Draft

Tropical cyclone models

Why track forecasts are better than strength forecasts, and what the cone of uncertainty does and does not show.

Track and intensity

Where a tropical cyclone goes is steered by the large-scale flow around it, which global models handle well, and track forecasts have improved steadily over decades. How strong it will be is much harder: the core of intense winds is only tens of kilometres across, smaller than the boxes of a global model, and it can strengthen or weaken quickly depending on small changes in the ocean heat below, the wind shear above and the storm’s internal structure.

The models involved

TypeRoleIn this catalog
Global modelsSteer the track; used by all warning centresECMWF IFS, GFS, ICON
Global ensemblesShow the range of tracks and strengthsECMWF ENS, GEFS
Hurricane-specific modelsVery fine grid that follows the storm, to improve strength and structureHAFS
Official advisoriesForecasters combine the models into the warning you should followNHC, CPHC, JTWC, RSMC

The cone of uncertainty

now12 h24 h48 h72 h96 hBlue cone: where the centre may be (it widens with time)Orange haze: strong wind and waves reach beyond the coneDotted lines: individual ensemble members
Schematic. The cone widens because track errors grow with time. The strong wind and waves around the storm extend beyond the cone.
  • The cone is built from the average track errors of the past few years: the centre of the storm has stayed inside it roughly two times in three. So one time in three it goes outside.
  • It shows the possible path of the centre only. It does not show the size of the storm. Strong wind, rough seas and heavy rain extend far from the centre, often hundreds of miles on one side.
  • Intensity is not in the cone at all. A weak storm and a major one look the same.

For sailors

  1. Follow the official warnings from the centre for your area, and read the full text, not only the map.
  2. Look at the ensemble tracks. A wide fan means the forecast is uncertain: keep an escape route in several directions. Ensembles and probabilities
  3. Use a margin. A common mariner’s rule, the 1-2-3 rule, allows about 100 NM at one day, 200 NM at two days and 300 NM at three days around the forecast track, in addition to the storm’s own wind radius. Recent average errors are smaller, but a single forecast can be much worse.
  4. Act early. Boats are slow; leave or secure the boat while the forecast is still days away, not when the warning is for tomorrow.