Meteorological records define “rain” as total annual precipitation, measured by the liquid equivalent in inches, which includes rainfall, snow, sleet, and hail. The United States has an enormous range of precipitation totals due to its varied geography and proximity to large bodies of water. Totals range from less than five inches annually in the arid Southwest to hundreds of inches in mountainous coastal regions. The highest annual totals are found by looking beyond the contiguous states to the nation’s most geographically extreme territories.
The Extreme Precipitation Leaders: Alaska and Hawaii
The highest annual precipitation totals in the United States occur outside of the Lower 48 states, specifically in Alaska and Hawaii. These locations demonstrate the magnitude of moisture captured by specific geographic features. The Little Port Walter research station in the Alaskan Panhandle, on Baranof Island, holds the record among current active stations. It averages nearly 246 inches of precipitation annually, spread over an average of 239 days each year.
Hawaii contains the most extreme precipitation location recorded in the country: Mount Waialeale on the island of Kauai. Although it lacks an active, long-term weather station, historical data from 1931 to 1960 shows it averaged an astonishing 460 inches of rainfall per year. This total surpasses any other measured location in the United States, making it one of the rainiest spots in the world. Another reliably wet Hawaiian location is Mountain View on the Big Island, which averages 229 inches of precipitation over 301 rainy days annually.
Highest Rainfall Zones in the Contiguous United States
Within the contiguous United States, the most significant precipitation totals are concentrated in two distinct geographical zones: the Pacific Northwest and the Gulf Coast. The Pacific Northwest, particularly coastal Washington and Oregon, experiences high annual totals, though they are significantly lower than those in Alaska or Hawaii. For example, the Aberdeen Reservoir in Washington State sees an average yearly precipitation of about 131 inches. This high precipitation results from moisture-laden air masses from the Pacific Ocean colliding with the coastal mountain ranges.
The other major zone is the Gulf Coast and Southeastern States, including Louisiana, Mississippi, Alabama, and the Florida Panhandle. These states consistently rank among the wettest in the nation based on statewide averages, with Louisiana and Mississippi exceeding 59 inches annually. Cities like Mobile, Alabama, and New Orleans, Louisiana, often average over 60 inches each year. Unlike the Pacific Northwest, where the wet season is winter, the Southeast receives much of its precipitation during the humid summer months through intense thunderstorms and tropical systems.
The nature of the precipitation differs significantly between these two regions. The Pacific Northwest typically experiences a long season of persistent, lighter rain and drizzle. Conversely, the Gulf Coast’s high totals are driven by intense, heavy downpours associated with warm, moist air and tropical cyclones. This difference means a Pacific Northwest coastal town might have more rainy days, but a Gulf Coast city receives a greater volume of water in a shorter period, especially during the summer. The high moisture content from the Gulf of Mexico contributes to this pattern of intense, convective rainfall.
Key Geographical Factors Influencing US Rainfall Extremes
Extreme precipitation is directly caused by the interaction of atmospheric moisture with local geography. One powerful mechanism is orographic lift, which drives the exceptional rainfall in Alaska, Hawaii, and the Pacific Northwest. Orographic lift occurs when prevailing moisture-rich air is forced upward by a mountain range. This forces the air to cool, condense, and precipitate heavily on the windward side. This process creates a noticeable “rain shadow” on the leeward side of the mountains, where the air descends and warms, resulting in much drier conditions.
In the Southeast, the primary drivers are tropical moisture and convective activity. The warm waters of the Gulf of Mexico and the Atlantic Ocean supply enormous humidity, leading to frequent and intense afternoon thunderstorms throughout the summer. Tropical cyclones, including hurricanes and tropical storms, also contribute substantially to annual precipitation totals, particularly along the immediate coast. Additionally, the Pacific Coast often experiences intense precipitation events from atmospheric rivers. These are long, narrow corridors of concentrated moisture transport that deliver massive amounts of water vapor from the tropics to the western US, boosting annual totals in coastal mountain areas.