Understanding PAGASA's public storm warning system, rainfall advisories

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Metro Manila (CNN Philippines) — The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) predicted that at least 17 tropical cyclones will hit the country from May to October this year after it declared the start of the rainy season last month.

To help citizens prepare for the onslaught of the storms, PAGASA issues Public Storm Warning Signals that indicate the intensity, the areas that will be affected and the forecast direction and speed of the tropical storm or typhoon.

The public storm warning system (PSWS) is as follows:

PSWS No. 1: The first signal indicates that the area affected should expect intermittent rains within at least 36 hours. Winds of 30 kph to 60 kph should be expected, although it is unlikely that they will cause significant damage. As a precaution, classes in all public and private pre-schools are automatically suspended.

PSWS No. 2: The second warning signal is raised in areas that will experience winds of 60 kph to 100 kph within at least 24 hours. Light to moderate damage is expected. Some trees may be uprooted and roofs blown away. People traveling by air and sea are cautioned, and disaster preparedness agencies should be alerting their respective communities. Classes from pre-school to high school are suspended.

PSWS No. 3: When the third warning signal is raised, people are advised to seek shelter inside strong buildings, evacuate low-lying areas, and stay away from coasts and riverbanks as moderate to heavy damage is expected. Winds of 100 kph to 185 kph are expected within at least 18 hours. The winds could topple trees and destroy crops and houses made of light materials. Widespread disruption of electrical power and communication services is also expected. Classes at all levels are automatically suspended.

PSWS No. 4: The fourth storm warning signal indicates that a very intense typhoon with winds of more than 185 kph may be expected within at least 12 hours. The typhoon is potentially very destructive. Large trees are expected to be uprooted and residential and institutional buildings could be severely damaged. Travels and outdoor activities should be cancelled.

PSWS No. 5: This recently added storm warning signal is raised when a super typhoon will affect an area. Very powerful winds of more than 220 kph may be expected in at least 12 hours. This typhoon is "extremely destructive or catastrophic" to the community as almost total damage to structures is expected. Most residential and institutional buildings may be severely damaged, and only a few crops and trees will be left standing. Evacuation to safer shelters should be completed early as it may already be too late if it hasn't begun. The disaster coordinating councils concerned and other disaster response organizations should be fully responding to emergencies by this time or completely ready to immediately respond to the calamity.

To further help citizens prepare for heavy rains and flooding, PAGASA also has the following color-coded rainfall or storm surge advisory system:

Yellow Rainfall Advisory: Citizens should expect flooding in low-lying areas as 7.5-15 mm of rainfall (8 liters per square meter/hour) is expected within one hour and is likely to continue in the next two hours.  There is also a possibility of a storm surge of .5-1 meter high.  Everyone is advised to monitor the weather condition because the rainfall warning could be raised.

Orange Rainfall Advisory: Intense rains of 15-30 mm (15-30 liters per square meter/hour) are expected within one hour and flooding is considered a definite threat in communities under this alert. Rainfall is expected to continue in the next two hours and storm surges 1 meter to 3 meters high are expected.

Red Rainfall Advisory: This rainfall advisory is issued when downpours constitute an emergency. It is raised when the torrential rainfall is more than 30 mm within one hour or if it has continued for the past three hours and has risen to more than 65 mm (30 liters per square meter/hour). Storm surges over 3 meters high are expected and will most likely cause severe damage to coastal and marine infrastructure.  Serious flooding is expected in low lying areas and evacuation to designated safe zones is recommended.