Have you noticed that sometimes the weather will repeat itself several days in a row. For example, the weather could be cold and rainy for several days in a row. Another example is a month of repeating hot and dry weather. Meteorologists refer to repeating weather as a weather pattern. It is common for the weather to become locked in a repeating pattern. The same weather will occur day after day. Then suddenly one day the weather changes and for example dry weather for 2 weeks becomes rainy weather for a week. When this happens it is referred to as a weather pattern change. The primary source that determines the weather pattern in the middle latitudes is the upper level flow pattern. An examination of the jet stream winds shows regions likely with below normal temperatures and above normal temperatures. If a forecast area repeatedly stays within a trough the weather will tend to be cooler than normal. If a forecast area repeatedly stays within a ridge the weather will tend to be warmer than normal. Troughs and ridges can become locked in position for many days or weeks in a row. When this happens some regions will have droughts while other areas have flooding. Some regions will continually have warm weather while other area much cooler weather. Regions in which the wind direction continues from a southerly direction will tend to be warmer than normal. Regions in which the wind direction continues from a northerly direction will tend to be cooler than formal. The region in transition from the trough to the ridge with winds from the south tends to have the best rain changes especially if the southerly flow is also bringing up moisture. However, if the southerly flow is from a very dry region such as a desert then the weather will become very dry. The region in the transition from the ridge to the trough with the northerly winds tends to have drier weather since the cooler air does not have as much moisture. Also, if the northerly winds are from the continent they tend to have lower dewpoints. Study the 300-mb jet stream winds each day. See if you can notice the relationship between the trough-ridge pattern and the temperature and precipitation pattern over the forecast area. http://weather.unisys.com/nam/300.php |