Quality of Forecast Results: Chaos Effect & Climate Factors
Eventually, improvements in the initial conditions for the forecast have little impact on the quality of the forecast, the so-called chaos effect. It is then difficult, if not impossible, to provide a detailed day-to-day variation of the forthcoming weather. Climatological factors then start to play a more significant role. Such factors include sea surface temperatures and atmospheric modes of variation, which change slowly. Factoring in these quantities, the medium to long range forecasts are much more general than the shorter range forecasts. They provide an overall indication of expectations based on a simple scale of above average, near average, and below average compared with the long term climate mean.
Factors Influencing the January 2012 Weather Forecast
In issuing its forecast, the National Weather Service has described the most significant features influencing the weather for January 2012.
One of the most important factors in US weather is the El Nino phenomenon. This is a strong warming of the Equatorial Pacific Ocean. At the moment, in this location the conditions are opposite (referred to as La Nina): a cooling of the ocean. Warming would tend to increase the production of cloud (convection) due to the rising of warm air, but conditions are presently opposite to that, leading to suppressed convection. The effect on the weather of El Nino is to increase the precipitation in southern USA, hence the forecast for precipitation (top figure right) shows the opposite for the current La Nina conditions.
Another important factor is the Arctic Oscillation, measured by the difference in pressure between middle latitudes and the polar regions. The Arctic Oscillation is an important mode of variation of the weather and has hemispheric influences. The Arctic oscillation is currently positive and expected to remain for most of the rest of December. A positive Arctic oscillation leads to ocean storms travelling further north. East of the Rocky Mountains, cold polar air does not penetrate as far south over the USA. This is seen in the forecast chart with higher temperatures forecast East of the Rockies.
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