A Hovmöller diagram compresses a sequence of forecast charts into a single image: longitude runs along one axis and forecast time down the other, with the field averaged across a latitude band. It makes the propagation and retrogression of Rossby waves far easier to read than flicking through individual charts, and it is the quickest way to spot a blocking pattern setting up or breaking down.
The charts below are averaged across 40–60°N. Deterministic runs show a single solution; ensemble mean charts smooth out low-confidence detail and are usually more useful beyond about day 7.
ECMWF, ECMWF ENS and GEFS Hovmöller charts are available to premium members. ECMWF generally has the edge over GFS in the medium range, and the ensemble mean charts strip out the low-confidence detail that dominates deterministic runs beyond about day 7 — making genuine pattern changes considerably easier to identify.
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Time increases downwards, so a feature that tilts to the right as you move down the chart is progressing eastwards; one tilting left is retrogressing. A feature that stays vertical is quasi-stationary — often the signature of a block. The steeper the tilt, the slower the propagation.
Comparing the deterministic and ensemble mean charts for the same run is a quick confidence check: where the two agree on the timing of a pattern change, that change is reasonably well supported.
This service is based on data and products of the European Centre for Medium-Range Weather Forecasts (ECMWF). Source: www.ecmwf.int. This ECMWF data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence.
The charts on this page have been modified from the original ECMWF data: fields have been averaged across a latitude band and rendered as Hovmöller diagrams by The Weather Outlook. ECMWF is not the source of these derived charts and has not endorsed them.
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GFS and GEFS data are produced by NOAA's National Centers for Environmental Prediction and are in the public domain.