(WGHP) – Hurricane Idalia left behind a trail of damage in Florida’s Big Bend and dumped heavy rain across the Southeast, but how were forecasters able to forecast Idalia’s landfall and the storm’s journey back out into the Atlantic?

What is the cone of uncertainty? 

Example of the cone of uncertainty for Hurricane Idalia. (WGHP)

The cone of uncertainty is drawn by the National Hurricane Center to show the possible forecast track and intensity of a tropical system. 

The cone shows the most likely track for specifically the center of the tropical system. This means that impacts will be felt even outside of the cone since it only tracks the center of a system. 

According to the National Hurricane Center, the center of a tropical system is expected to remain within the cone of uncertainty 60% to 70% of the time, based on their forecast path accuracy over the past five years. 

One of the ways forecasters at the National Hurricane Center are able to draw the cone, or the forecast path, of a tropical system is by running the data hundreds of times, taking into account current conditions in the atmosphere. These data runs spit out possible paths for the system.

The data runs are referred to as “spaghetti plots” because they look like spaghetti noodles on a map. Each line is a different model that shows a path that the center of a tropical system could take.

In the above image, spaghetti plots showed the different scenarios for Idalia’s future path over a 24-hour period. (WGHP)

While the National Hurricane Center takes into account the tropical system’s trajectory based on the computer models, they’re also looking at the big picture of what’s happening in the atmosphere that could “steer” the system in a certain direction. 

How do forecasters know where to draw the forecast track for tropical systems? 

The big picture means that forecasters are looking at the direction in which winds are blowing at different heights in the atmosphere, as well as weather patterns that may be taking shape. 

Since we’re using Idalia as an example, forecasters were taking into account a high-pressure system that was located over the Dominican Republic and Haiti. The winds around a high-pressure system like this move in a clockwise direction. 

So, when Idalia was located to the west of the high-pressure system in the Caribbean, the “steering” winds, as we call them, were south winds, blowing south to north. 

Idalia moved from just north of Cuba in the Gulf of Mexico to the Big Bend in Florida due to the direction of the “steering winds” around the high-pressure system. 

Another factor in Idalia’s movement, especially after landfall in Florida, was a cold front that moved from west to east. The winds behind a cold front are north winds, which means they are coming in from the northwest and moving southeast. 

As Idalia made landfall and continued into the southeastern United States, the position of the winds around the high-pressure system and the approaching cold front pushed Idalia in an eastward direction, “steering” the system back into the Atlantic. 


This “steering flow” is also the reason why North Carolina’s Piedmont Triad region barely saw any rainfall from Idalia. The cold front moved through the western half of North Carolina before Idalia arrived and steered the tropical system towards the North Carolina coast, bringing them a significant amount of rainfall in comparison.