Sunday, April 20, 2008

Signs of a Warm Summer?

April, while the cruelest month, can also be a relatively safe harbinger of forecasting summer weather. Among the last 38 Aprils there is a 60% correlation between warmer-than-average Aprils and warmer-than-average Summers, with a stunning 75% correlation between cooler-than-average Aprils and cooler-than-average Summers.

April, like October and November in the Fall, is a key transition month in weather as winter patterns ease and summer patterns take hold. Any patterns that tend to develop, such as strong ridges of high pressure or cut-off lows and troughs, tend to replicate themselves throughout the summer months as patterns are called patterns for a reason -- they like habits and consistency more often than not and once a pattern sets up, it tends to hold serve for a while.

As we see the first vestiges of the warm season pattern shape up we can begin to take stock of what is happening in the atmosphere and look for similarities to past years. For instance, if the current ridge in the East continues to hold serve (despite occasional upper level lows rambling through, there is a ridge in place) and the bulk of heaviest moisture sits out in the Midwest along the jet stream boundary between the ridge and Western trough, a summer similar to 1995 or 1999 is possible in terms of heat and potential humidity. Both summers were quite hot in Philadelphia and both (except the last three weeks of August 1999) were very, very dry.

However, April 2004 was very similar to this one (relatively warm, ridge in the East) and while that pattern persisted into May we saw a major change in June, with a relatively cool summer in Philadelphia as a trough developed and parked itself over the Midwest and Great Lakes while Alaska and the Central Pacific, along with the Central Atlantic, had strong high pressure.

It's still early in the game with regards to determining summer weather but the next few weeks will offer more clues, perhaps blatant hints, about the weather that's to come.