Weather Information

This overview of weather information is an extract from the Royal Institute of Navigation booklet Electronic Navigation Systems Guidance for safe use on leisure vessels .

GRIB

GRidded Information in Binary (GRIB) is a data format that allows large amounts of historic and forecast weather data to be stored and transferred in relatively small files. It is used by most meteorological centres worldwide to generate their weather prediction models.

Weather prediction models

Global forecast data models include the Global Forecast System (GFS) model, produced by the US National Oceanic and Atmospheric Administration (NOAA), and the European Centre for Medium Range Weather Forecasting (ECMWF) model. There are other global models as well as many regional models. Data points in the global models are relatively spread apart, typically 9km or more between data points. Regional models reprocess the global data on a much finer grid of closer to 1km between data points, factoring in a more detailed representation of land features.

Weather apps

Weather forecasts and weather routeing capabilities may now be packaged together in a single app. The variety of such apps has markedly improved the access to, and use of, digital weather information. All of the various weather apps source their data from one or more of the data models. Some things to be aware of
  • A relatively low resolution model, with data points spread 9km or more apart, is usually adequate offshore, away from the effects of land
  • Regional interpolations are better for coastal sailing, but meaningful data can still only be produced for land features that are bigger than the data grid. So, even at the much higher regional resolution any localised coastal effects, such as acceleration zones off high peaks, around headlands or up and down river valleys, are unlikely to be factored in with any accuracy
  • Forecast timing is important. Some apps do not use the most recent data set and, depending on the frequency at which source data is published, there may be up to a twelve hour time lag in what we see in an app
  • High resolution interpolations tend to give unstable results after about 36-48 hours, so using them as the weather input for a routeing app passage plan for longer passages is usually counterproductive.

The best apps use recent, original data, at the best available resolution, from a reputable source. It is easy to get sucked into the comfort of professional displays and easy-to-use routeing programs without understanding the data behind them, but it is still just as important as ever to understand and monitor basic weather patterns in order to judge how accurately the screen predictions match reality. We should monitor a variety of forecasts for several days before a passage and then continue to monitor them, as much as possible, during the passage itself.

When weather systems are more settled the forecasts will tend to align. When systems are more chaotic the different models tend to interpolate differently and there may be diverse presentations of forecast weather. In itself this is a good indicator of how trustworthy, or not, any of the predictions might be.

Weather Sources

The list below can be downloaded for offline use as a Progressive Web App using this link.

