On a radar loop of Spain’s Valencian coast, several patches of heavy rain travel inland. Each storm moves, yet for a time they all seem to start in the same place. To understand why, look at the wind near the ground, including the area where rain has yet to fall.
Schematic cross-section of a convergence line near the coast.
Convergence occurs where horizontal flow brings air together. When this happens along a narrow, elongated strip, meteorologists call it a convergence line. Winds can meet at an angle, or a faster current can catch a slower one. Near the surface, some of the incoming air is forced upward.
Rising air expands and cools. If it reaches saturation, clouds form. For a thunderstorm to develop, the air also needs enough moisture and an atmospheric profile that allows it to keep rising. Some convergence lines produce little more than a row of small cumulus clouds.
As the land warms during the day, a sea breeze can carry marine air inland across the Valencian coastal plain. Where its leading edge encounters a wind blowing out from the interior, lift may strengthen along the boundary. Valleys channel the flow and the terrain affects how far the breeze penetrates. The active strip can lie near the shore or several kilometres inland.
Storms can create another boundary of their own. Rain-cooled air spreads outward when it reaches the ground. Where its leading edge meets warm, humid marine air, a new storm cell can develop.
The boundary and the storms above it can move at different speeds. If the boundary stays nearly stationary while winds aloft carry cells away, a succession of downpours may cross the same narrow strip. A town a few kilometres to one side may collect far less rain. A small forecast error in the position of the boundary can therefore change which town lies under the storms.
Centre the radar on the Valencian coast and play several frames. Follow one storm away from its starting point, then look back. If another develops there, you have found a region of repeated storm initiation. Check the wind on both sides and compare it with surface observations.
Open the Meteorix map ↗Radar shows where precipitation is occurring. Wind observations on both sides of the band help establish whether air is actually coming together there.
For official forecasts and warnings, consult your national weather service.
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