Areas of precipitation and thunderstorms are observed by radar on a routine basis. Radar weather reports (RAREPs) or storm detections (SDs) are issued by radar stations at 35 minutes past the hour, with special reports issued as needed.
RAREPs provide information on the type, intensity, and location of the echo top of the precipitation. [Figure 12-11] These reports may also include direction and speed of the area of precipitation, as well as the height and base of the precipitation in hundreds of feet MSL. RAREPs are especially valuable for preflight planning to help avoid areas of severe weather. However, radar only detects objects in the atmosphere that are large enough to be considered precipitation. Cloud bases and tops, ceilings, and visibility are not detected by radar.
A typical RAREP will include:
- Location identifier and time of radar observation
- Echo pattern
- Line (LN)—a line of precipitation echoes at least 30 miles long, at least four times as long as it is wide, and at least 25 percent coverage within the line.
- Area (AREA)—a group of echoes of similar type and not classified as a line.
- Single cell (CELL)—a single isolated convective echo such as a rain shower.
- Area coverage in tenths
- Type and intensity of weather
- Azimuth, referenced to true north and range, in nautical miles from the radar site of points defining the echo pattern. For lines and areas, there will be two azimuth and range sets that define the pattern. For cells, there will be only one azimuth and range set.
- Dimension of echo pattern—given when the azimuth and range define only the center line of the pattern.
- Cell movement—movement is coded only for cells; it will not be coded for lines or areas.
- Maximum top of precipitation and location—maximum tops may be coded with the symbols “MT” or “MTS.” If it is coded with “MTS,” it means that satellite data, as well as radar information was used to measure the top of the precipitation.
- If the contraction “AUTO” appears in the report, it means the report is automated from WSR-88D weather radar data.
- The last section is primarily used to prepare radar summary charts, but can be used during preflight to determine the maximum precipitation intensity within a specific grid box. The higher the number, the greater the intensity. Two or more numbers appearing after a grid box reference, such as PM34, indicates precipitation in consecutive grid boxes.
TLX 1935 LN 8 TRW++ 86/40 199/115
20W C2425 MTS 570 AT 159/65 AUTO
^MO1 NO2 ON3 PM34 QM3 RL2=
The radar report gives the following information: The report is automated from Oklahoma City and was made at 1935 UTC. The echo pattern for this radar report indicates a line of echos covering 8⁄10 of the area. Thunderstorms and very heavy rain showers are indicated. The next set of numbers indicates the azimuth that defines the echo (86° at 40 NM and 199° at 115 NM). The dimension of this echo is given as 20 NM wide (10 NM on either side of the line defined by the azimuth and range). The cells within the line are moving from 240° at 25 knots. The maximum top of the precipitation, as determined by radar and satellite, is 57,000 feet and it is located on the 159° radial, 65 NM out. The last line indicates the intensity of the precipitation, for example in grid QM the intensity is 3, or heavy precipitation. (1 is light and 6 is extreme.)
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