Intra-seasonal variability (bottom panel) can raise water levels a moderate amount over long periods (here 3 weeks in 2009). Hurricanes and nor'easters (e.g., Irene in 2011) generate higher, shorter peaks. "Superstorm Sandy" (top panel) was unusual in generating a storm surge that persisted through numerous tidal cycles. All data are from the Chesapeake Bay Bridge Tunnel. MLLW = Mean Lower Low Water.