Storm science

Atmospheric rivers

An atmospheric river transports more water than the Amazon. When one hits the West Coast, floods, mudslides, and infrastructure failures follow. Here is the physics.

The definition

An atmospheric river (AR) is a long, narrow band of concentrated water vapor in the atmosphere. Typical dimensions: 250-500 miles wide, up to 3,000 miles long.

One moderate AR carries 7-15 times the average water flow of the Mississippi River. A strong AR can carry 25 times that.

The rating scale (AR Scale)

AR 1 — Weak

Primarily beneficial. Ends drought without significant flooding.

AR 2 — Moderate

Mostly beneficial. Some minor flooding.

AR 3 — Strong

Balance of beneficial and hazardous. Ongoing flooding likely.

AR 4 — Extreme

Primarily hazardous. Major flooding, mudslides.

AR 5 — Exceptional

Historical. Widespread catastrophic flooding.

The scale was introduced in 2019 by Scripps Institution of Oceanography. It considers water vapor transport (IVT) and duration.

The Pineapple Express

The Pineapple Express is a specific AR type: a subset that originates over the tropical Pacific near Hawaii and slams into the Pacific Northwest.

These carry warm subtropical moisture and produce heavy rain at all elevations — including melting mountain snowpack. That combination made the 1996-97 Northern California floods particularly destructive.

Notorious atmospheric rivers

1861-62 Great Flood of California — 45-day sequence of ARs. Central Valley became inland sea. Modern estimates: this event would kill 3+ million if repeated today.
1996-97 New Year's Floods (CA) — 21 dead, $3B damage.
2005 winter storms (PNW) — landslides, coastal flooding.
2015-16 water year — helped end California drought.
2022-23 winter — 12 consecutive ARs in California. Historic snowpack. $30B damage estimated.
2024 January AR — Southern California mudslides.

Where they hit

Pacific Northwest — receives 30-50% of annual precipitation from ARs.
Northern California — 50%+ of annual precipitation from ARs.
Southern California — smaller but crucial share.
Rocky Mountains — inland ARs bring snowpack.
US East Coast — occasional Atlantic ARs.
UK / Ireland — Atlantic ARs. Iceland-Faroes floods.
Chile — Pacific ARs on west coast.
South Africa — occasional.

The impacts

Flooding

Both flash and river. Reservoir overtopping.

Mudslides

Saturated hillslopes fail catastrophically. Post-wildfire terrain worst.

Coastal erosion

Combined storm surge + wave action.

Infrastructure

Levee overtopping, dam release, transportation failures.

Water supply

Positive: reservoirs fill.

Snowpack

Positive: fills mountain reservoirs for summer melt.

Agriculture

Mixed: fills wells but floods early crops.

Wildfire mitigation

Positive: increases summer moisture.

The ARkStorm scenario

The ARkStorm is a hypothetical worst-case AR scenario developed by USGS.

Modeled after 1861-62 event.
Would deposit 10-20 feet of rain over 40 days.
Estimated $725 billion damage in California.
Would displace 1.5 million people.
Central Valley would flood 40-300 feet deep in places.
Considered as likely or more likely than California's "Big One" earthquake within 40 years.
Climate change increases AR intensity 25-50% per °C warming.

Forecasting and warning

GEFS ensemble is best for medium-range AR forecasts.
ECMWF also excellent.
IVT (Integrated Vapor Transport) is the key metric — kg/m/s.
IVT > 250 = weak AR. IVT > 750 = extreme.
CW3E at Scripps issues formal AR forecasts.
NWS Weather Prediction Center issues rainfall/AR outlooks.
Emergency managers use AR probability forecasts for reservoir management.

Safety during ARs

Prepare for extended flooding.
Move to higher ground if in low-lying areas.
Check dam and levee alerts.
Stock supplies for extended power outage.
Post-fire terrain: evacuate any burn scar with debris flow risk.
Do not drive through flooded roads — same rule as flash flood.