JMA High-Resolution Precipitation Nowcast

What the product is

The Japan Meteorological Agency's High-Resolution Precipitation Nowcast, 高解像度降水ナウキャスト or HRPN, is not read from a data file at all. We sample the public map tiles the official viewer itself draws from: 256-by-256 8-bit RGBA PNGs in standard Web Mercator XYZ, refreshed every 5 minutes, at jmatile/data/nowc/{basetime}/none/{validtime}/surf/hrpns/{z}/{x}/{y}.png. Each pixel is one of nine intensity buckets from JMA's own published legend, so getting a rate means matching the pixel colour against that palette rather than parsing a value out of a grid.

Coverage is not a bounding box either. Japan is a 3000 km arc, and a single rectangle wide enough to hold the outlying Pacific islands also swallows large volumes of open ocean no radar reaches. A distance-to-nearest-radar gate is the better shape, and it should err generous: a gate that is too wide only costs a less precise status label, while one that is too narrow silently discards a real reading.

A fixed zoom goes silent in dry weather

JMA renders a tile only where it has something to draw. A tile with no echo anywhere inside it is not served as blank white paint; it is served as a fully transparent, 334-byte stub, with HTTP 200. This adapter originally requested a single zoom finer than HRPN's native grid, which sounded like the more precise choice. It was not: a zoom nobody renders returns nothing to sample, so in dry weather the adapter reported unavailable for the whole of Japan, every time, from the day it shipped. Nothing in the response says so. You get HTTP 200 and no pixels.

The fix is to walk several zooms, finest first, and take the first that yields a pixel. Which zooms are worth trying, and how much coverage each one buys, is a question for measurement against the live tiles rather than for the published grid resolution. The trade is resolution for coverage, and it is only paid where the finer tile had nothing to give: falling back to a coarse pixel only happens when no echo exists for tens of kilometres in any direction, where the smearing costs nothing because there was nothing there to smear.

Two transparent pixels that are not the same

A tile that does get rendered can still answer transparent for a given pixel, and here the fully unpainted stub reappears in a more dangerous form. Sampled live on 2026-07-30 against six points at one basetime: Tokyo, the Pacific 600 km southeast of it, the Sea of Japan and Minami-Tori-shima on the edge of the coverage box all came back (0,0,0,0), byte-identical, 334-byte tiles. Sapporo, raining at the time, and Naha both carried (255,255,255,0) pixels alongside their rendered precipitation.

(255,255,255,0)occurs only on a tile that also carries rendered precipitation somewhere: it is the in-domain paint for "a radar looked here and found no echo", a real measurement that resolves to 0 mm/h. (0,0,0,0) is a tile with nothing drawn on it at all, and metropolitan Tokyo and open ocean hundreds of kilometres outside radar range produce the same bytes. An adapter that reads either transparent variant as zero cannot tell a dry city from a point no radar has ever reached; this one used to, and the bounding-box gate alone did not catch it, since Minami-Tori-shima and the mid-Pacific points above sit inside that gate and are plainly outside radar range. Only the painted no-echo variant is a measured zero; the unpainted one, and any colour matching no known palette entry at every zoom tried, resolves to no reading at all.

A bucket that straddles the no-fly line

HRPN's nine intensity values are bucket midpoints, not exact readings, and the lowest non-zero bucket spans 0.1-1 mm/h. That range straddles the 0.3 mm/h precipitation threshold the flight score uses to call a no-fly condition: a true 0.15 mm/h drizzle and a true 0.8 mm/h shower land in the same bucket and both report as 0.5 mm/h. Any echo in that bucket is therefore classified conservatively as no-fly, a deliberate false positive in the safe direction rather than a rounding error, and it stays that way until the snapshot type can carry a range instead of a single number.