There’s a lot more in a cone map than meets the eye. The map below shows the latest forecast track for Tropical Storm Barry. Here’s a short tutorial on some other things you can get out of it.

As you can see, the trajectory continues to favor landfall in Louisiana and then movement up through Arkansas. However, these maps convey much more information that may not be apparent at first glance.

What Cone Map Symbols Mean

This graphic shows an approximate representation of coastal areas under a hurricane warning (red), hurricane watch (pink), tropical storm warning (blue) and tropical storm watch (yellow).

The orange circle indicates the current position of the center of the tropical cyclone.

The black line, when selected, and dots show the National Hurricane Center (NHC) forecast track of the center at the times indicated.

The dot indicating the forecast center location will be black if the cyclone is forecast to be tropical and will be white with a black outline if the cyclone is forecast to be extratropical.

If only an L is displayed, then the system is forecast to be a remnant low. The letter inside the dot indicates the NHC’s forecast intensity for that time: 

D: Tropical Depression – wind speed less than 39 MPH
S: Tropical Storm – wind speed between 39 MPH and 73 MPH
H: Hurricane – wind speed between 74 MPH and 110 MPH
M: Major Hurricane – wind speed greater than 110 MPH

Forecasting Uncertainty

NHC tropical cyclone forecast tracks can be in error. This forecast uncertainty is conveyed by the track forecast “cone”, the solid white and stippled white areas in the cone map graphic.

The solid white area depicts the track forecast uncertainty for days 1-3 of the forecast, while the stippled area depicts the uncertainty on days 4-5.

Historical data indicate that the entire 5-day path of the center of the tropical cyclone will remain within the cone about 60-70% of the time. So about 2 out of 3 times.

How the NHC Forms Cone Maps

To form the cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96, and 120 h positions, where the size of each circle is set so that it encloses 67% of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles.

Big Difference Between Cone Size and Storm Size

Realize also that a tropical cyclone is not a point. Their effects can span many hundreds of miles from the center.

The area experiencing hurricane force (one-minute average wind speeds of at least 74 mph) and tropical storm force (one-minute average wind speeds of 39-73 mph) winds can extend well beyond the white areas shown enclosing the most likely track area of the center.

Other Linked Graphics Complete the Story

The distribution of hurricane and tropical storm force winds in this tropical cyclone can be seen in this linked Wind History graphic.

Considering the combined forecast uncertainties in track, intensity, and size, the chances that any particular location will experience winds of 34 kt (tropical storm force), 50 kt, or 64 kt (hurricane force) from this tropical cyclone are presented in tabular form for selected locations and forecast positions. This information is also presented in graphical form for the 34 kt, 50 kt, and 64 kt thresholds.

All things considered, the latest forecast shows that there’s only a 5% chance that Houston will see tropical storm force winds from this event.

If you have relatives or friends living closer to the center of the projected path, the tabular data above, will show you the exact percentages for dozens of cities.

To Find the Latest Cone Maps

Go to the National Hurricane Center and click on two-day or five-day. Then scroll to the bottom of the page for many linked graphics that tell the story of the storm.

Rainfall Graphic

Posted by Bob Rehak with Info from the National Hurricane Center on July 12, 2019

682 Days since Hurricane Harvey