Around this time every year, hurricane forecast maps pop up all over television and the internet — including on your favorite weather blog — but your friendly neighborhood weatherman rarely has enough time to explain the forecasts to you. Here's what those hurricane forecast maps mean.

Basic Information

Every hurricane forecast map includes a legend that tells viewers crucial facts about the storm. The most important facts in the legend are usually the wind speed and direction of movement. The wind speed tells you the strongest sustained wind within the storm — in this case, Bertha had 50 MPH winds at the 2PM advisory. That doesn't mean that the wind are 50 MPH throughout the entire storm, just in the strongest part.

On maps created for The Vane, the legend will also include a text listing of watches and warnings in effect as of that advisory, if any.


Watches and Warnings

Watches and warnings are usually denoted by color-coded shading along the coastline of the landmasses impacted by the storm. As a general rule of thumb, everyone in the Atlantic and eastern Pacific use the National Hurricane Center's yellow/blue/pink/red color coding. These watches and warnings often extend farther inland than just the coast.

  • A tropical storm/hurricane watch means that tropical storm/hurricane conditions are possible within the next 48 hours
  • A tropical storm/hurricane warning means that tropical storm/hurricane conditions are possible within the next 36 hours.

The map above shows the watches and warnings in effect this afternoon for the Antilles in advance of Tropical Storm Bertha.


The Cone of Uncertainty

The cone of uncertainty is the most important part of a hurricane forecast map. The cone shows you where the center of the storm may wind up at any point within the forecast period (usually 5 days).

Weather forecasting is still an inexact science, and hurricane forecasting is even less certain. The cone of uncertainty accounts for the historical margin of error in the National Hurricane Center's forecast track. The points on a forecast map are timed out every 12 hours, so the margin of error increases in 12-hour increments. The margin of error is measured with circles radiating out from the forecast location of the storm's center, and then smoothed, creating the "cone" that everyone is familiar with.

The easiest way to understand the cone of uncertainty is to remember that, historically, the center of a tropical cyclone stays inside the cone of uncertainty 66% of the time. The center moved so far away from the forecaster's predicted path that it fell outside of the cone 33% (well, I guess it'd be 34%) of the time.

For example, at the 24-hour mark, the National Hurricane Center's historical margin of error is 52 nautical miles. This means that 66% of the time, the actual location of the center of the storm wound up within 52 nautical miles of the NHC's predicted location. This also means that 34% of the time, the center wound up outside of that 52-nautical-mile circle.

Last night when Tropical Storm Bertha formed, I mentioned that there was a slight threat that the storm could impact the eastern United States, as the cone of uncertainty covered about a hundred miles of coastline across North Carolina. As the map at the top of this post shows, that forecast has since shifted, steering the storm out to sea, but it's still worth keeping an eye on.

For a historical example, Hurricane Isaac from 2012 shows us the importance of the cone of uncertainty, as well as a great reminder that the center winds up outside of the cone about 33% of the time.

Isaac was a notoriously hard storm for forecasters to predict. I lived in Mobile at the time and the center's predicted path came straight over us for several days, but the storm wound up making landfall in Louisiana.

Speaking of which, that brings us to the next major aspect of hurricane forecasts.

The Center's Predicted Path

When maps show the line stretching through the middle of the cone of uncertainty, that's the forecaster's predicted path for the center of the storm. The eye. The tropical storm and hurricane force winds, storm surge, heavy rains, and tornadoes can extend hundreds of miles away from the center of the storm. Just because the center of Hurricane Arthur last month missed Wilmington, North Carolina, it didn't save the city from getting a good wallop. Even though the center of Tropical Storm Bertha is expected to graze the southwestern coast of Puerto Rico, the northeastern side of the island is still under a Tropical Storm Warning.

The winds around the center of a tropical storm or hurricane are usually most intense, but the cyclone's adverse effects can extend well beyond the center. Case in point: Hurricane Sandy.

Just before Sandy made landfall and completed its transition from a hurricane to an extratropical cyclone, its wind field was absolutely enormous. Even though the center made landfall in New Jersey, its tropical storm force winds extended from Myrtle Beach, South Carolina to Bangor, Maine, and its hurricane force winds stretched from Atlantic City, New Jersey down through Virginia.

Sandy is an extreme case, but it perfectly illustrates the fact that the center's path isn't always the entire story.

As we head into the peak of hurricane season this month and next, keep these points in mind when you watch The Weather Channel or see someone on social media post those hurricane forecast maps. Understanding these maps is the difference between staying informed and panicking (or, even worse, a false sense of security).

[Map of Hurricane Sandy by the NHC, all other graphics created by the author]

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