Lorenz passed away in 2008, but I was fortunate to meet him when I was a young professor and swap stories about exploring Utah. He was an avid hiker, climber, and cross-country skier who would have fit in well with the Wasatch Weather Weenies.
|Ed Lorenz. Source: Realclimate.org.|
Lorenz showed that atmosphere is a chaotic system. It cannot be predicted like the phases of the moon or the tides. Small differences in the initial state of the atmosphere can yield dramatically different forecasts. This places a limit on forecast accuracy as we project farther into the future. However, how predictable the atmosphere is varies somewhat from day-to-day and region-to-region. Some patterns are more predictable than others. The idea behind an ensemble forecast system is to produce lots of forecasts with somewhat different initial analyses (or in some cases model configurations) to get a handle on forecast confidence and the range of possibilities during the forecast period.
The National Centers for Environmental Prediction Global Ensemble Forecast System (GEFS) provides global ensemble forecasts out to 16 days in the future. The GEFS consists of 21 forecast members, one being the Global Forecast System (GFS) model forecast, the other 20 additional forecasts produced with slightly different initial conditions. One way that meteorologists visualize the output from all of these models is using spaghetti diagrams that include a couple of selected contours from all the ensemble members on a single plot. For example, we can plot the 5280 and 5700 m geopotential height contours at 500-mb from this morning's model analyses.
The upper-level flow roughly parallels these contours. Because they lie nearly on top of one another, the initial analyses in these forecasts are quite similar. The weak northward bulge in the 5700 m contour over the Intermountain West reflects the weak upper-level ridge that sat over us this morning.
As we go forwards in time, the forecasts begin to diverge, just as chaos theory suggests.
This is a 5-day forecast based on the analyses above and is valid at 0500 MST Wednesday 4 January. Although there is clear divergence of the forecasts, it varies geographically. There is good correspondence between the ensemble members over the western United States. They all produce a ridge of comparable amplitude. This is one reason why we have high confidence that a ridge will build over Utah following the cold front passage tonight. Over the northeast United States and eastern Canadian Provinces, however, there is a mess of contours indicating great uncertainty in the timing and amplitude of the upper-level trough. This is an area where forecast confidence is lower, especially with regards to the timing and amount of any precipitation that might accompany the upper level trough.
Now, lets go way out in the future and examine the 10 and 16 day forecasts.
If you were forecasting in that area, you might have a closer look by examining postage stamps from the ensembles. Those below from the Penn State e-Wall provide a glimpse of a subset of 12 of the ensemble members. Many show a long-wave trough over the eastern United States and a ridge over the west, but there are variations in amplitude in position and a couple favoring a trough that is shifted a bit to the west.
|Source: Penn State e-Wall|