KNMI publishes a number of maritime bulletins. Background information on these bulletins and explanation is given below.
The Marifoon forecast covers the coastal waters around the Netherlands up to 30 nautical mile offshore and the greater inland waters. This area is divided into the following districts: Vlissingen, Zierikzee, Hoek van Holland, IJmuiden, Texel, Harlingen, Rottum, Delfzijl, IJsselmeer and Markermeer (see map). The Marifoon is issued four times a day and covers a period of 24 hours beginning at 00:00, 06:00, 12:00 and 18:00 UTC. The forecast contains wind warnings in force (≥6 Bft) and a forecast of: general synopsis, average wind direction and force, wind gusts (if ≥ 10 knots above average wind), weather and visibility for the districts mentioned above.
General synopsis
The location and direction of the low and high pressure systems and significant troughs and ridges.
Wind direction
Indicates the direction from which the wind is blowing. Is indicated in compass zones. The changing of the wind direction is mentioned in the forecast when the change in wind direction is expected to be more than 45 degrees.
Cyclonic and Variable
Changeable wind direction in a low and high pressure area respectively.
Veering
Changing of the wind direction clockwise.
Backing
Changing of the wind direction anticlockwise.
Wind Force
The strength of the wind in Beaufort. This is the hourly average wind force. The increasing/decreasing of the wind force is mentioned in the forecast when the change in wind force is expected to be more than 1 Bft.
Wind Gusts
Is included in the Marifoon if the wind gusts are ≥ 10 knots above the hourly average wind force, e.g. if the average wind force is 4-5 Bft (11-21 knots), then the wind gusts are mentioned if they are ≥ 31 knots.
Weather
Such as rain, drizzle, hail, snow, sleet, showers, thunder and lightning, freezing rain/drizzle and ice accretion.
Visibility
Fog
Visibility less than 1000 metres.
Poor
Visibility between 1 and 4 km and below 1000 m in precipitation.
Moderate
Visibility between 4 and 10 km.
Good
Visibility more than 10 km.
The NAVTEX forecast covers the Dutch continental shelf and is divided into 4 districts: Thames, Humber, German Bight and Dogger. The NAVTEX is issued twice a day at 02:00 and 14:00 LT and covers a period of 24 hours beginning at 03:00 and 15:00 UTC respectively. The forecast contains wind warnings in force (≥7 Bft) and a forecast of: general synopsis, wind direction and force, weather, visibility and significant wave height for the districts mentioned above.
General synopsis
The location and direction of the low and high pressure systems and their core pressure in hPa. Secondly the location and direction of the significant troughs and ridges.
Wind direction
Indicates the direction from which the wind is blowing. Is indicated in compass zones. The changing of the wind direction is mentioned in the forecast when the change in wind direction is expected to be more than 45 degrees.
Cyclonic and Variable
Changeable wind direction in a low and high pressure area respectively.
Veering
Changing of the wind direction clockwise.
Backing
Changing of the wind direction anticlockwise.
Wind Force
The strength of the wind in Beaufort. This is the hourly average wind force. The increasing/decreasing of the wind force is mentioned in the forecast when the change in wind force is expected to be more than 1 Bft.
Wind Gusts
-If there are significant wind gusts the hourly averaged wind is raised by 1 Bft
-If there are significant wind gusts associated with showers, the term “squally showers” is used
Weather
Such as rain, drizzle, hail, snow, showers, thunder and lightning, freezing rain/drizzle and ice accretion.
Visibility
Fog
Visibility less than 1000 metres.
Poor
Visibility between 1 and 4 km and below 1000 m in precipitation.
Moderate
Visibility between 4 and 10 km.
Good
Visibility more than 10 km.
Significant wave height
The mean wave height of the highest third of the waves.
