Bonneville Dam grammar (#0460)

At the visitor center, an interesting (but grammatically incorrect) illustration of weather patterns along the Columbia River gorge, including an average annual of 40" in Portland, 75" here at the dam, but only 14" in The Dalles, which is just 38 miles (60 KM) east.

What’s the grammar problem? “Drought” refers to a below-average period of dryness, not a normal pattern of dryness (‘deluge’ is not as clearly an unusual amount of rain, so the grammar there is okay).