Weather Radar Shadowing in a Line of Cells
You are at FL350 in a twin-jet, 40 NM from a line of convective cells. The weather radar shows a strong red core, but immediately behind it the display is black. The tilt is set correctly for the range. ATC reports the cell tops at FL400 and offers a deviation 20 NM to the right. Fuel is 35 minutes above final reserve, and there is traffic 15 NM to the left at FL350.
1. Explain why the area behind the red core appears black and what hazard it may hide.
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The black area behind a strong red core is most likely a radar shadow. The radar beam is attenuated as it passes through heavy precipitation, so little energy reaches beyond the core and the return from that region is weak or absent. The display therefore shows black even though hazardous weather may exist there.
The hazard is that the shadow can hide a second, equally strong cell, turbulence, or a developing storm. A black area behind a strong core must never be read as clear air. The correct interpretation is that the region is unknown, not safe, and the aircraft should not plan a path through or immediately behind the core on the assumption that the display shows a gap.
2. Evaluate the offered deviation and decide on a course of action.
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The right deviation moves away from the cell line and has no conflicting traffic at FL350, whereas the left side has traffic 15 NM away. The right deviation is therefore the safer option, provided it is coordinated with ATC and the aircraft remains clear of the cell tops at FL400; at FL350 the aircraft is below the tops, so the deviation must give adequate lateral clearance, typically at least 20 NM from the strong core.
Request the right deviation, confirm the assigned heading and any altitude change, and keep the radar tilt adjusted to monitor the cell as you close in. Do not attempt to thread between the core and the shadow. The decision is to take the clear side with the greater margin, not the shorter path.
3. What would you do if ATC could not approve the right deviation immediately?
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If the right deviation is not immediately approved, slow down or request a hold clear of the cell line while the conflict is resolved. Reducing speed buys time and reduces the closure rate on the storm. Do not turn right without clearance in controlled airspace with traffic nearby, and do not accept a route through the shadow.
If approval is delayed and the aircraft is closing on the cell, declare the situation and ask for priority or an alternative altitude. With 35 minutes above final reserve there is limited scope for a long hold, so the crew should press for a decision and be prepared to divert if no safe clearance is available. The priority is to stay well clear of the storm, not to preserve the schedule.