The Typhoon Rule Can Predict Chilly Wet Air In North America.

In an age where computer models are believed to be the only viable long range weather forecasting tool, there remains a very effective technique for looking ahead a couple of weeks that had its roots in World War Two. 

It is called the “Typhoon Rule”. When the far western Pacific is active in terms of tropical disturbances, it is a great way to determine what the jet stream pattern will look like over the US with a lead time of one to two weeks. 

Military forecasters in the Pacific theatre dealt with typhoons pretty much year round, due to the persistence of water warm enough to form and sustain such storms in that part of the ocean.  One of the meteorologists noted that after a typhoon either re-curved over or east of Japan or tracked straight west into mainland Asia, a change in the weather would result in a week or two in Washington, D.C. and thus the correlations were born.  Here is how the Typhoon Rule works:

All tropical disturbances across the globe act like a cork in a stream.  They are warm systems without the warm and cold fronts that drive mid-latitude low pressure systems, and are therefore more influenced by jet stream level winds in terms of where they will move.  When a typhoon in the southwest Pacific encounters an upper ridge to the north—centered over Korea and Japan—the clockwise circulation around the ridge directs the storm westward, often through the Philippines and into Asia. 

A ridge in that position correlates with an upper level ridge and tranquil weather over central and eastern North America with a lead time of one to two weeks.  However, when the axis of the Pacific ridge is set up further east by several hundred miles or more, over the waters well east of Japan, the typhoons tend to run around the perimeter of the circulation, leading to a track that heads for Japan before turning north and then northeastward. 

This happens in a fashion similar to Atlantic storms threatening the east coast before turning around the perimeter of the Bermuda high as they head for New England and the Maritimes—when the ridge is far enough to the east, the storms miss the U.S.  A re-curving Pacific typhoon around a ridge well east of Japan correlates with a central/eastern North American trough, often leading to colder and stormy weather.

As we wait anxiously for the new season to start, the Typhoon Rule can give us a hint as to when colder air masses will be available for early season snowmaking or natural snow.  As I write this on Oct. 22, the Typhoon Rule is set up for a cold air mass to dive into the center of the country later in the week of the 28^th.  The cold will also spread into the East, though in modified form.  Take a look:

There are nuances to the rule, and one of them comes into play here.  This strong October 22 storm will not make a dramatic turn to the northeast toward the Aleutians, but it will turn north of east. This track suggests that the trough/cold shot will be centered in the middle of the country with the chilly air eventually spreading east. 

The Killington Women’s World Cup is about five weeks away, and it looks as though the snowmakers will be able to get started there around Halloween. The Typhoon Rule that an observant WWII forecaster discovered says so.