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This "GDD forecast" analyses past years and combines regular and seasonal forecast. Seasonal GDD forecasts can be compared with characteristic past years to estimate if the upcoming season will be warmer, colder or roughly the same.

meteoblue NEMS or ECMWF ERA5 are used as a base for analysis. Seasonal weather forecasts are displayed from various national weather services to give a wide overview. The GDD forecast graph contains 4 sections:

On the left side of the graph, the past days are displayed as a blue line. This data is either meteoblue NEMS or ECMWF ER5 data. meteoblue NEMS is selecting a high-resolution weather model for each location individually. Resolutions vary from 4 to 30 kilometres. ERA5 is a global reanalysis dataset with 30 km resolution.
In purple a 7-day weather forecast is included. The 7-day weather forecast is only available with meteoblue NEMS.
After 7 days of forecasts, different statistical analysis of meteoblue NEMS or ERA5 are shown as percentiles. For the entire period, the past 30 years are analysed to detect hot, cold or average years. "p10" represents the third-coldest year out of 30 years. "p50" an average year. "p90" the third-hottest year. Please note that this only applies to the analysed season and not entire year. With this statistical distribution, it is certain that the GDD seasonal forecast will stay close or within this spread.
As pink graphs, different seasonal forecasts from various national weather services are displayed. Typically, seasonal forecasts indicate if the temperature might be slightly warmer or colder for the next 6 months. This trend is applied to past data from meteoblue NEMS or ERA5 to calculate GDDs. Therefore, if the weather model is switched, seasonal GDD forecast results are adapted as well.

As a general recommendation seasonal forecasts should be used with care. The presented GDD forecast is merely a trend to estimate how this season might develop. Please consider some recommendations:

Because GDDs have a lower and upper boundary (like 10° to 30°C for maize) there is a sudden start and stop when temperature is accumulated. Especially for colder temperature and the growing season is about to start, this leads to higher GDDs in seasonal forecasts.
All data is based on weather models with 4x4 or 30x30 km resolution and therefore will not represent small-scale effects. If compared to a single local weather station, absolute numbers will be different. However, the trend of a warmer or colder season should be accurate.
Growing season are not perfectly "linear" from year to year. Weather changes in larger patterns. Some seasons start colder than average, but quickly become and stay warm. Other seasons start early with warm weather but a long low-pressure system keeps temperatures down. This can be seen in these graphs as well. A warm "p90" year might be colder than average in the beginning of the season.