Northern Hemisphere Forecast Viewer
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Purpose. Shows the large-scale weather pattern over the Northern Hemisphere for the past five days and the next ten, and follows the upper-level lows (L) and highs (H) at about 5.5 km altitude (500 hPa) as they move. Choose a field to shade and one to contour, then use the slider or the arrow keys to step through time; click an L or H to see how far it has travelled. Variables: u and v, the west–east and south–north wind; T, temperature; Z, geopotential height (the altitude of a pressure surface); ω, vertical motion in pressure units (negative = rising); q, specific humidity.
Data. Latest complete run of the NOAA GFS 0.25°, the ECMWF AIFS-single 0.25° or IFS 0.25° (open data, CC BY 4.0; the 06/18 UTC IFS cycles end at 144 h), or the ECCC GDPS 15 km (00/12 UTC runs; interpolated to 0.25°; contains information licensed under the Data Server End-use Licence of Environment and Climate Change Canada), subsampled to 0.5° over 0–90°N, forecast hours 0–240 every 6 h, preceded by the analyses (f000) of the previous five days. Variables u, v, T, Z, ω, q at 850, 500 and 250 hPa; wind speed and the 500-hPa height anomaly (relative to the 1991–2020 NCEP/NCAR daily climatology) are derived in the browser. Upper-level low (L) and high (H) centres are extrema of the smoothed 500-hPa height (or of its anomaly), linked between 6-h steps. Updated every 6 h from this dataset.
Method notes
Fields. GRIB2 pressure-level fields (HGT, TMP, UGRD, VGRD, VVEL, SPFH) from the NOAA Open Data GFS bucket, quantised to 16 bits per value (relative precision better than 2×10−5 of the field range) and stored as gzip-compressed arrays. Shading is bilinearly interpolated to the screen; contours are computed by marching squares on the 0.5° grid.
Centres. Z500 is smoothed with a Gaussian (σ = 1°). A cyclone (anticyclone) centre is a local minimum (maximum) within 800 km whose depth relative to the mean on the 600–1000-km ring exceeds 40 gpm, restricted to 20–88°N. The anomaly basis applies the same rule to Z500 minus the NCEP/NCAR Reanalysis 1 daily long-term mean (1991–2020, 2.5°, bilinear). Centres are linked between 6-h steps by nearest-neighbour matching from a position extrapolated with half the previous displacement, accepting matches within 700 km. Displacements are accumulated along the track (zonal leg on the mean latitude of each step).