Global & Disaster Medicine

North American Monsoon: June 15 – September 30

Monsoon Season or “The Monsoon”
Definition: Large scale weather pattern that causes summer thunderstorms to develop.  Moisture transported northward from Gulf of Mexico and eastern Pacific due to a reversal in wind patterns, resulting in a distinct “rainy season” over western North America. Thunderstorms with heavy rain can cause severe flooding. These storms can be powerful and sudden.

Typical Monsoon Season:

• Rainfall is not continuous over space or time

• Varies considerably and usually comes in distinct “burst” periods of heavy rain

• Accounts for over 50% of annual precipitation in northwest Mexico and Southwest U.S.

Associated Hazards

• Deadly, unexpected lightning

o A clap of thunder indicates a thunderstorm is within 10 miles

o Lightning can strike up to 60 miles away from the nearest rainfall • Local damaging straight-line winds (100+ mph)

• Flash floods, especially at low water crossings

o Water can rise quickly downstream from heavy rain, even when the thunderstorm is miles away

o Six inches of fast-moving flood waters can knock you off your feet

• Excessive heat (higher humidity and hot temps)

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