1. What do the symbols on the wind forecast charts indicate?
2. Why do your wind forecasts underestimate the wind speed of hurricanes / tropical cyclones?
3. What weather data do you use to produce your weather charts?
4. When are the weather charts updated?
5. What does UTC time mean?

1. What do the symbols on the wind forecast charts indicate?

These symbols are called "wind barbs", and they indicate both the wind speed and the wind direction at a height of 10m above sea level.  The wind direction is indicated by the angle of the barb, with the barb "flowing" from the wind direction. The speed of the wind is represented by the number of lines and/or flags on the tail. Each full line equals 10 knots and a smaller half-line is 5 knots. A flag (triangle) is equal to 50 knots. Every item should be added together to determine the speed of the wind.

The wind charts show average wind speeds. Wind gusts may be up to 40% stronger.

In each of the following examples the wind is blowing from the NW:

2. Why do the wind forecasts underestimate the wind speed of hurricanes / tropical cyclones?

The difference in wind speeds between our wind forecasts and the tropical cyclone (tropical depressions, tropical storms, hurricanes, typhoons, etc.) forecasts and warnings issued by official government agencies (such as the US National Hurricane Center) has to do with terminology and the averaging method used. Our wind charts show average wind speeds, while the tropical forecasts and warnings indicate "maximum sustained winds", which are based on maximum wind gusts, but use a shorter averaging period. These maximum sustained winds tend to be 30-40% higher than average wind speeds. Added to this is the fact that the official government agencies tend to exaggerate the intensity of tropical cyclones, to "be safe". Therefore, the maximum sustained winds from an official tropical cyclone forecast can be significantly higher than the average wind speeds that our wind forecasts indicate, sometimes as much as 60% higher.

3. What weather data do you use to produce your weather charts?

The weather data that we use to produce our weather forecast charts come from the most trusted and reliable sources available.  

The majority of our forecasts (surface wind, surface pressure, visibility, cloud cover, precipitation) are derived from the 0.5 degree GFS weather model from NOAA/NCEP.

For North America, we create higher-resolution Surface Wind (10m above sea level) charts using data from the 12 km (~0.12 degree) NAM model from NOAA/NCEP.

We also run higher-resolution Surface Wind forecasts for parts of Western Europe and the Mediterranean Sea, using the 18 km (~0.2 degree) COAMPS model, developed by the Naval Research Laboratory.

The global wave forecasts are derived from the WW3 wave model from NOAA/NCEP.

The Gulf Stream forecasts are derived from the RTOFS model from NOAA.

The Sea Surface Temperature analysis data comes from NOAA using satelite data from NASA.

For more information on the above data sources and models, please go to the About page.

4. When are the weather charts updated?

The GFS model forecast charts (surface wind, surface pressure, visibility, cloud cover, precipitation) are updated 4 times a day, usually at approximately 0445, 1045, 1645 and 2245 UTC.

The NAM wind forecast charts are updated 4 times a day, usually at 0245, 0845, 1445 and 2045 UTC.

The COAMPS wind forecast charts are updated twice a day, usually at 0330 and 1530 UTC.

The global WW3 wave forecast charts are updated 4 times a day, usually at 0530, 1130, 1730 and 2330 UTC.

The Mediterranean and Black Sea WAM4 wave forecast charts are updated once a day, usually at 1345 UTC.

The Gulf Stream forecast charts are updated once a day, usually at approximately 1115 UTC.

The Sea Surface Temperature forecast charts are updated once a day, usually at 1900 UTC. 

5. What does UTC time mean?

Coordinated Universal Time (Temps Universel Coordonné) The international time standard (formerly Greenwich Mean Time, or GMT). Zero hours UTC is midnight in Greenwich, England, which is located at 0 degrees longitude. Everything east of Greenwich (up to 180 degrees) is later in time, everything west is earlier.

The current UTC time is displayed in the upper right corner of the PassageWeather website.

If using the PassageWeather forecast images when offline, the easiest way to find the UTC time is to check your GPS, which displays UTC time very accurately.