Information Shown
on the California Regional Weather Server's
U.S. Weather-Radar Maps

(Sketch map: U.S.)

Weather-radar maps show snapshots of precipitation patterns, precipitation intensity, and other information about precipitation. (See general background information about weather radars for more info.) In particular, the weather-radar maps presented by the California Regional Weather Server show the following information:

  1. RCM ("radar coded messages") radar echoes, which show the greatest precipitation intensity (in inches/hour) recorded within each square region, 12 kilometers on a side, within the 100-200 mile range of each radar. The precipitation intensities are color-coded as listed below.

    (Note, however, that some echoes on these maps come from nearby mountains, buildings, or other reflective objects that are not precipitation. Be careful about interpreting these false echoes as precipitation--look for other, confirmatory evidence such as satellite imagery and surface weather reports. False echoes are usually of the weakest intensity, coded as blue on the maps, though much real precipitation can also be of light intensity.)

    (with symbol)
    Intensity in
    Intensity in
    Blue Light ("-") 0.0 to 0.1 in/hr 0.0 to 0.2 in/hr
    Cyan Moderate ("") 0.1 to 0.5 0.2 to 1.1
    Green Heavy ("+") 0.5 to 1.0 1.1 to 2.2
    Yellow Very Heavy ("++")   2.2 to 4.5
    Magenta Intense ("x")   4.5 to 7.1
    Red Extreme ("xx")   greater than 7.1
    White Unknown    

  2. Echo tops, expessed in hundreds of feet (so that "170" would be 17,000 ft., for example). Within a cloud, the echo top represents the highest altitude reached by precipitation particles large enough to reflect the radar beam and produce an echo; the cloud top would always be at least this high.

  3. Watch boxes, which indicate areas where the National Weather Service considers the development of some form of severe weather (associated with severe, possibly tornadic thunderstorms) to be likely in the near future. When they are issued, watch boxes are shown as white rectangles or parallelograms and labeled according to (a) the nature of the watch (e.g., "T" for tornado, "S" for severe thunderstorm); (b) the watch number; and (c) the time at which the watch expires (in UTC--see below).

(The time and date appearing on these maps is in Universal Time Coordinates (UTC), formerly known as Greenwich Mean Time (GMT). (See comments about time labels for more information.)

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