RCCPF PublicationCountrySource 
AllWorldwide http://weather.gmdss.org/ Full Details
AllWorldwide http://weather.mailasail.com/Franks-Weather/Home Full Details
AllWorldwide https://magicseaweed.com/ Full Details
AllWorldwide https://sailmail.com/ Full Details
AllWorldwide https://windy.app/ Full Details
AllWorldwide https://winlink.org/ Full Details
AllWorldwide https://www.passageweather.com/ Full Details
AllWorldwide https://www.predictwind.com/ Full Details
AllWorldwide https://www.rain-alarm.com/ Full Details
AllWorldwide https://www.theyr.com/app.asp Full Details
AllWorldwide https://www.weatheronline.co.uk/ Full Details
AllWorldwide https://www.weathertrack.us/intro.html Full Details
AllWorldwide https://www.windfinder.com/ Full Details
AllWorldwide https://www.windguru.cz/53 Full Details
AllWorldwide https://xcweather.co.uk/ Full Details
AllWorldwide Full Details
AllWorldwide https://www.squid-sailing.com/en/ Full Details
North AfricaAlgeria https://meteofrance.com/ meteo-marine Full Details
 Antigua and Barbuda antiguamet.com/ Full Details
South Africa to the CaribbeanAntigua and Barbuda http://www.antiguamet.com/ Full Details
Pacific Crossing GuideAustralia bom.gov.au/marine/lite/ Full Details
Pacific Crossing GuideAustralia http://www.bom.gov.au/marine/index.shtml Full Details
 Belgium afdelingkust.be/en/data/sea-and-coastal-weather-forecast Full Details
 Belgium https://www.meteo.be/en/belgium Full Details
Atlantic IslandsBermuda http://weather.bm/ Full Details
BrazilBrazil https://portal.inmet.gov.br/ Full Details
South China SeaBrunei http://www.bruneiweather.com.bn/weather Full Details
Cape Horn and Antarctic WatersChile http://web.directemar.cl/met/jturno/indice/english.htm Full Details
South China SeaChina (Taiwan) https://www.cwb.gov.tw/V8/E/M/TownCoastal/TownCoastal_Map.html Full Details
Baltic SeaDenmark http://ocean.dmi.dk/english/ Full Details
Baltic SeaEstonia https://www.ilmateenistus.ee/meri/mereprognoosid/mereilm/?lang=en Full Details
Cape Horn and Antarctic WatersFalkland Islands "https://www.metoffice.gov.uk/ weather/specialist-forecasts/ coast-and-sea" Full Details
Fiji Lau GroupFiji https://www.met.gov.fj/ Full Details
Fiji Lau GroupFiji http://metvuw.com/ows/ Full Details
Fiji Lau GroupFiji http://vmgd.gov.vu/vmgd/index.php Full Details
Fiji Lau GroupFiji https://www.weather.gov/marine/ Full Details
Baltic SeaFinland https://en.ilmatieteenlaitos.fi/marine-weather-and-baltic-sea Full Details
Channel Islands, Cherbourg Peninsula & North BrittanyFrance https://meteofrance.com/meteo-marine Full Details
Baltic SeaGermany https://www.dwd.de/EN/Home/home_node.html Full Details
Crete : A Yachting Pilot for CreteGreece http://www.emy.gr/emy/en/navigation/naftilia Full Details
South China SeaHong Kong http://www.hko.gov.hk/en/wservice/tsheet/pms/mariners.htm Full Details
 Ireland https://www.met.ie/ Full Details
 Italy http://www.meteoam.it/meteomar/burrasca Full Details
Corsica and North SardiniaItaly http://www.meteoam.it/meteomar/burrasca Full Details
 Jamaica http://metservice.gov.jm/marine-forecast/ Full Details
South Africa to the CaribbeanJamaica http://metservice.gov.jm/marine-forecast/ Full Details
Pacific Crossing GuideJapan http://www.jma.go.jp/en/seafcst/ Full Details
South China SeaJapan http://www.jma.go.jp/en/seafcst/ Full Details
Baltic SeaLatvia https://marine.meteo.lv/ Full Details
North AfricaLibya https://meteofrance.com/meteo-marine Full Details
Baltic SeaLithuania http://www.meteo.lt/en/sea-forecasts Full Details
South China SeaMalaysia http://www.met.gov.my/?lang=en Full Details
North AfricaMorocco https://meteofrance.com/meteo-marine Full Details
South Africa to the CaribbeanNamibia www.meteona.com Full Details
 Netherlands https://www.knmi.nl/nederland-nu/maritiem Full Details
 New Zealand metvuw.com/ows/ Full Details
Pacific Crossing GuideNew Zealand http://metvuw.com/ows/ Full Details
NorwayNorway https://www.yr.no/hav_og_kyst/ Full Details
South China SeaPhilippines http://www.typhoon2000.ph/ Full Details
South China SeaPhilippines http://bagong.pagasa.dost.gov.ph/marine Full Details
Baltic SeaPoland http://maps.meteo.pl/ Full Details
Baltic SeaPoland https://baltyk.imgw.pl/ Full Details
Atlantic IslandsPortugal http://www.ipma.pt/pt/index.html Full Details
Atlantic Spain & PortugalPortugal http://www.ipma.pt/en/index.html Full Details
Baltic SeaRussia https://meteoinfo.ru/en/ Full Details
South China SeaSingapore http://www.weather.gov.sg/home/ Full Details
South Africa to the CaribbeanSouth Africa http://marine.weathersa.co.za/Forecasts_Home.html Full Details
South Africa to the CaribbeanSouth Africa http://wavenet.csir.co.za Full Details
Atlantic IslandsSpain http://www.aemet.es/es/eltiempo/prediccion/maritima Full Details
Atlantic Spain & PortugalSpain http://www.aemet.es/es/eltiempo/prediccion/maritima Full Details
 St Lucia https://met.gov.lc/ Full Details
South Africa to the CaribbeanSt Lucia https://met.gov.lc/ Full Details
Baltic SeaSweden https://www.smhi.se/en Full Details
South China SeaThailand https://www.tmd.go.th/en/shipping_forecast.php Full Details
 Trinidad and Tobago metoffice.gov.tt Full Details
South Africa to the CaribbeanTrinidad and Tobago http://www.metoffice.gov.tt/ Full Details
Black SeaTurkey https://mgm.gov.tr/eng/forecast-cities.aspx# Full Details
Channel Islands, Cherbourg Peninsula & North BrittanyUnited Kingdom https://www.metoffice.gov.uk/ weather/specialist-forecasts/ coast-and-sea Full Details
Isles of ScillyUnited Kingdom https://www.met.ie/ Full Details
Isles of ScillyUnited Kingdom https://www.metoffice.gov.uk/weather/specialist-forecasts/coast-and-sea Full Details
Alaska SE to Vancouver IslandUnited States https://www.weather.gov/marine Full Details
Atlantic IslandsUnited States https://www.weather.gov/marine/ Full Details
Pacific Crossing GuideVanuatu vmgd.gov.vu/vmgd/index.php Full Details