On 1 November 2020, Super Typhoon Rolly (international name Goni) made landfall in Catanduanes and crossed Albay and the Bicol region. It is among the strongest tropical cyclones ever recorded at landfall anywhere.

Two weeks later, Typhoon Ulysses hit Luzon. Bicol had not finished cleaning up from the first when the second arrived.

Two hazards, one event

The wind was the headline. In Catanduanes it stripped roofs and flattened structures across whole towns.

The killing hazard in Albay was different. Rolly's rain fell on the slopes of Mayon Volcano, where loose volcanic material sits on steep ground, and that material came down as mudflow. Communities in Guinobatan and elsewhere were buried in a slurry of ash, sand, and boulders.

That is a compound hazard: a weather event triggering a volcanic-material hazard on ground that a volcano prepared decades earlier.

What a volcano leaves behind

An eruption deposits ash and fragmented rock on the slopes around it. That material is loose, it is thick, and it stays for years. When enough rain falls on it, it moves as lahar or mudflow, following channels down the slope and spreading where the ground flattens.

The trigger is rain, not eruption. That is the part people miss. A volcano can be quiet for years and its deposits can still kill during a typhoon. The mechanics are in lahar arrives years after the eruption and, for the Pinatubo case, in Pinatubo's lahars.

What to screen around a volcano

1. Distance to the nearest mapped volcano at the exact coordinates. See the volcanic proximity brief.
2. The debris flow and alluvial fan flags, which is where mudflow deposition shows up in the mapped layers. See the debris flow and alluvial fan brief.
3. Distance to the nearest mapped waterway, because lahar follows channels. A dry gully on the slope of a volcano is not a feature, it is a route.
4. Slope at the coordinates and above them.
5. Flood susceptibility and elevation on the surrounding flat.

Then check the current PHIVOLCS alert level and any declared danger zone yourself. Those change, a screening does not, and PHIVOLCS is the authority. The Albay-specific version is in buying in Albay.

Wind, which is not a mapped layer either

There is no wind hazard rating on a hazard screening. Wind exposure is a structural design question governed by the National Structural Code and handled by a licensed structural engineer, using PAGASA's wind zone information.

What Rolly established, in Catanduanes especially, is that the design event is not hypothetical. For any property on the eastern seaboard, roof structure and fixing details are worth a professional look. See roof design for typhoon wind and buying in Catanduanes.

Checklist for a Bicol or volcano-adjacent property

1. Screen the exact coordinates for volcanic proximity, debris flow flags, waterway distance, slope, and flood susceptibility.
2. Check the current PHIVOLCS alert level and any declared danger zone yourself.
3. Identify every channel or gully between the volcano and the property.
4. Ask what happened here in November 2020, and again two weeks later.
5. Ask whether the property or the channel near it has been dredged, walled, or diverted since.
6. Have the roof structure and fixings assessed by a licensed structural engineer.
7. Confirm the evacuation route and how the warning reaches the household.

Two storms in two weeks

The most useful thing about that fortnight is the reminder that events do not queue politely. A property assessment that assumes you get a full year to recover between events is optimistic.

Ask how long recovery actually took, not just how bad it got. People answer that in months.

The honest limit

PHIVOLCS is the authority on volcanoes, alert levels, and danger zones, and PAGASA on storms and wind. A hazard screening reports the distance to a mapped volcano, mapped debris flow and alluvial fan flags, and terrain at your point. It does not rate wind, forecast eruptions, or model lahar for a specific channel.

CheckHazard does not replace a professional geotechnical or engineering survey.