After The Forecast 8 September to 29 September

Rain check · 8 September to 29 September 2026

KNMI Harmonie NL timed tomorrow's rain best at De Bilt; 3 days ahead, no model scored above 55.0.

Rainfall totals and rain timing are different tests. This report checks the models on both.

Edition No. 5 · The Netherlands · 22 days of hourly rain at De Bilt

From 8 September to 14 September 2026, De Bilt recorded 36.4 millimetres of rain. At least one millimetre fell on 5 of those days; 8 September 2026 was the wettest with 18.1 millimetres. Over the whole period from 8 September to 29 September 2026, the gauge collected 48.0 millimetres, with at least a millimetre on 9 of its 22 days.

Every model was scored on the same 22 days, at every notice its runs reach out to seven days ahead, for timing and for amount. One day ahead, KNMI Harmonie NL timed the rain best, with a score of 68.8; ICON followed at 67.5.

The table below reads each model from a week out down to the day before. One day ahead, the forecasts for those 22 days ranged from 25.4 millimetres by ICON to 52.2 by ECMWF AIFS, against 48.0 measured. With only 37 wet hours in the sample, one shower caught or missed moves a score by several points, so cells a few points apart do not separate two forecasts.

Apple Weather and Foreca have no public archive to replay, so their columns are the forecasts we collected ourselves, from 16 July 2026 and 24 July 2026. Their hourly figures are stamped by a different convention from the archive's, so they are never scored hour by hour: their cells carry only the rain they forecast over the same hours.

Rain at De Bilt, 8 September to 29 September, at every noticeFig. 1 · rain score, with forecast rainfall beneath

Rain score and forecast rainfall for every day of the period, by notice
Notice	KNMI Harmonie NL Netherlands	ICON Germany	GFS United States	ECMWF IFS Europe	ECMWF AIFS Europe, AI	Apple Weather United States, commercial	Foreca Finland, commercial
7 days ahead	beyond horizon	beyond horizon	29.629.4 mm	38.565.0 mm	40.851.6 mm	amount only41.3 mm	beyond horizon
6 days ahead	beyond horizon	34.835.0 mm	34.821.6 mm	44.552.3 mm	44.953.7 mm	amount only43.6 mm	amount only54.9 mm
5 days ahead	beyond horizon	31.941.9 mm	34.631.3 mm	38.834.3 mm	36.663.3 mm	amount only51.5 mm	amount only46.1 mm
4 days ahead	beyond horizon	25.643.0 mm	24.726.2 mm	41.261.6 mm	41.354.9 mm	amount only49.8 mm	amount only50.2 mm
3 days ahead	beyond horizon	55.039.4 mm	47.750.6 mm	51.047.2 mm	48.454.9 mm	amount only45.9 mm	amount only55.0 mm
2 days ahead	beyond horizon	58.716.2 mm	56.625.8 mm	55.750.6 mm	46.852.5 mm	amount only40.7 mm	amount only45.5 mm
1 day ahead	68.838.1 mm	67.525.4 mm	66.427.3 mm	63.539.6 mm	48.952.2 mm	amount only51.3 mm	amount only44.2 mm

Every cell scores the same 22 days at De Bilt: 528 readable hours, 37 of them wet, 48.0 millimetres measured. The large figure is the rain score, the small one the rain that run forecast over those hours. Public models are read from the 00 UTC run issued that many days before each day. A model whose run does not cover every day at a notice has no cell there. The commercial columns use the forecast collected on each notice day nearest 6 a.m.

The wettest day, hour by hour

On 8 September 2026, De Bilt measured 18.1 millimetres across 7 of the 24 hours it could read. The strip below sets each model's run from the day before against the gauge, hour by hour. GFS caught 7 of those 7 wet hours, with no false alarms.

