Guide
Reading a hurricane forecast
A storm's category tells you its peak wind and nothing else. The forecast cone tells you where the centre may go and nothing about where the damage will be. Here is how to read both, in the National Hurricane Center's own words.
The Saffir-Simpson categories
NHC says the scale is based only on maximum sustained wind speed and does not take into account storm surge, rainfall flooding or tornadoes. Wind speeds in this table are sustained winds, in the National Hurricane Center's units. The Center reports wind in knots, and the app also shows miles per hour rounded to the nearest 5 the way the Center does.
| Category | Sustained wind | In km/h | Note |
|---|---|---|---|
| Category 1 | 64 to 82 knots | 119 to 153 km/h | |
| Category 2 | 83 to 95 knots | 154 to 177 km/h | |
| Category 3 | 96 to 112 knots | 178 to 208 km/h | Major hurricane |
| Category 4 | 113 to 136 knots | 209 to 251 km/h | Major hurricane |
| Category 5 | 137 knots or higher | 252 or higher km/h | Major hurricane |
Category 3 and above are called major hurricanes on the Center's page. Storm surge, rainfall flooding and tornadoes are the hazards the Center names as left out of the scale, so a category alone is not a measure of how dangerous a storm is.
The cone
The Center's own explanation of the cone is that it is formed by enclosing the area swept out by a set of circles along the forecast track, where two-thirds of historical official forecast errors over a five-year sample fall within each circle. It also says the entire track of a tropical cyclone can be expected to remain within the cone roughly 60 to 70 percent of the time. Read that carefully: by that figure the track leaves the cone roughly 30 to 40 percent of the time.
Forecast hours
The Center counts forecast hours from the synoptic time (00, 06, 12 or 18 UTC) at or before the advisory was issued. For an advisory issued at 15 UTC, the 96-hour point is valid at 12 UTC four days later. The app works out each point's time this way and rejects the file if its result disagrees with the time printed in the Center's own data.
Storms outside the National Hurricane Center's area
Outside the areas the Center covers, the app uses the Global Disaster Alert and Coordination System. It gives one wind figure for a whole storm, and that figure can be higher than the current wind. In one check, it listed 250 km/h for a storm the Center said was at 185 km/h. The app labels it as the highest wind listed for the storm.
A storm that crosses the dateline is drawn with longitudes past 180 in the Center's files. The app wraps them so the track and cone appear on the right side of the map.
Sources
- NOAA National Hurricane Center. Current storms and forecast tracks
- NHC Saffir-Simpson Hurricane Wind Scale. The category table
- NWS disclaimer and public domain statement. The terms for using NOAA and NWS information
- GDACS. GDACS terms of use. GDACS is the source for storms outside NHC's areas