AROME surface forecasts by initialization

Use the native public Météo-France files when forecast initialization and lead must be retained. The rolling server retains approximately fourteen days; older runs require a local mirror of the same file layout. It provides the 0.025° AROME grid and hourly forecast leads from 0 to 51 hours.

import numpy as np
from baro.met.arome_surface import AromeSurfaceSource
from baro.met.surface import SurfaceEnricher

valid_time = np.datetime64("now", "h")  # current UTC hour
source = AromeSurfaceSource.latest(valid_time)
result = SurfaceEnricher(source, cache_dir="/tmp/weather-cache").enrich(
    np.array([valid_time]),
    lat=[48.85], lon=[2.35],
)
print(result.fields, result.run, result.lead)
source.close()

The example selects a run that is available now for the current UTC hour. For a reproducible historical run, pass an explicit run=np.datetime64(...) to AromeSurfaceSource and use a local mirror if the run has left the rolling server's retention window.

The canonical fields are u10, v10 (m/s), t2m (K), precipitation (mm) and gusts (m/s). Map output names with fields={"temperature": "t2m"} to retrieve only selected variables. Heights are above the model ground; queries require no altitude. SurfaceEnricher also returns wind_speed and wind_direction, calculated from the interpolated horizontal components. Direction is the meteorological wind-from direction, in degrees clockwise from true north; it is undefined (NaN) for exactly calm wind.

Precipitation is the total water equivalent during the preceding hour, obtained by subtracting cumulative values from the same initialization, including when the previous lead occupies another GRIB file. Gusts retain the native preceding-hour maximum. Instantaneous fields interpolate in time; period fields select the full native interval containing the query. At initialization itself, preceding-hour precipitation and gusts are unavailable. The final native lead is queryable without requiring the next forecast hour.

AromeSurfaceSource.latest(valid_time) selects the newest run whose necessary files can be found now. Its observed_at records when that check happened. Initialization is distinct from publication: this method cannot reconstruct the last forecast available at a historical cutoff. Use an explicit run when an external publication record establishes that choice. A stitched Open-Meteo archive cannot supply initialization or lead provenance.

The reader scans GRIB headers once to discover message offsets, then downloads only selected field messages. Those full-grid compressed messages are cropped after decoding. With a range cache, subsequent reads reuse immutable message bytes and saved indexes. This is selective by variable and forecast step; GRIB messages do not permit the regional byte selection of the Open-Meteo archive's tiled format.

See the official AROME specification for the grid, hourly horizon and run publication schedule, and the Open-Meteo downloader for native package naming and variable mapping.