8 September 2026, 18.1 millimetresFig. 2 · one day ahead, hour by hour

Rain on 8 September 2026 hour by hour: the gauge, then each model's forecast from the day before
Model	00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	Day
Gauge	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	0.1	0.4	1.2	2.2	2.7	10.6	0.8	Dry	18.1 mm
GFS	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Within an hour	Caught	Caught	Caught	Caught	Caught	Caught	Caught	Within an hour	3.4 mm
ICON	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	False alarm	False alarm	False alarm	False alarm	Within an hour	Caught	Caught	Caught	Caught	Caught	Caught	Caught	Dry	6.1 mm
ECMWF AIFS	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	False alarm	False alarm	False alarm	False alarm	False alarm	False alarm	Within an hour	Caught	Caught	Caught	Caught	Caught	Caught	Caught	Within an hour	9.3 mm
KNMI Harmonie NL	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	Dry	False alarm	False alarm	False alarm	Within an hour	Caught	Caught	Caught	Caught	Caught	Caught	Missed	Dry	5.2 mm
ECMWF IFS	Dry	Dry	Dry	Dry	Dry	Dry	False alarm	Dry	Dry	Dry	Dry	Dry	Dry	False alarm	False alarm	Within an hour	Caught	Caught	Caught	Caught	Missed	Missed	Missed	Dry	6.3 mm

The top row is the gauge at De Bilt, in millimetres per hour; each column is the hour ending at the time above it. Below it is each model's run from the day before. A wet gauge hour is caught when the model had rain within an hour of it and missed when it had none. A forecast wet hour with no rain measured within an hour is a false alarm; one beside measured rain is timing inside the tolerance. The last column is the day's total.

Which model timed tomorrow's rain best?

Across all 22 days, KNMI Harmonie NL offered the best balance between missed rain and false alarms, with a score of 68.8. ICON followed at 67.5. ECMWF AIFS finished last at 48.9: it caught 37 of the 37 wet hours, with 138 false alarms. How ready a model is to forecast rain shows in the hours it called wet, from 54 to 204 over the same 528 hours. For rain at De Bilt one day ahead, KNMI Harmonie NL was the best choice in this test.

Rain, 8 September to 29 SeptemberFig. 3 · one day ahead

Rain standings, one day ahead
Model	Rain score	Caught	Missed	False alarm	Hours called wet
KNMI Harmonie NL Netherlands	68.8	30	7	25	62
ICON Germany	67.5	29	8	22	54
GFS United States	66.4	32	5	41	89
ECMWF IFS Europe	63.5	30	7	43	90
ECMWF AIFS Europe, AI	48.9	37	0	138	204

Scores for one-day-ahead forecasts across 528 readable hours at De Bilt; 37 of them were wet. The score rewards hits and penalises misses and false alarms.

About this comparison

The period's wet spell runs from 8 September 2026 to 14 September 2026; its wettest day, 8 September 2026 with 18.1 millimetres, is drawn hour by hour. The other two figures cover every day of the period. Forecasts come from the public models' 00 UTC runs and are compared with the gauge at De Bilt.

How rain is judged

An hour counts as wet from 0.1 millimetres. A forecast may be 1 hour early or late; when the gauge has no reading, that hour is left out for every model. This is a test at one gauge in De Bilt, not a verdict on a whole country.

A day enters the sample only when the gauge could be read and all 7 runs before it are in the archive, so every notice is scored on the same 22 days. A model whose run does not cover every one of them at some notice has no score there, rather than a score on fewer days. Each notice is judged on its own. There is no row for how far ahead a model first had the rain: showers do not reduce to one number that a forecast either lands near or misses.

At De Bilt, KNMI's R1H field is the rain during the preceding sixty minutes. The reading taken on the whole hour is compared with the model value for the hour ending there. The other ten-minute readings overlap and are not added.

Selection and coverage

Apple Weather and Foreca have no public archive. Their columns come from forecasts VeriSky has stored since 16 July 2026 and 24 July 2026; a different time convention means they are not scored hour by hour.

We compare archived model output with independent station observations. Data is at data.json, the full method at methodology and corrections at corrections.

The live version of this report is an app

After The Forecast comes out on a schedule. VeriSky keeps the rolling scores current for the places you care about and shows how models change with forecast distance.

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