.
Contours of echo reflectivity, also known as "VIP levels" (for Video Integrator and Processor), are plotted on the radar charts as a color coded display. The colors within these contours provide an indication of the precipitation intensity and the size depicts the areal extent of the detected precipitation. Based on empirical studies, six standard VIP levels are related to the rainfall rate for the steady "stratiform" precipitation typical of wintertime storms or warm frontal systems and for showery "convective" precipitation of representative summertime storms or cold front systems.
COLOR VIP RAINFALL RATE PRECIPITATION
LEVEL [inches per hour] INTENSITY
STRATIFORM CONVECTIVE
Blue 1 < 0.1 < 0.2 Light -
Cyan 2 0.1 to 0.5 0.2 to 1.1 Moderate
Green 3 0.5 to 1.0 1.1 to 2.2 Strong +
Yellow 4 1.0 to 2.0 2.2 to 4.5 Very Strong ++
Magenta 5 2.0 to 5.0 4.5 to 7.1 Intense X
Red 6 > 5.0 > 7.1 Extreme XX
White Unknown
Typically, the blue colors indicate light rain or snow; the green
colors identify light thunderstorms/moderate rain showers; yellows
identify moderate thunderstorms, while the colors of magenta grading
to red are reserved for the severe thunderstorms that can cause
flooding rains. On a local scale (not possible on the national
summary), an echo with red can also indicate highly reflective
water-coated hail.
The areas encompassed by an official severe weather watch issued by the Storm Prediction Center are plotted on the radar summary chart. These areas appear as a rectangular "box" outlined by dashed lines, conforming to the map coordinates of the affected area. Weather watch numbers associated with each box are placed in the box.
The weather watch numbers are sequentially numbered throughout the year and have a letter prefix to identify the type of watch: a letter "S" indicates severe thunderstorm watch, while a "T" indicates tornado watch. As an example:
T335 A tornado watch, #335.
S336 A severe thunderstorm watch, #336.
The radar summary chart helps fill in the precipitation regions between surface observation stations. Hence, the meteorologist can obtain an additional perspective of precipitation types, intensities and movements that are not provided by the conventional surface analysis. However, care must be exercised and surface observations should be inspected, because radar summaries may contain echoes of precipitation that does not reach the ground. This condition, called "virga", may exist because the air would be too dry and evaporation of the rain droplets has taken place. The inclusion of current severe weather watch boxes on the chart helps identify regions having weather conditions that are potentially conducive for severe weather.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.