Friday, April 26, 2024

Japan Weather — A Sample of What We Can Do For All Global Waters

We had an excellent question come up in class asking simply what are the best weather resources for the waters of Japan?  In our textbook (Modern Marine Weather) and in our training and resources app (Weather Trainer Live) we list all resources available worldwide and even have sections on specific regions, but realize it could be valuable to just focus in and list specific solutions for a sample area, with details needed to actually obtain the data underway.

So we use Japan for this example, stressing that these same sources (or counterparts) are available for essentially any part of the world.


(1) Model forecasts in grib format
The main workhorses we use anywhere will be the global model forecasts from GFS and ECMWF.  These data are available from saildocs with an email request to with this in the body of the message:



These files can then be viewed in any navigation app, such as qtVlm, OpenCPN, TimeZero, Coastal Explorer,  or Expedition or in a dedicated grib viewer such as XyGrib. Background on use of gribs at the Grib School. Luckgrib is a state of the art app for downloading and viewing grib data. 

Sample model forecasts. Red is GFS, blue is ECMWF

(2) Graphic weather maps
We need to check the pure model data from above with actual maps made by human meteorologists, and we have several sources of those. 

Above is a sample surface analysis (12z Apr 28) and below is the corresponding OPC map, which is as far west as they go (135E).

(2a) Weather maps from Japan

Analysis chart

24-hr forecast

48-hr forecast

These are pdfs of about 550 kb, too large for sat phones as a rule, but it won't be long till all mariners have high speed internet offshore, then this type of link becomes more valuable. In the meantime, a supporter on land can download the file, copy the image from the pdf, reduce the file size, and email it to you on the boat.

Graphic weather maps are also available by HF radiofax if your boat happens to have the SSB radio and antenna set up. Japan stations are listed in the Worldwide Marine Radiofacsimilie Broadcast Schedules.  The many JMA maps available this way are listed at the JMH radio Station.

The other important radio related resource for international voyages is the NGA Pub 117, Radio Navigational Aids.  This tells, for example, what time of day you get VHF storm and navigation warnings for different parts of Japan. NAVTEX broadcast times are also given.

HF Fax is frankly an outdated technology (replaced by satellite communications), but if we could access the folder JMA store the images in we could request the same maps by email the way we do the US maps.

(2b) US OPC maps covers NW Japan waters

US maps only go to 135E (sample above), but they could be helpful on the approach from the east. See the Starpath Pacific Briefings page for examples and links. These are easy to obtain by email from Saildocs or FTPmail.

(2c) You can see UK maps of Japan waters at and choose Eastern Asia region. These maps, however, might be just their model output plotted, which does not add knowledge.

(3) Satellite cloud pics
Japan has an excellent satellite image program (Himawari). See index to files here, which is also where you learn the file name you need to ask for.

The latest visible image (b13) for Japan area can  be requested by email from Saildocs is  

The last four digits are the UTC of the image, available every 10 min, i.e., 0000, 0010, 0320 etc.  Himawari data are also excellent throughout the South Pacific.

This sample image is from 2 days later than other examples shown.

(4) Near live ASCAT winds

To get near live ASCAT winds, follow articles we have online about it and use links like the following for Central Japan waters:

You can use the same set of links for Northern Japan by changing the file number to 253, and for Southern Japan use 242.  You can get these online or ask for them from Saildocs by email. For background see

Sample ASCAT pass

(5) Live ship reports
You can also get a list of all ship reports near Japan by sending a blank email to and put the central Lat Lon in the subject line, such as 37.0 N, 140.2 E. (See This gets a list of all the reports plus a GPX file of the reports that can be loaded into a nav app to see actual locations and data.

(6) Ocean Currents.
You can get ocean currents and SST for that region from RTOFS request to Saildocs


For background on currents see

(7) Waves and sea state.
GFS is best for this, again available from Saildocs. There are many sea state parameters (see Grib School list), but these are likely of interest most often:

Significant Wave Height of the Combined Seas (HTSGW)  

Primary Wave, Direction it comes from (DIRPW)

Primary Wave, Mean Period (PERPW)


(8) Tropical cyclone warnings and reports
Primary source is Japan Meteorological Agency, which is also the Regional Specialized Meteorological Center (RSMC).

To get the latest Japan waters text reports from Saildocs, use send Met.11por

To see how to get reports for other parts of the large metarea XI, use send metarea

Other metareas around the world...
(9) Special sources (with thanks to Mark D'Arcy for this reminder)
JMA, like other maritime nations, has weather models of their own, but the grib format is a paid service. You can see their MSM model at, but the higher-res LFM is paid only.

Many maritime nations also have universities or other agencies that run a localized version of the Weather Research & Forecasting Model (WRF). These high-res data can be very useful when available.  Japan and South Korea have WRF data available from selected resources, such as the weather and nav app Expedition


Mike said...

In addition to that Japan's JMH transmits lots of various weather charts, including sea state, tropical cyclones etc. via HF radiofax.
Kagoshima's fishery JFX provides weather maps transmission as well.

David Burch said...

Thanks for the reminder. I have relied on HF fax for many thousands of ocean sailing (some years ago), and should have remembered that, but it is not very common these days. Nevertheless, as long as it is there it might be helpful in some areas if the boat has the radio, antenna, and receivers set up. I added the main reference for that. Thanks again.