How to Read a Forecast Cone: The Line Is Not the Forecast, and the Cone Is Not the Storm
Every time a typhoon approaches, the same picture goes around: a blob over the ocean, a red line running toward land, and a widening white shape around it.
Almost everyone reads that picture wrong, and the two most common mistakes are the ones that matter most if you are travelling.
The first is thinking the line is where the storm will go. The second is thinking the cone is how big the storm is.
Neither is true, and understanding why will tell you more than any headline.
What the cone actually is
The cone is a statement about forecast error, not about the storm.
Agencies look at how far off their track forecasts have been over the previous several years. At 24 hours the average miss might be a certain distance, at 72 hours a much larger one. They draw a circle of that size around each forecast point and connect the circles.
That is the cone. It is built from past mistakes, not from this storm's wind field.
The standard is roughly two out of three. The centre of the storm stays inside the cone about 66 to 70 percent of the time. Which means it goes outside it something like one time in three.
That is not a rare event. It is a normal outcome that the graphic does not visually communicate at all.
Why the cone gets wider
It widens with time because the forecast gets less certain with time, not because the storm gets bigger.
A cone drawn for tomorrow is narrow because 24-hour track forecasts are quite good now. A cone drawn for five days out is enormous because five-day forecasts still carry real uncertainty.
So a narrow cone near the start is not confidence about landfall. It is just the near term.
The line in the middle is the most misleading part
The centre line is the single most likely track. It is not a promise, and small shifts in it produce very different outcomes on the ground.
A storm centre passing 80km to one side of a city versus 80km to the other side is the difference between a windy afternoon and a serious event. On the graphic, that difference is a few pixels.
Some agencies have started removing or de-emphasising the centre line for exactly this reason. People anchor on it and stop reading anything else.
If you take one thing from this article: look at the cone, not the line, and then stop looking at the cone too.
The cone says nothing about how big the storm is
This is the mistake that gets travellers caught.
The cone shows where the centre might be. It says nothing about how far out from that centre the damaging winds and rain extend.
A compact storm might produce strong winds only 100km from its centre. A large one can spread tropical-storm-force winds 400km or more. Both get drawn with a cone of the same width, because the cone is about forecast error, not size.
So being outside the cone does not mean you are unaffected. It routinely means the opposite.
What to look at instead
Wind radii. Most agencies publish the radius at which winds reach tropical-storm force, usually plotted as a separate shaded area. That shaded area is the one that tells you whether your city gets weather.
The right side matters more. In the northern hemisphere the storm's forward motion adds to its rotation on the right-hand side of the track. That side gets stronger winds, higher surge and heavier rain. Being on the left of the track is meaningfully better than being on the right.
Rainfall forecasts, which are almost entirely separate from wind. Some of the worst typhoon disasters in Asia have come from slow-moving, unimpressive-looking storms that sat over mountains and dropped enormous rain totals. Intensity category tells you very little about flooding.
Timing. Arrival time of tropical-storm-force winds is published by several agencies and is far more useful for travel decisions than landfall time, because that is when airports stop operating and roads close, often many hours before the centre arrives.
The word landfall is doing too much work
Landfall means the centre crossing the coast. That is all.
Serious conditions begin long before it and often continue well after. In a large storm, the difficult period can start 12 hours before landfall.
Planning around a landfall hour is how people end up at an airport that stopped functioning the previous evening.
Spaghetti models are not what they look like
You will also see charts with dozens of coloured lines, each one a computer model run.
Two things about them.
They are not equally credible. Some of those models are consistently better than others, and the good ones are weighted heavily by forecasters. A chart that draws them all with the same thickness hides that.
Their spread is a rough measure of uncertainty, which is useful. When the lines cluster, confidence is higher. When they fan out across a thousand kilometres, nobody knows yet, whatever a headline says.
They are a tool for reading uncertainty, not for picking the line you like best.
Where this site has been wrong
Being honest about this is part of the point of writing here.
On this site we have described forecast spread as though a tightening cluster of models meant a settled outcome. That is half right. Clustering does raise confidence, but models can agree with each other and still be wrong together, particularly when the steering pattern itself is what is uncertain.
The safer phrasing, and the one we will use going forward, is that agreement lowers the range of likely outcomes without eliminating the chance of a shared error.
Where to check this yourself
Rather than reading about a storm second-hand, these are the sources the forecasts come from.
Japan Meteorological Agency: https://www.jma.go.jp/en/typh/
Korea Meteorological Administration: https://www.kma.go.kr/eng/weather/typoon/typhoon_05.jsp
Joint Typhoon Warning Center: https://www.metoc.navy.mil/jtwc/jtwc.html
Hong Kong Observatory: https://www.hko.gov.hk/en/index.html
China National Meteorological Center: http://www.nmc.cn/publish/typhoon/typhoon.html
Pick the one whose area you are actually in, and read that one. Comparing agencies against each other produces confusion rather than clarity, for reasons worth their own article.
A short checklist for travellers
Is my location inside the tropical-storm-force wind radius, at any point in the forecast? That is the question, not whether I am in the cone.
Am I on the right side of the track or the left?
When do those winds arrive, not when does the centre arrive?
What is the rainfall forecast, separately from the wind?
Has my airport or airline said anything yet, and have I turned on notifications?
None of those questions are answered by the picture everyone shares.
Photo credits
Photographs in this post come from Wikimedia Commons and are used under their respective Creative Commons licences.
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