|Source: http://www.onlinecasinosdir.com. Of course the Wasatch Weather Weenies do not endorse gambling, except when it comes to the weather.|
Ensemble forecast systems (see previous post "Forecast Tools: Ensemble Forecast Systems") produce many forecasts with slightly different initial conditions to get a handle on the range of future possibilities. For example, below are all the 168 hour 500-mb geopotential height forecasts produced by yesterday afternoon's run of the NCEP Global Ensemble Forecast System.
|Source: Penn State E-wall|
Unfortunately, we still have a problem. First, since we don't know the initial conditions precisely, we don't know how to generate all the differing initial conditions for the ensemble precisely either, so there is a very real possibility that the range of possibilities differs from that indicated above. Further, the model used for those forecasts isn't perfect either, and that is an additional source of error. As a result, we tend to use ensembles of ensembles for long-range forecasts. So, in addition to the GEFS, we look at the ensemble produced by the Canadian Meteorological Center (and in practice, others as well).
In the Canadian Meteorological Center ensemble forecast valid for the same time, we see a bit more spread. A few of the forecasts put a trough over western North America or along the west coast, but there are others that put a ridge over western North America (e.g., upper right).
|Source: Penn State E wall|