This year (2007) a mild El Nino changed the climate patterns on the West Coast early in the year. It turns out that the mild El Nino earlier this year was part of what is known as a decadal pattern. That means weather events have a tendency to happen at ten or twenty year intervals. To study these repeating pattern it is useful to look for analogs to the present year.

The ClimaTrends analog years for the spring of 2007 are 1988 and 1998. During those years the geometries used by our researchers to form their long range forecasts were very similar to the geometries of this past spring of 2007. In both May of 1988 and in May of 1998 unusual low pressure patterns emerged to threaten the flowering of the grapes. However, only in May 1998 did the low pressure result in a bad harvest due to untimely rains. This article addresses the similarities between these two years and the critical differences.

1 Fall 1987 El Nino in central Pacific

In the fall of 1987 a strong El Nino warming of the central Pacific along the equator had occurred

2 May 1988 La Nina event in the eastern Pacific

However by May 1988 a very cold tongue of water had rapidly replaced the warmth of the fall. The shift was accompanied by the formation of a large mass of low pressure air just off of the West Coast between British Columbia to the north and central Oregon to the south. The counter clockwise circulation around this low kept moisture moving to the northwest corner of the US but even though the large low was a dominant feature during the normal blossoming period of the grapes little rain fell out of the trough. The weather was cooler than normal and unsettled but little rain fell below Mt. Shasta with most of the precipitation along the coast of Washington State.

3 Fall 1997 El Nino in eastern Pacific

Similar to the fall of 1987, in the fall of 1997 a strong El Nino emerged in the Pacific. However, this warmth event took place along the west coast of Peru rather than in the mid Pacific. The warmth tongue from this event extended for thousands of miles west along the equator from Peru to the longitude of Hawaii.

4 May 1998 remnant of El Nino still present in the eastern Pacific

By May of 1998 there was still a remnant of this enormous warmth event with a smaller tongue of warm water still in place near South America. A fringe of warm water also was found along the West Coast of the United States. The warm water allowed a strong low to be built up for the month of May 1998. The counter clockwise circulation around this low kept fronts moving into the West Coast during the important time of grape flowering. As a result a problematic harvest came in that year for the wine growers on the West Coast.

These two patterns are similar in that they both resulted from an El Nino situation in the Pacific. They differ in that the El Nino of 1988 took place at mid ocean and the El Nino of 1998 took place in the far eastern Pacific. In 1998 the warmth was retained in the eastern Pacific setting up the emergence of a low at low latitude along the West Coast that was supported by the warmth fringe along the coast. This is in contrast to the cold Sea Surface Temperatures along the equator in the 1988 event. The low that formed against the coast at that time was active in the latitude of British Columbia. This high latitude placement kept the damaging rains to the north even though the unusually strong low persisted off of the Canadian coast for almost the whole month of May.