Luis and Maymay Were Both Gone Within a Day. The Flooding They Left Behind Ran for Two Weeks, and the Thing Actually Doing It Has Its Own Name.

There is a word you pick up fast in the Philippines, and it is not bagyo.

Bagyo is the storm, and that is the part the rest of the world knows about. The named system, the track map, the wind signal numbers climbing province by province. Habagat is the other thing. It has no name, no number and no track, it does not get a cone drawn around it, and this month it is the one that put half a million people out of their houses.

The photographs in this article are stock images used for illustration, not pictures of the specific places or events described below.

Two storms that barely counted

Look at what actually crossed Luzon this month and it is almost underwhelming.

Tropical Depression Luis came in with maximum sustained winds of 55 kilometres per hour, made landfall in Isabela, and had degenerated into a low pressure area by 3 August. Tropical Storm Maymay landed in Ilocos Sur early on 6 August with maximum sustained winds of 75 kilometres per hour, crossed the island, exited past Isabela and fell apart the next day.

Neither of those is a serious storm by Philippine standards. A country that gets twenty named systems in an average year does not stop for a 75 kilometre per hour tropical storm. Both were over almost as soon as they arrived.

Now look at what the same fortnight produced. By 10 August the national tallies had 585,993 individuals, or 170,593 families, affected across nine regions from the combined effects of the enhanced southwest monsoon and those two cyclones. The Office of Civil Defense counted more than 180 flooded areas in northern Luzon. The toll as of that Monday stood at twelve dead, eight missing and eleven injured.

Twelve people, from two systems that a forecaster would describe as weak and short-lived. The arithmetic does not work unless you know what habagat is.

So what is habagat

It is the southwest monsoon. That is the whole definition, and it is not exotic.

From roughly June through September the prevailing flow over the Philippines comes up from the southwest, off the South China Sea, and it is carrying an enormous amount of water vapour. That flow hits the western side of Luzon, runs into the mountains, and has nowhere to go but up. Air that goes up cools, cools past the point where it can hold its moisture, and rains.

This happens with no storm involved at all. It is the background weather of a Philippine summer, and it is why the wet season on the west coast of Luzon looks nothing like the wet season on the east coast.

On an ordinary habagat day this is unremarkable. Grey morning, heavy afternoon, everything wet, life continues. Nobody in Manila treats plain habagat as an event, in the same way that nobody in London treats drizzle as an event.

The trouble starts when something reaches in and turns the tap.

The word forecasters use is enhanced

A tropical cyclone is, mechanically, a hole in the atmosphere that air rushes toward. Put one north or northwest of Luzon and the southwest flow underneath it does not just continue at its usual pace. It gets pulled. The storm's own circulation grabs the monsoon and drags it harder and faster across the country.

So the storm is a pump, and the monsoon is the hose.

This is why the geography of the damage looks wrong to people reading about it from outside. The rain does not fall where the storm is. It falls hundreds of kilometres away, on the side of the country facing the incoming flow, and it keeps falling for as long as the pressure pattern holds. The storm can be weak. The storm can be somewhere else entirely. The storm can have died two days ago and left the pattern behind it.

That last one is the case that gets people. A tropical depression that never came closer than a few hundred kilometres, that was downgraded and forgotten by the news cycle, can still have set up a monsoon surge that goes on dumping water long after anybody stopped watching it.

And unlike a typhoon, which comes through and leaves, this arrangement can sit there. Two weeks of it is not unusual. That is what turns an ordinary flood into a disaster: not the depth on any single day, but the fact that the ground never gets a dry hour to recover in.

The signal number is the wrong number

Here is the practical consequence, and it is the reason I wanted to write this one down.

PAGASA raises tropical cyclone wind signals, numbered one through five. Those signals are about wind. They tell you how hard it is expected to blow in a given province, they are tied to a specific named storm, and when the storm dissipates the signals come down.

Habagat flooding does not produce a wind signal. There is nothing for a signal to attach to. You can have no signal anywhere in the country and still have Metro Manila under water, because the flooding hazard is being carried by rainfall, and rainfall lives in a completely different set of warnings.

Those exist. They are the rainfall advisories, colour coded yellow, orange and red, issued per area and updated through the day, and they are the product that matters when the water is the problem rather than the wind. Red means flooding is expected in low-lying areas and you should be acting on it.

Almost nobody outside the country reads them. Visitors check whether there is a storm, find that there is not, and conclude that the weather is fine. Then they wonder why the highway is a river.

I have watched foreign coverage of Philippine floods do exactly this, treating a two-week monsoon event as though it were a typhoon that had somehow been mislabelled. It is not a mislabelling. It is a different hazard with a different warning system, and the storm is a supporting actor.

This has happened before, badly

August 2012 is the reference case. No landfall on Luzon, no signal over Metro Manila for most of it, and the capital region went under for days. Something like eighty percent of Metro Manila flooded at the peak. It is remembered locally as a monsoon event, not a typhoon, and it is the reason Filipinos take the word habagat seriously in a way that surprises outsiders.

There was a storm involved in that one too, sitting away to the north and pulling the flow. Same mechanism, larger scale.

So this month is not a freak. It is the normal behaviour of a monsoon country in monsoon season, running a little worse than average because two systems came through in the same week and kept the pattern locked in place.

What is genuinely different now is the water infrastructure sitting in the middle of it, which is a separate problem and one I will come back to.

If you are going in August, check both

Two products, not one. Check whether there is a tropical cyclone, and separately check the rainfall advisory for the specific area you will be in. PAGASA publishes both and they are not the same page.

Watch the west coast of Luzon in particular. Manila, Zambales, Pangasinan, La Union, Ilocos. Habagat rain is orographic, so it stacks up on the windward side, and the same day can be miserable in Subic and pleasant in Legazpi.

Take a dead storm seriously for another two or three days. The pattern outlasts the system that set it up, and the news stops covering it the moment the name goes away.

And if you are moving around Luzon in August, budget for the road being closed rather than for the flight being cancelled. The airports mostly keep working through this. The highways do not.

Anyway, the thing I would tell anyone arriving in August is that you are not really watching for a typhoon. You are watching for a fortnight of ordinary-looking grey rain that never stops, which is much harder to take seriously and does considerably more damage.

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