Summer 2008 "Forecast"
Having spent the last few days crunching numbers, index data, and looking at atmospheric patterns, it seems to me that we may be setting ourselves up for a modest summer in terms of heat.
We presently are watching La Nina wretch its last throws of life in the Eastern Pacific. In the last few weeks (click the graphic above) temperatures in the Eastern Pacific have moderated markedly thanks to warming of the waters off of the East Coast of Mexico. The Nina that wreaked havoc on Philadelphia's relatively snowless winter will be a distant dying memory.
One of the factors I looked at in seasonal forecasting, although as a secondary factor, was the ending of a La Nina and a turn in the Pacific towards a neutral pattern. The developing Nina last summer helped to carve out a ridge of high pressure over the Southeast, setting up a drought that persisted in the Carolinas and Georgia into the winter months, while helping dry out our region and turning portions of Delaware and South Jersey brown for several weeks in the late summer. With Nina dying out, consideration needs to be given to those years that featured a weakening or weak Nina going into the summer...those years include 1951, 1976, 1985, 1989, 1996, 2001, and 2006.
Another factor is the March-April temperature couplet. While there is a weak correlation between April temperatures and the proceeding summer temperature, the correlation does provide value in helping establish potential patterns in the atmosphere for the upcoming summer season. Since 1950, there have been only three years that were not only in a dying or weak La Nina and had temperatures above normal in March and April. Those years are 1976, 1985, and 2006.
The dominant pattern in these years was the presence of a ridge of high pressure across the Intermountain West, along with a trough that persisted in the East that brought slightly cooler than normal conditions to our part of the world.
The reds on the map above correlate to mid-atmospheric high pressure with blues and purples correlating to lower pressure compared to a near thirty year "average" for June-August.
This correlates pretty well with the temperatures seen below.
The warmest temperatures in those three years, when blended together, are confined to the intermountain West and High Plains while we were slightly below normal. Keep in mind that the 1970's and early 1980's featured a cooler global temperature pattern than the early 2000's have so the results don't look similar on a month-to-month basis within each year.
Out of the three years sampled above the temperature breakdown is as follows.
June: Two months above average, one month below.
July: One month above, two months below.
August: One month above, two months below.
The "average" on the graphic above illustrates the 1971-2000 average for number of 90 degree days and the average temperature for each of the summer months. My forecast calls for a slightly warmer August than the consensus blend of the three years as I do put some weight into recent temperature trends in nudging temperatures upward. I do think we'll see temperatures for the upcoming summer the will average out just slightly below average for the first time since 2004, but not by any significant amount, as I feel a cool July will help offset the warmer June and near normal August we will see. Precipitation is much harder to forecast for in the summer due to localized thunderstorm development but there appears to be no sign of a significant drought shaping up in the East as none of the three years showed significant trends of below normal precipitation. June 2006 did feature flooding rains but there were no summer long droughts in any of the years.
We presently are watching La Nina wretch its last throws of life in the Eastern Pacific. In the last few weeks (click the graphic above) temperatures in the Eastern Pacific have moderated markedly thanks to warming of the waters off of the East Coast of Mexico. The Nina that wreaked havoc on Philadelphia's relatively snowless winter will be a distant dying memory.
One of the factors I looked at in seasonal forecasting, although as a secondary factor, was the ending of a La Nina and a turn in the Pacific towards a neutral pattern. The developing Nina last summer helped to carve out a ridge of high pressure over the Southeast, setting up a drought that persisted in the Carolinas and Georgia into the winter months, while helping dry out our region and turning portions of Delaware and South Jersey brown for several weeks in the late summer. With Nina dying out, consideration needs to be given to those years that featured a weakening or weak Nina going into the summer...those years include 1951, 1976, 1985, 1989, 1996, 2001, and 2006.
Another factor is the March-April temperature couplet. While there is a weak correlation between April temperatures and the proceeding summer temperature, the correlation does provide value in helping establish potential patterns in the atmosphere for the upcoming summer season. Since 1950, there have been only three years that were not only in a dying or weak La Nina and had temperatures above normal in March and April. Those years are 1976, 1985, and 2006.
The dominant pattern in these years was the presence of a ridge of high pressure across the Intermountain West, along with a trough that persisted in the East that brought slightly cooler than normal conditions to our part of the world.
The reds on the map above correlate to mid-atmospheric high pressure with blues and purples correlating to lower pressure compared to a near thirty year "average" for June-August.This correlates pretty well with the temperatures seen below.
The warmest temperatures in those three years, when blended together, are confined to the intermountain West and High Plains while we were slightly below normal. Keep in mind that the 1970's and early 1980's featured a cooler global temperature pattern than the early 2000's have so the results don't look similar on a month-to-month basis within each year.
Out of the three years sampled above the temperature breakdown is as follows.June: Two months above average, one month below.
July: One month above, two months below.
August: One month above, two months below.
The "average" on the graphic above illustrates the 1971-2000 average for number of 90 degree days and the average temperature for each of the summer months. My forecast calls for a slightly warmer August than the consensus blend of the three years as I do put some weight into recent temperature trends in nudging temperatures upward. I do think we'll see temperatures for the upcoming summer the will average out just slightly below average for the first time since 2004, but not by any significant amount, as I feel a cool July will help offset the warmer June and near normal August we will see. Precipitation is much harder to forecast for in the summer due to localized thunderstorm development but there appears to be no sign of a significant drought shaping up in the East as none of the three years showed significant trends of below normal precipitation. June 2006 did feature flooding rains but there were no summer long droughts in any of the years.
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