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May 26, 2019

Bombogenesis and pineapple express: Funny names for serious flood risks


Snow and ice accumulation, heavy rains, snowmelt runoffs — severe winter weather, and spring thawing often produce prime conditions that can lead to potential flood risks. Even places where flooding has not been an issue historically are still at risk. Read more about some lesser-known winter and spring weather conditions that are capable of unleashing flood-producing moisture.

Atmospheric Rivers and the Pineapple Express  

An atmospheric river (AR) is a flowing, river-like vapor of condensed water from the tropics. Atmospheric rivers are part of the Earth's ocean water cycle and are tied closely to both water supply and flood risks. In the US, 30% - 50% of average annual precipitation on the West Coast happens as a result of just a few AR events. ARs are present somewhere on Earth at any given time and average around 250-375 miles wide.

When an AR makes landfall, the water vapor is released in the form of rain or snow and can cause extreme rainfall and floods. A well-known example of an AR is the Pineapple Express, a narrow region of atmospheric moisture that builds up around Hawaii and impacts the West Coasts of the US and Canada with heavy rainfall and snow.   

In 2017, strong storms associated with the Pineapple Express brought flooding and mudslides to California, destroying homes and closing many roads. The storms brought record-breaking snow to the Sierra Nevada and significant flooding to Southern California. 

Bombogenesis Events and Nor’easters

A bombogenesis or “bomb cyclone” — a fierce and fast-growing storm resulting from warm air colliding with cold air — typically occurs between late fall and early spring. A bombogenesis forms because the atmospheric pressure drops rapidly as the two air systems create a vacuum. The North Atlantic region is often the recipient of bombogenesis events, but they occur over much of the US.  A bombogenesis often produces intense wind, blizzards, and rain in addition to snow that can create river flooding.

Nor’easters are storms that occur along the East Coast of North America where Arctic winds from the Northeast meet warmer air carried up via the Gulf Stream. A nor’easter may also turn into a bombogenesis when the pressure sinks quickly. The moisture from the ocean feeds the storm that can cause massive flooding.

One of the most notorious nor’easters was the 2011 Halloween Nor’easter, impacting the Mid-Atlantic and Northeastern US. This intense storm dumped more than 30 inches of snow in Western Massachusetts and Southern New Hampshire, causing billions of dollars in damage, and to the disappointment of many children, the cancellation of Halloween in many towns due to flooding concerns.  This is a reminder that severe weather events can cause flooding year-round.

Get Prepared

A better understanding of perennial flood exposures and their potential impact can help individuals and families make informed decisions about preparedness steps to take and how best to protect yourself and your property. Learn more in our flood whitepaper.