Similar to tropical cyclones, extra-tropical storms cause an offshore rise of water.
However, unlike most tropical cyclone storm surge, extra-tropical storms can cause
higher water levels across a large area for longer periods of time, depending on the
system. This is due to many factors, such as storm size and different steering winds,
which could keep a system in a storm-surge prone area for longer periods of time.
Another component of extra-tropical storm surge is the phenomenon of negative water levels.
If strong winds are blowing offshore, situations can arise where mean water levels in a
bay fall significantly, which poses a serious threat for ships tied up at piers.
If negative water levels are severe enough, ships tied up at docks can actually sit
on the seafloor, preventing them from leaving port.
In North America, extra-tropical storm surges may occur on the Pacific and Alaska coasts, and north of 31°N on the Atlantic Coast. Extra-tropical storm surges may be possible for the Gulf coast mostly during the wintertime, when extra-tropical cyclones affect the coast, such as in the March 1993 Storm of the Century.