How high the river is likely to be this term and over the next few, compared with a usual year. Updated daily.
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ECMWF's extended-range forecast (51 members) for weekly rain over the upper Thames, Monday to Sunday: middle value and 10th–90th percentile, against the 1993–2025 normal (CRU TS). Not yet corrected for bias or tested here.
ECMWF's seasonal forecast for the upper Thames, corrected using its forecasts for past years: middle value and 10th–90th percentile, against the 1993–2025 normal (CRU TS). Only the first month or two have shown any skill.
This uses only the river's own flow at Farmoor since 1993, with no weather forecast. A river that has been high or low tends to stay that way for weeks, because the ground and groundwater take time to fill or drain. So we take today's flow and see how each of the last 33 years carried on from a similar starting point. That gives 33 possible futures, and the odds come from counting them. For the next few weeks, the years whose rain best matches ECMWF's three-week forecast count for more; this fades out for periods further off. The monthly seasonal forecast was tested this way and made it worse, so it is shown for information only. Each past year was forecast from the others to check it works.