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The Cone of Uncertainty: Tropical Storm and Hurricane Forecasting Tool

To keep track of,  and to help the public, emergency and disaster management decision-makers understand where a tropical depression, tropical storm or hurricane is headed, meteorologists give each of these systems a forecast track called the cone of uncertainty (CoU) or in some quarters the cone of error.

The CoU is a tropical cyclone forecasting and warning tool used to show the likely path the center of a tropical depression, tropical storm or hurricane is forecast to take and it also shows that the further out the forecast is projected, the greater the track error will be. Given that it is a likely path, it is not an exact path, and should not be seen as a sure thing. In terms of correctness, the CoU is known to be accurate on or about two thirds of the times, or put another way, it is correct 2 out of 3 times. This means, that in general, it is expected that tropical storms and hurricanes will stay within the forecast CoU two thirds of the times. The take away message here is that one third of the times, or 1 out of 3 times, the actual track or path of the center of tropical storms or hurricanes may move outside of the CoU but this is not shown. One third is a relatively large portion of times.

Figure 1. A cone of uncertainty for Tropical Storm Elsa on Thursday July 01, 2021

The CoU starts as a point at the current location of the tropical cyclone and extends out to 5 days showing track points where meteorologists believe the center of the storm will be located, at 12 hour intervals, over the 5-day period. These track points are usually, but not always, connected by a central line that is drawn solid for the first three days and broken for the fourth and fifth days. Just to be clear, neither the track points nor track line in the cone are exact forecast paths. They can change rapidly and have changed rapidly at times because they are based on computer models, satellite data and movements of the particular storm.

Also, the CoU does not provide the size of the tropical storm or hurricane because tropical storms and hurricanes vary in size. Therefore, the cone does not give the exact locations where the storm or hurricane hazards, such as extreme rainfall amounts, flooding, storm surge, damaging winds, or tornadoes, may impact. To be clear, CoU says nothing about where impacts will be experienced, usually there are impacts such as strong wind, heavy rain, storm surge and tornadoes extending beyond the cone, even for the most accurately forecasted track. This means, impacts from a tropical storm or hurricane can occur outside of the cone. It is because of these reasons that it is important to take nothing for granted, if Trinidad and Tobago is originally placed outside of the cone of uncertainty. 

When developing the shape of the CoU, meteorologists use a set of circles of increasing size along the forecast track to show increasing uncertainty in the possible storm track over increasing time (see figure 2 for red circles). Within the CoU, there is a circle centered on each of the following hours, ahead of the storm current center: 12, 24, 36, 48, 60, 72, 96, and 120, as shown in the figure 2. Usually, these circles are not visually displayed or seen on the cone. The size of each circle is set, so that two-thirds of the National Hurricane Center official forecast track errors over the last five (5) years, fall within the circle. The radii for the circles which are used to develop the forecast for the CoU during the 2021 hurricane season are based on error statistics from 2016-2020. For the 2021 season, the 12-, 24-, 36-, 48-, 60-, 72-, 96- and 120-hour circles have the following radii 50 km, 74km, 102km, 128km, 159 km, 274km, 370km, respectively. It must be noted that in general, the entire track of the tropical cyclone is expected to remain within the cone, roughly 60-70% of the time, which has increased over the years as hurricane track forecasting became more accurate. In a given year, the size of the cone is fixed for every forecast but when the tropical cyclone is moving quickly, the cone tends to become more elongated and when it is moving slowly, the cone appears more compact, leading to the appearance of a difference in cone size, but it is the exact same cone.

Figure 2. A cone of uncertainty for Hurricane Irma, Thursday August 31, 2017.

What does all this mean? It means that at 12 hour out from the time of the forecast, the center of the tropical depression, tropical storm or hurricane is expected to be within 50 km of the forecasted storm location, two out of three times. Within a day and half (36 hours), the center of the tropical storm or hurricane is expected to be within 102 km of the forecast location, two out of three times. One can interpret this to mean that within 36 hours, the uncertainty in terms of distance will double, two out of three times. At 5 days out (120 hours), the uncertainty is likely to expand significantly, and the storm center may be found within 370 km from its forecasted location.

To put this into perspective, the island of Trinidad is 143 km in length from north to south. Thus, the error associated with the likely deviation of the tropical cyclone from the forecasted center of the tropical cyclone 5-days out, is a little more than two and a half times the north-south length of Trinidad and more than the distance between the north coast of Trinidad and the southwest coast of Grenada. The cone of uncertainty is an important forecasting and tracking tool to get a rough idea of where the storm center may go, and the current wind watches and warnings for coastal areas that may arise.


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