Your own GRIB, and weather at sea

1 Which of the two you want

They solve different problems and are two buttons on the same card, so it is worth being clear about which is which before you press one.

All three are in SETTINGS → WEATHER AND DATA → WEATHER on the tablet, on one card, because a crew looking for their weather should look in one place. The third is covered in Weather, and so is everything about what the platform fetches on your behalf.

2 What Race Control can read

GRIB edition 2 only, and within it a specific set of packings. The table is the whole of it, and it is worth reading before you pay for a file rather than after.

The tablet names which of these it found, in these words, on the card. It does not say “unsupported format” and leave you to work out which part was unsupported.

The grid has to be a regular latitude/longitude one, which is what every service sends for a passage. Rotated, Gaussian, Lambert conformal and polar stereographic grids are declined rather than resampled, because resampling one would be Race Control inventing a forecast and putting the model’s name on it.

3 Which fields are used

10 m wind is required. A file with no wind is refused, not imported empty. The rest are taken when they are there and simply absent when they are not.

• 10 m wind — the u and v components. Required. Ask your service for UGRD and VGRD at 10 m, or whatever it calls them.
• Gust at the surface. Some models publish an instantaneous gust and some a maximum over a window; Race Control reads which off the file rather than assuming, because the two are different claims.
• Mean sea-level pressure.
• Precipitation, when it is a total or a mean over a stated period. An instantaneous rain rate is a different quantity and is left out.
• Surface current, if the file carries one.

2 m temperature is read and then left out, and the summary says so. Race Control’s weather store carries a fixed set of fields and temperature is not one of them.

The wind height matters: it is the 10 m wind, because that is the height a polar is measured against. A file whose only wind is at 100 m has no 10 m wind as far as Race Control is concerned, and says so.

4 Getting the file onto the tablet

There are two ways in and they end in the same place.

1. From the card. SETTINGS → WEATHER AND DATA → WEATHER → YOUR OWN GRIB → IMPORT GRIB. The tablet’s file picker opens; choose the file. Typical names are .grb, .grb2, .grib and .bin, plain or .gz.
2. From another app. In your mail client, on the attachment, choose Open with or Share and pick Race Control. It reads the file straight away; the summary is waiting on the WEATHER card when you next open SETTINGS.

Compressed files are unpacked automatically, and the name is not what decides. Race Control looks at the first bytes of the file rather than at its suffix, because mail clients rename attachments and satellite terminals strip suffixes. A .gz that is really plain, or a plain name that is really compressed, both work — and the same forecast saved both ways is stored once, not twice.

The file picker offers everything on purpose. Most apps report a GRIB as a generic file with no type at all, so a picker that filtered by type would hide your own forecast from you. Race Control identifies the file by reading its contents, and says plainly when what you picked is not a GRIB2.

The import runs on the tablet. It does not upload your file, it does not need a connection, and nothing about it reaches the platform. That is deliberate: this feature is for a boat offshore with a mail client and no working connection to the platform.

5 Reading the summary

A successful import prints a line you will recognise from the rest of the card, then what it cost, then anything the file held that the forecast does not.

Below those, one line per item the file contained that the forecast does not: a temperature field Race Control cannot store, fields in a packing it cannot read, fields on a different grid from the wind, fields from a different model run. None of them stops the import — they are listed so you know what you are sailing on before you leave.

A GRIB may hold more than one grid and more than one run. Race Control’s forecast is one of each, so the wind decides both and anything on another grid or from another run is counted and left out. It is named on the summary rather than dropped quietly.

6 Living with an imported forecast

• It appears in the model picker under the model’s own name — NCEP GFS 00z, and so on — beside anything the platform fetched, with your own file against it on the settings card.
• PATH AHEAD routes on it like any other model. Two forecasts aboard means two routed paths, and the spread between them is the confidence display.
• Nothing deletes it but you. Fetched bundles are dropped when a newer run supersedes them; nothing supersedes a file you brought aboard yourself, so it stays until you press FORGET on its row. There is no confirmation, because what it removes is a copy of a file you still have.
• The accuracy rules do not change. A missing barb is missing data and never a calm; gust windows differ between steps and between models; nothing is extrapolated past the end of the file. All of that is in Weather, and it applies to a file you supplied exactly as it applies to one Nordic Stars fetched.

Switching the tablet to another boat clears the weather it is carrying, imports included. If you move a tablet between hulls, expect to import again — it is one press and you still have the file.

7 Refreshing the weather at sea

REFRESH WEATHER NOW reads the catalogue and then downloads forecast files, the same as FETCH WEATHER. Three things about it are different, because you are sailing rather than standing on a pontoon with fast free wifi.

• The smallest useful file goes first. Ashore the freshest run is fetched first because there is time for all of them. At sea a transfer that finishes is worth more than one that is newest, so the order inverts. Ties go to the newer run.
• A stopped download carries on where it left off. If the link drops, press it again: the bytes already aboard are kept and the transfer continues from there. Not every connection supports this — when it does not, the card says so plainly rather than claiming a saving you did not get.
• It does not download a corridor you are not sailing. A forecast whose area does not cover your loaded route is listed and not downloaded. Ashore that wastes wifi; at sea it wastes money.

What it would cost is printed above the button, in megabytes, before you press anything — the total and the size of the first file. You need that figure before you decide, not after.

The per-download limit

Beside the button is a limit: NO LIMIT by default, and tapping it steps through 2, 5, 10, 25 and 50 MB. It is remembered per boat, because it describes the connection that boat has.

A file over the limit is refused, not trimmed. There is no such thing as most of a forecast. The card names how many were kept out and how big the largest one is, so raising the limit is a decision you make with the number in front of you rather than a setting you discover was costing you a model.

8 When something does not work

Nothing is written to the tablet when an import is refused, so there is nothing to clean up and no half-read forecast to find later.

9 Where to go next

• Weather — what the platform fetches for you, the models and their limits, and how to read the wind layer once it is aboard.
• Race plans — drawing the route a corridor is built around, which is what a refresh checks its coverage against.
• Race Control on the boat — the tablet from the licence to the first gun.