FORECAST JAPAN
JMA's own models, rendered the way they deserve — six forecasts from Yonaguni to Wakkanai and out across the Pacific, in the colours the agency draws them in.
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JMA's mesoscale model — 5 km over Japan and its seas, run eight times a day, hourly to a day and a half and to three days on the 00 and 12Z runs. The finest grid the agency publishes openly, and what Japanese forecasts of the next twelve hours are actually built on. Temperature, rain, wind, pressure, cloud and sunlight.
GSM, Japan window
The global model delivered on a Japan window at about 10 km, hourly for five and a half days and three-hourly to eleven. Where MSM stops at three days this carries the forecast on, on the same frame and in the same colours — so the handover changes the model and nothing else.
Sea State
JMA's nested coastal wave model at about 5 km — significant height, primary period and direction, three-hourly to three days. Fine enough to tell the Sea of Japan side of a prefecture from its Pacific side, which is most of what a Japanese marine forecast is read for.
The Pacific
The whole run on the western North Pacific frame — the basin Japan's weather arrives from. A typhoon three days out sits at 20 N and 135 E, off the bottom of the national map and squarely in the middle of this one.
Ocean swell
The open-ocean sea state to eleven days. Long-period swell reaches these coasts days before the storm that made it, and the period field — deliberately not coloured on the intensity ladder — is where you see it coming.
The outside benchmark
ECMWF's operational deterministic run, widely held to be the most accurate global model there is, drawn on the Japanese frame in the same colours as JMA's own. Here so the two can be read against each other at the same valid hour rather than in two different tabs.
Why this site exists
Six products, one instrument
JMA publishes its whole operational GPV stream openly, and nowhere is it drawn on one frame. Flip MSM against the GSM against ECMWF HRES at the same valid instant and every difference on screen is a real disagreement between forecasts, not an artefact of two different maps. Switching model holds the valid time, never the forecast hour.
JMA's own colours
The rainfall ladder here is the agency's: pale blue, blue, deep blue, then the hue changes rather than the shade — yellow at 20 mm in the hour, red at 50, dark violet at 80. It is the scale a Japanese reader already reads without a key, and it is the default rather than an option. Every other table in the network's collection of eleven hundred is one click away.
The picture is the data
Every frame is a palettised PNG whose palette is the colour ramp, so the image is a reversible encoding of the forecast: point anywhere on the map and the value comes back out, with no extra bytes published. Ramps are pinned to absolute values, so a loop shows the weather changing and never the palette re-fitting itself.
Part of a network
Forecast Japan is the Japanese member of a family of model sites: Forecast Norway reads MET Norway, Forecast Britain reads the Met Office, Forecast Italy reads Agenzia ItaliaMeteo, Forecast Euro reads ECMWF, Forecast Canada reads the MSC suite, and weather.city covers places rather than models. The global model and global wave frames here reach from the South China Sea to the dateline, so the maps already take in far more than Japan.