Understanding the Tropical Cyclone Cone of Uncertainty

Make sure you understand what the “cone of uncertainty” can—and can’t—tell you about tropical weather threats.

This image depicts a "cone of uncertainty", which shows the forecast track of a tropical cyclone.

Image: NWS National Hurricane Center

What Is the Cone of Uncertainty?

The cone of uncertainty is a graphic depicting the path that a tropical cyclone (such as a tropical storm or hurricane) is forecast to take. This information can help you stay ahead of an impending storm.

  • The cone of uncertainty shows the forecasted path for the center of a storm for up to five days
  • It does NOT show the size of storm
  • It does NOT attempt to measure or precisely locate storm impacts (storm surge, flooding, rainfall amounts or damaging wind potential)

How Accurate Is It?

Cones of uncertainty issued by the National Hurricane Center are accurate about two thirds of the time. They are drawn with the expectation that the entire track of the center of a tropical cyclone will remain inside the cone two times out of three. But, that also means that the center of the storm will move outside the cone at some point about one third of the time—an important point to remember.

How Do Meteorologists Determine the Shape of the Cone?

The cone’s shape is based on where meteorologists think the center of the storm will be at various points during the forecast period, with an expected two-thirds accuracy rate. Meteorologists also consider forecast track forecast errors from the previous 5 years (2014-2018) for storms in the Atlantic Basin. Central Pacific cones utilize errors from 2013-2017.

The cone’s shape is actually composed of several smaller circles. Each circle is centered on a specific forecast point (12 hours out, 24 hours out, etc.). The forecasted storm track runs through the centers of these circles.

A Storm’s Cone Size Grows As Its Forecast Uncertainty Grows

Each year, the National Hurricane Center determines the radii of these circles based on current data and previous forecast errors. For 2019: at 12 hours out, the center of the circle will always have a 26-mile radius with a 2/3 probability of accuracy.

Or, put more simply: 12 hours into the forecast, the storm’s center will be located within 26 miles of the forecasted point, two times out of three.

By 48 hours, forecast uncertainty increases a bit and that circle’s radius grows to 68 miles. At 120 hours (5 days) out, the cone of uncertainty has a significantly larger 198-mile radius. That is roughly the distance from New York City to Baltimore.

Better Forecasts Result in a Smaller Cone

The accuracy of tropical cyclone track forecasts has improved dramatically over the years. This has allowed the cone of uncertainty to get smaller. Moreover, a smaller cone allows emergency planners and businesses to better focus their resources on storm preparation, and it reduces the number of people that will spend unneeded time, money and emotional energy preparing for a storm that would realistically have a little-to-no chance of affecting them.

How Much Smaller is the Cone Now?

Forecast Period
(Hours)
2011 Circle Radius
(Nautical Miles)
2019 Circle Radius
(Nautical Miles)
Change in
Cone Size
12 36 26 -27%
24 59 41 -30%
36 79 54 -31%
48 98 68 -30%
72 144 102 -29%
96 190 151 -20%
120 239 198 -17%

Outside the Cone—But Not Out of Danger

Tropical cyclone behavior can be erratic. The size and intensity of a storm can change with little or no warning. A small deviation in the storm’s track can determine if a location will experience non-severe thunderstorms, or have to withstand damaging winds and life-threatening flooding.

Remember:

  • The cone only indicates where the center of the storm could be at any given point
  • Hurricane force winds (74 mph or greater) can extend to more than 150 miles from the center of a large hurricane
  • Tropical storm force winds (39 mph to 73 mph) can extend up to 300 miles from a large hurricane’s center
  • The most severe winds occur near the eyewall
  • The relatively calm eye of the storm can measure 20-40 miles across

Thus, it is imperative not to focus too much on the exact location on the cone. It is an only an estimation of the center of storm’s possible path, and those in—or anywhere near—the cone should always take precautions to ensure their safety.

PREPAREDNESS TIP: PLAN FOR WATER NOW

Power outages and infrastructure damage can render local water systems inoperable after the storm, and stores (if they are even open) are likely to be sold out of bottled water. Make sure you have a plan for potable water in place—Berkey® gravity-fed purification systems are portable and can purify raw, untreated water sources. No tools, plumbing, electricity, or water pressure are required. For more information, read our articles:

Hurricane Preparedness: A Plan for Potable Water
After the Hurricane: Contaminated Flood Water Precautions
What is the Difference Between a Tropical Depression, a Tropical Storm, and a Hurricane?

2019-09-17T11:06:10-05:00September 17th, 2019|Whatever the Weather|

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