Compared with the record total of 543 tornadoes in May of 2003 and another 509 last year, this year's unofficial total of 137 tornadoes makes this May a quiet one. There are a number of reasons why this month has not been conducive to tornado development, probably the most influential is the anomalous trough over the east coast of the US. This feature has acted to keep really warm and juicy gulf air from advecting north into the midwest. It also materializes in a predominate NW flow aloft over the midwest. If you look at the 500 hPa height anomaly for 2004, you will notice a ridge in the east, which helps to pump gulf of mexico moisture north and promotes a strong SW jet aloft.

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