First, a definition---a numerical weather forecast begins with a description of the atmosphere at one time and then using a computer model of the atmosphere to predict the exact state of the atmosphere at future times. Like the temperature in Cleveland at 2 PM three days from now. Or the exact position and pressure of a low pressure center in four days. This compares with predicting climate--by which we the temporal mean or average state of the atmosphere over some location or region.
At the UW we run the WRF model out to 180 h (7.5 days) in weather forecasting mode.
The National Weather Service runs the GFS global model (which have shown on this blog several times) out to 16 days. The European Center runs their high-resolution global model out to 10 days.
So why go so far out in time when forecast skill really drops after 5-7 days? The only reason is that in some situations useful forecast skill can hold out 7-14 days or more.
The major weather prediction entities (National Weather Service, European Center, UKMET office, and others) also regularly run global forecasts out to SIX MONTHS. To run that long they have to use both atmosphere AND ocean models, coupling them together.
So if forecast skill fades after roughly one week, why do they do this? The reason is that the forecast centers hope that some average aspects of the weather are predictable out several months. For example, some aspects of the future atmospheric conditions are driven by sea surface temperatures, which change relatively slowly...perhaps that might provide some skill. In fact, that is exactly what we are doing subjectively when we talk about El Nino and La Nino weather....that the temperatures of the tropical Pacific drive weather in the midlatitudes (and elsewhere) for the months ahead. Or perhaps the slowly changing snowpack over Asia might perturb that atmosphere during the next few months in a coherent way...offering some predictability.
OK, lets check out the goods! Here are the monthly forecasts from the National Weather Service Coupled Forecast System Version 2 (CFSv2) for the next 6 months. This figure shows the surface temperature difference from normal (the anomaly) in degrees C.
A bit cooler than normal for March over us, but April is above normal and the remainder of the spring and summer are normal or above normal. Wonderful news if true.
I can't show you the ECMWF seasonal forecasts (they charge!), but I can show you the long-term predictions from the excellent UKMET office coupled seasonal prediction system. Here are the forecasts of surface temperature for the early spring and early summer for an ensemble (average) of a number of forecasts they made: