Be prepared for torrential rain - Guerrilla-gohu and Oh-ame
In the summer season, what to keep in mind during your trips in Japan is typhoons that sometimes come up from the Pacific Ocean. Also, remember that you should watch out for the Guerrilla-gohu and Oh-ame heavy rainfalls, which frequently happens these days.
What are the Guerrilla-gohu and Oh-ame?
Guerrilla-gohu ゲリラ豪雨 is a downpour; the heavy rain suddenly and intensively falls as if it was like a Guerrilla attack. Oh-ame 大雨 literally means “big rain”, the rainfall that is a raining not suddenly but so heavily, more than cats and dogs.
Typhoons and Oh-ame are predictable by checking weather forecasts, and you can plan when and where to evacuate beforehand. On the other hand, Guerrilla-gohu is hardly forecasted as it suddenly hits, and tends to involve tourists in disasters.
Here you find some information about how to cope with such disasters.
Check weather forecast frequently while on your trip, especially on a mountain hike.
While both Guerrilla-gohu and Oh-ame often occur at the point where a warm air mass meets a cold air mass, knowing the movement of air masses will help you to expect the risk.
Check the nearest evacuation places for emergencies
School buildings, community centers, commercial facilities, etc.
Is there a safe place nearby where I can evacuate to?
Stay away from the riverside
The river's water level will quickly rise as the intensive rainwater is coming down from the upper stream. Once a flood occurs, you will hardly find roads underwater, and you will lose a way to evacuate. Otherwise, you may fall into the submerged river. At places on the riverside in the mountains, like BBQ sites, people should be aware of water discharge from the dam, which will rapidly raise the river's water level.
Leave the valley as soon as it starts raining (When on hiking in the mountains)
Even if it is not much raining at the site you are, the chances are that it is heavily raining at the upper place on the mountain, and it may bring a flood in the valley you are. A flash flood may also happen if a landslide occurs.
A caution board indicates "it alarms when a water discharge of dam is conducted."
A caution board indicates "the water level of this river may suddenly increase".
It is too risky to get out of buildings if a flood has already happened, especially at night.
On the submerged road, you can hardly find where to walk safely, and you may fall into a manhole, resulting in death in most cases. If you find yourself too late to evacuate, try not to take such risk and stay in a safe place as possible as you can.
At a building near mountains, avoid staying in a room on the side of the mountain slope
The heavy rain may cause landslides on mountains, with large rocks rolling downhill. If you have to stay at a building near a mountain slope, evacuate in the room on the opposite of the mountain slope.
Stay on the second floor of a building
Water floods into rooms very quickly, maybe much quicker than you expect. The first floor will be submerged in minutes.
If you need to drive a car in the middle of/after the torrential rain, watch out for water level on the road.
You can hardly tell the depth of water on flooded roads, especially half-submerged slope roads, which looks like a flat road where drivers tend to try going through, but they cannot. The car breaks down at the bottom of the slope, where the driver cannot open doors due to water pressure. In that case, the driver is to break the window screen with a hammer to get out of the car. If the driver has nothing to break the glasses, the driver needs to wait while inside of the car is filled with water so that it enables to open the door manually, usually at the very last minutes, in a touch-and-go situation before the car fully gets sunk.
Related Japanese words and phrases
Heavy rain, Downpour
Temporary evacuation area
Is there a place I can evacuate near hear?
Alert level 5
Best action to protect your life
Alert level 4
Everyone evacuate from hazardous places immediately
Alert level 3
Prepare to evacuate/The elderly and others begin evacuation
The elderly and others evacuate from hazardous places immediately.
Alert level 2
Check how to evacuate using a hazard map, etc.
Alert level 1
Raise disaster preparedness
About disaster alert by Japan Meteorological Agency (In Japanese)
Japan Meteorological Agency Website
Check the water level in rivers in Tokyo
TOKYO METROPOLITAN GOVERNMENT operates real-time monitoring system and offers information on rainfall and the water level in rivers.
Multilingual Information on Disaster Mitigation
Available in 15 languages by Japan Meteorological Agency
Last updated : Sep 2020
This article is also available in Japanese : Watch out for Oh-ame and Guerrilla-gohu