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The seasonal climate forecast for the region of Nijmegen shows the monthly mean temperature and precipitation anomalies for the next 6 months in the top panel. The forecast is regional for areas of 100 km by 100 km or larger.

Seasonal weather forecasts, which would indicate what happens on a particular day, are not at all possible. You probably noticed the unreliability of a 10-day weather forecast and predicting several months is clearly more difficult. Hence, only a climate forecast, providing statistics such as mean values or anomalies for an entire month can be presented. Anomalies are deviations from the climatological mean. Thus, a negative temperature and precipitation anomaly indicates cooler and drier than average conditions, respectively. As only climatological information is possible, we cannot learn much about the character of the actual weather we can expect. Assume a month with a positive anomaly of +1 degree. It is very unlikely that every hour of this month is 1 degree warmer. A more realistic scenario is that some days are significantly warmer than average while others are on average. Most importantly there might also be some days that are colder or even significantly colder than average, so the positive anomaly is not at all a guarantee to have e.g. no frost.

Unfortunately, also these climatological forecasts are rather unreliable. It happens, that the 6 months forecast computed today is very different from the seasonal forecast computed yesterday.

To better understand how quickly the seasonal forecast is changing, new forecasts (actually an entire ensemble) are computed every day. We average all forecasts computed within a ten-day period, giving three periods of different forecast age for the last 30 days. The most current period (0-10 days) thus includes todays forecast and the forecasts from the previous 10 days. This most recent forecast period is used to draw the maps for the next 3 months in the lower panels. If you see that forecasts of different age are very different, and even contradict each other, then there is very little hope of forecasting the season at the moment.

There are some regions and situations where seasonal forecasts can be quite accurate. The most famous examples are El Nino and La Nina situations. NOAA and ECMWF compute the seasonal forecasts presented here.