Track forecasting has improved substantially over recent decades. Intensity forecasting has improved far less, and the hardest case is a storm that strengthens dramatically within a single day close to land.
What the environment has to supply
Rapid strengthening requires deep warm water, because a hurricane draws energy from the ocean and its own winds mix cooler water up from below.
It requires weak wind shear, since a sheared storm has its heat and moisture displaced away from the core rather than concentrated within it.
It requires a moist middle atmosphere, because dry air entrained into the circulation cools the storm through evaporation and disrupts the thunderstorms that sustain it.
Why favorable conditions are not sufficient
Many storms sit in an entirely favorable environment and never intensify quickly, which means the environment sets an upper limit rather than an outcome.
What determines whether that potential is realized happens inside the core, where the eyewall organizes and the circulation becomes symmetric.
Those processes operate over a few tens of miles, and small differences in how thunderstorms arrange themselves early on can lead to very different outcomes a day later.
Where the models run out of resolution
Global forecast models divide the atmosphere into a grid, and the spacing of that grid has historically been coarser than the features doing the work in a hurricane core.
An eyewall that occupies only a few grid cells cannot be represented in detail, so the model approximates its effects rather than simulating them.
Specialized high-resolution models nested around the storm improve on this, but they still depend on an accurate initial picture of a core that observations sample sparsely.
Why the initial conditions are so thin
Satellites see cloud tops well and the interior structure poorly, and the ocean surface beneath a storm is obscured entirely.
Reconnaissance aircraft supply direct measurements of pressure, wind and thermal structure, but they sample a limited number of tracks through a large and rapidly changing system.
Between flights the model runs forward on an incomplete picture, and small initial errors in the core amplify quickly.
Why the forecast is written the way it is
Because rapid change cannot be ruled out, advisories emphasize preparing for a stronger storm than the current forecast shows.
Evacuation decisions require lead time measured in days while intensity can change in hours, so plans are made against the plausible upper end rather than the central estimate.
A storm that arrives weaker than feared is the expected outcome of a system designed around the consequences of being wrong in the other direction.