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The Pineapple Express and Atmospheric Rivers

California’s Great Flood of 1862

This past weekend, Northern California passed from drought to flood as a narrow band of fast-moving air, supercharged with moisture, dumped its wet load. Welcome to the Pineapple Express, an atmospheric river bringing moist tropical air from Hawaii to the West Coast of the USA. The National Weather Service called the storm a “dangerous, high-impact weather system.

An atmospheric river is a narrow, elongated flow of moist air in the lower atmosphere. The flow corridor measures up to 2,000 km in length and is typically 400–600 km wide. The heavily moisture-laden air is in the lower atmosphere stretching up to 3 km above the earth’s surface.

Atmospheric rivers, like the Pineapple Express, form along the front edge of slow-moving, low-pressure weather systems related to the polar jet stream. The cyclone nature of these weather systems in the northern hemisphere causes winds to flow from southwest to northeast. Hence, the warm moist air from the tropics reaches the US West Coast traveling as far north as Washington and Oregon.

The current atmospheric river drenching California brought heavy, record-setting rainfall to areas in the lowlands and thick snow in higher elevations. The storm measures as a Level 5 event on a scale of 1 to 5. The Center for Western Weather and Water Extremes uses this scale as a measure of the amount of moisture in the air. The scale relates to the amount of water vapor moving onshore each second. A level 5 event moves many tons of water onshore each second, and the water flow of an atmospheric river often exceeds the water flow from the mouth of the Mississippi River.

Historical Disaster of Biblical Proportions

California’s great flood of 1862 resulted from a prolonged battering of the West Coast by a series of atmospheric river storms. During December of 1861 and January of 1862, areas from Oregon to San Diego were inundated with rain and snow from the Pineapple Express.

Over 43 days, the equivalent of 10 feet of rain fell on California. The precipitation came in the form of both rain and snow. The USGS estimates this series of super-storms produced precipitation amounts seen only every 500 to 1000 years. The rain was so heavy it melted many of the existing adobe brick homes in the region into piles of mud.

The Central Valley of California became an inland sea stretching 300 miles from north to south and 20 miles across. The water was 30 feet deep in some areas.

The culmination of this event was a blast of very warm air, which melted the high mountain snowpack. Conditions were already dire from heavy flooding caused by the rains, and the additional influx of meltwater from the mountains created a catastrophe. Whole towns disappeared under the floodwaters, and a quarter of the State’s 800,000 cattle perished. The Great Flood of 1862 was the worst disaster in California’s history. The cost of such a storm in today’s dollars is estimated at $725 billion.

Then and Now

The current situation in California is dangerous and destructive, but it only represents a fraction of how bad conditions could get if another megaflood like 1862 occurred. Today, similar to 1862, the flooding came after a prolonged drought and the State’s farmers were praying for rain. The old adage, “be careful what you ask for because you may get it,” held true both then and now.

But not everyone was lured into complacency before the great flood. By December of 1861, the indigenous tribes, who relied on the accumulated wisdom of 10,000 years, had moved to higher ground. They recognized the early warning signs.

Fortunately, today we have weather satellites and other monitoring systems to warn us days in advance of impending disaster. Both then and now, those living at low elevations and close to rivers or streams encounters the greatest risk. The basic wisdom in hydrology is: “When you live on a floodplain, you will experience flooding. It’s not a matter of if; it’s only a matter of when.” This common-sense warning is one of the most ignored facts in urban planning, and it has become the bane of many homeowners as climate change accelerates.

Related Article:

Evaluating Flood Risk is Not One of Our Strong Points (by WM House; Medium & ArcheanWeb)

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General Information about Atmospheric Rivers

An ‘extreme and possible historic atmospheric river’ is battering California(By Matthew Cappucci, Diana Leonard and Jacob Feuerstein; The Washington Post)

Atmospheric River Scale Forecast Products (Source: Center for Western Weather and Water Extremes )

Atmospheric Rivers (NASA — Earthdata)

What are atmospheric rivers? (Source: NOAA)

The Great Flood of 1862 left devastation in its path across the state (Mark Landis — The Sun)

California Megaflood: Lessons from a Forgotten Catastrophe (By B. Lynn Ingram; Scientific American)

Remembering the Great Flood that Put Northern California Under 30 Feet of Water (Source: Active)

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.