TALLAHASSEE, Fla. (WTXL) — As the forecast for Tropical Storm Ian get further refined and more precise, it is inevitable that the National Hurricane Center's cone for possible movement for the center of the storm will change, either slightly or more obviously, with each updated forecast.
The NHC's forecast cone — sometimes called the cone of uncertainty — depicts the margin of error involved in the position of a tropical system. Naturally, the farther out in time a forecast goes, the broader the room for error, which can be over 200 miles east of west at the Day 5 forecast position.
When forecasts are released every six hours, they take into account various adjustments in many computer models' projections of anticipated movement of a storm.
There are times when the various guidance sources are in general agreement about how a tropical storm or hurricane's path will evolve. In these cases, there may not be much difference in how much territory a forecast cone can cover. Wider swings in periodic model data can influence a bigger adjustment in a forecast path.
Those wider swings can be more pronounced in the longer-range portion of the forecast cone, which advances in time with each new update and remains expansive. But as time passes, and the forecast data get more precise, the size of the cone narrows.
Some areas that are covered in the day 5 portion of the cone may not necessarily stay included in it by day 3 or closer. If particular areas are in the cone in the nearer part of the forecast period, the risks for direct storm impacts are greater and perhaps more certain.
It can be easy to be unsettled and anxious by forecast cone adjustments, especially in the long-term perspective of forecasting. But staying informed of how the outlook changes throughout the process can eventually relieve some of the stress that the early uncertainty creates, and it can give you a clearer perspective of how to decide your own course of action in response to a nearby tropical storm or hurricane threat.