After The Forecast 1 July to 12 August

Verification report · 1 July to 12 August 2026

ECMWF AIFS held the 33 degrees on 13 August within the accuracy margin continuously from 10 days ahead.

We compared 7 forecast models at 3 weather stations in the Netherlands, using 960 observation hours across 40 complete station-days.

Edition No. 1 · The Netherlands · 6,720 forecast-hour comparisons

On 13 August 2026, De Bilt recorded 33 degrees as the heat arrived in the Netherlands. The earliest continuous accurate forecast was 10 days ahead, from ECMWF AIFS. One day ahead, ECMWF AIFS forecast 32.3 degrees.

At 11 days ahead, no model was within 2 degrees of the measured 33. Forecasts ranged from 22.2 to 25.3 degrees. Even one day ahead, the furthest forecast was 36.1 degrees from GFS.

13 August 2026, measured at 33 degreesFig. 1 · forecast daily maximum

Forecast daily maximum for 13 August 2026 at each notice, against the 33 degrees measured
Notice	ECMWF AIFS Europe, AI	Apple Weather United States, commercial	Foreca Finland, commercial	ICON Germany	KNMI Harmonie NL Netherlands	ECMWF IFS Europe	GFS United States
11 days ahead	25.3-7.7	Not available	Not available	Not available	Not available	23.2-9.8	22.2-10.8
10 days ahead	32.8-0.2	Not available	Not available	Not available	Not available	26.7-6.3	20.1-12.9
9 days ahead	34.5+1.5	30.6-2.4	Not available	Not available	Not available	26.6-6.4	32.7-0.3
8 days ahead	34.1+1.1	24.3-8.8	Not available	Not available	Not available	27.3-5.7	31.3-1.7
7 days ahead	34.3+1.3	31.0-2.0	32.7-0.3 · 16 h	Not available	Not available	34.1+1.1	39.9+6.9
6 days ahead	32.0-1.0	34.2+1.2	32.4-0.6	34.2+1.2	Not available	29.8-3.2	36.8+3.8
5 days ahead	31.7-1.3	32.1-0.9	32.8-0.2	34.1+1.1	Not available	30.2-2.8	35.5+2.5
4 days ahead	31.7-1.3	32.7-0.3	31.5-1.5	34.6+1.6	Not available	30.2-2.8	35.9+2.9
3 days ahead	32.3-0.7	32.8-0.3	31.2-1.8	34.1+1.1	Not available	31.3-1.7	34.9+1.9
2 days ahead	32.3-0.7	33.5+0.5	31.7-1.3	34.3+1.3	Not available	31.0-2.0	36.1+3.1
1 day ahead	32.3-0.7	33.0+0.0	31.5-1.5	33.1+0.1	33.3+0.3	30.7-2.3	36.1+3.1
Inside 2 degrees from	10 days ahead	7 days ahead	7 days ahead	6 days ahead	1 day ahead	never	never

Each cell is the highest hourly temperature that forecast gave for 13 August 2026 at De Bilt, with its distance from the 33 degrees the station measured. Cells inside 2 degrees are marked. The last row is the longest unbroken run of those, counted back from one day ahead, so a single lucky column does not count as having seen the day. Public models are read from the 00 UTC run of each notice day, the commercial columns from the forecast collected that day. The grid reaches back as long as any model still had the day inside the band, and ends with the first notice at which none did. At 7 days ahead, Foreca's value uses the 16 hours its archived forecast supplied for that date, through 15:00 local time; its highest supplied value was 32.7 degrees.

Apple Weather and Foreca are included alongside the public models. As neither has a public archive, VeriSky uses forecasts it collected itself: Apple Weather since 16 July 2026 and Foreca since 24 July 2026. An empty cell means no forecast was stored for that lead time. The event table normally uses the snapshot nearest 6 a.m. local time. If that snapshot stops before the afternoon, it uses a later snapshot from the same notice date that reaches the daytime maximum window and marks the partial hour count.

Temperature and wind, day by day

Across 40 complete station-days at De Bilt, Rotterdam, and Schiphol, ECMWF IFS had the highest temperature score at 90.5, with a typical error of 1.42 degrees. Apple Weather led for wind at 81.6, with a typical error of 4.12 km/h.

KNMI Harmonie NL ranked 6th of 7 for temperature and 4th for wind. This result applies only to De Bilt, Rotterdam, and Schiphol during 1 July to 12 August; it does not show how the model performs across the Netherlands.

ECMWF AIFS is trained on historical weather and works differently from the physics-based models. It ranked 5th of 7 for temperature and 2nd for wind.

For the hot day, it was within the margin from 10 days ahead. One day ahead it forecast 32.3 degrees, compared with 33 on the day itself. Strong average performance does not automatically produce the best forecast for an extreme.

