In a pasadena star news article on february 22, it was stated that while we are in a mild el nino, the recent storms have not followed the el nino pattern of coming directly from hawaii to southern california, but have come from the north. the reason for this is found in the greek letter omega, which is written thus: . meteorologists often refer to the 500 millibar mb. ; chart, because it is considered to be the steering level. surface storms will tend to follow the circulation at this level. sometime a particular pattern will form at 500mb, which looks the greek letter . it is called an omega block, and looks something like the sketch in figure 1. surface weather systems enter from the left and are diverted up and around the high. thus the pattern blocks the point labeled 1 from surface weather. figure1. example of an omega block the middle region h ; has fair weather and dry air. the outer regions l ; indicate rain. this is a very stable pattern and can persist for months. it is very efficient at transporting warm air from the tropics to the poles and cold air from the poles to the tropics. quite often, this pattern will develop in mid december to january with pasadena located at 2. think rose parade! ; it usually breaks down in february, giving southern california it's rainy season. this february, pasadena is located at 3! the forecasting problem is to determine when the omega block will break down and the 500 mb flow will return to normal. figure 2. 500 mb analysis 00z, 23 february figure 2 is a 500 mb analysis at 00z, 23 february 1600 pst, 22 february 2005 ; . the high is very skewed to the northeast, the low to the lower left of the omega block is missing, and the low to the right is very deep. it gives the impression of an omega block being destroyed.

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