Temperature and wind, 1 July to 12 AugustFig. 2 · one day ahead

One-day-ahead temperature and wind standings
Model	Temperature score	Typical error	Wind score	Typical error
ECMWF IFS Europe	90.5	1.42 °C	73.6	4.97 km/h
Foreca Finland, commercial	88.7	1.31 °C	75.1	4.97 km/h
Apple Weather United States, commercial	88.6	1.29 °C	81.6	4.12 km/h
ICON Germany	87.7	1.35 °C	77.8	4.48 km/h
ECMWF AIFS Europe, AI	82.6	1.56 °C	79.2	4.46 km/h
KNMI Harmonie NL Netherlands	81.7	1.76 °C	77.2	4.38 km/h
GFS United States	77.5	1.89 °C	75.1	4.61 km/h

One-day-ahead forecasts on 960 common hours at De Bilt, Rotterdam, and Schiphol. Every row uses exactly the same 40 complete station-days. Scores run from zero to 100; the measured root mean square error is printed beside each score.

Distance separates the models

The chart shows how accuracy changes as forecasts look further ahead, using the same stations throughout. A line stops below 95 percent coverage, or when the national model's own forecast ends.

Accuracy against distanceFig. 3 · temperature by days ahead

ECMWF IFS archive thins out after day 5ForecaApple WeatherICON archive thins out after day 4ECMWF AIFSKNMI Harmonie NL archive thins out after day 1GFS

View the chart values as a table

Temperature score and observation hours at each lead time
Model	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6
ECMWF IFS	90.5 960 h	88.4 960 h	82.2 960 h	62.8 960 h	69.4 960 h	Not available
Foreca	88.7 960 h	86.0 960 h	78.0 960 h	71.0 960 h	63.9 960 h	42.6 960 h
Apple Weather	88.6 960 h	84.7 960 h	74.5 960 h	66.4 960 h	60.8 960 h	40.9 960 h
ICON	87.7 960 h	83.1 960 h	85.0 960 h	75.9 960 h	Not available	Not available
ECMWF AIFS	82.6 960 h	79.7 960 h	75.1 960 h	76.5 960 h	64.4 960 h	57.7 960 h
KNMI Harmonie NL	81.7 960 h	Not available	Not available	Not available	Not available	Not available
GFS	77.5 960 h	73.4 960 h	75.8 960 h	70.1 960 h	60.9 960 h	54.1 960 h

Temperature score at each lead time, on the same station-days as Fig. 2. Every visible point carries at least 95 percent of the 960-hour sample; its exact hour count is in the data table and the interactive tooltip.

The sample, not just the score

The results are based on the stations and periods below, showing the true size of the sample.

Where the evidence came fromFig. 4 · common station sample

Station-days included in the common sample
Station	Days	Hours	First included	Last included
De Bilt 06260	14	336	30 July 2026	12 August 2026
Rotterdam EHRD	13	312	30 July 2026	12 August 2026
Schiphol EHAM	13	312	30 July 2026	12 August 2026

Only station-days with all 24 temperature and wind observations enter the candidate pool. The final 40 station-days are the intersection also served completely by every published model. Partial station-days and analysis-backed locations are left out.

What to take from this

At 6 days ahead, ECMWF AIFS has the best score across the full sample at 57.7. That is 32.8 points below the best one-day forecast. Accuracy declines clearly as the forecast range increases.

The 7 models on this page were selected in advance. Another 22 models were scored over the same hours and are listed in the accompanying dataset.

Editions are published for closed periods, once every day in them has been scored. The next one covers September.

About this comparison

Temperature and wind come from NOAA Aviation Weather Center METAR and KNMI Data Platform station observations at De Bilt, Rotterdam, and Schiphol. Only reports within 20 minutes of a whole hour and days with all 24 hours are included.

The heat section uses 13 August 2026, named in advance as the day the heat arrived rather than taken as the period maximum. The hottest complete station-day in the period was 4 August at 33 degrees. The comparison uses De Bilt's record, the public models' 00 UTC runs and commercial forecasts selected from each notice date using the rule below.

Selection and coverage

For Apple Weather and Foreca, VeriSky normally uses the station forecast collected nearest 6 a.m. local time. For an event maximum, if that snapshot ends before the afternoon, a later snapshot from the same notice date may be used when it reaches the daytime maximum window; the table prints its partial hour count. Collection began on 16 July 2026 and 24 July 2026; the later date sets the start of the common sample. Commercial forecasts are newer than the public models' 00 UTC runs.

Only models with at least 95 percent coverage qualify. Each row is then scored over the same 960 available hours.

AccuWeather is excluded because its terms prohibit ranked comparisons. Other commercial services and VeriSky's own blend are also omitted. The remaining 22 scored models are listed in the dataset.

Limits and sources

This edition covers only De Bilt, Rotterdam, and Schiphol during 1 July to 12 August. The results are not a verdict on the whole of the Netherlands or on long-term performance.

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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