Islanders see normal summer averages with unusual weather
The overall precipitation and temperature averages this summer were consistent with those of the last five summers, but the manner in which those averages were achieved was much different than in years past.
Recent summers began with a cool and mostly dry June, with less than two inches of precipitation in four of the past five summers. Despite the summer of 2006, when it rained an impressive nine inches, the average rainfall for June is just under three inches. This year was no different, with six storms producing one and half inches of rain. The end of the month saw three storms with significant lightning.
What was uncharacteristic about June was the temperature. The five-year temperature average for the month of June is 73.4. This summer's average hovered at 75. Despite that slight increase, the daily temperatures were more stratified than in years past. The Island experienced nine days above 80 in June, compared with just three in 2007 and two in 2006.
July had the most unusual qualities and saw the most drastic weather patterns for the summer. The average temperature for this month was 80, two degrees above the monthly average, making July the warmest month of the summer. Although the highest temperature recorded was only 84, on the last day of the month, it was consistently in the high 70s and low 80s all month.
These high temperatures fed weather patterns, called mesoscale convective systems (MCS), which are somewhat unusual for the Island. As the cooler, dry air in the jet stream collided and interacted with the warmer air flowing up from the south, organized storm systems were created, according to Bill Simpson, a meteorologist from the National Oceanic and Atmospheric Administration (NOAA) in Taunton.
"The jet stream shifted out of its normal southern pattern and was flowing out of the northwest instead," said Mr. Simpson, in a phone interview with The Times. "When this happens you usually have a couple days of rain, but it tends to clear up pretty quickly. But that pattern had locked in for a while. Really, that has been the only unusual thing about this summer in terms of weather; the pattern held for so long."
These systems caused July to be the wettest month of the summer. The five-year average for July is just three inches, but the MCS patterns dumped just under five inches of rain on the Island.
They also produced the successive, powerful thunderstorms seen in July and the beginning of August. Over the month of July and in the beginning of August, the Vineyard experienced 10 separate systems that produced violent thunderstorms with heavy rain and severe lightning.
"Its not a major increase in rain in purely meteorological terms," said Mr. Simpson. "It may appear that it has been wetter, but in reality what we have been seeing are several storms dumping a bunch of rain but not for extended periods of time. Again, it's not very unusual or record breaking, but it has been a noticeably wet summer. What was odd was that nowhere on the Cape has reached 90, which is indicative of the shift in the jet stream. The last time this has happened was in 2000."
Mr. Simpson could not say with confidence what could cause the jet stream to shift, but he said it could be a mix of several external factors including sunspots, water currents, and global climate change.
Although these storms may have affected beachgoers and those who do not like thunder and lightning, it did not disturb farmers, who depend on the weather for their livelihoods.
"It started out relatively dry in the beginning of the summer but obviously that has changed in late July and early August," said Andrew Woodruff of the Whippoorwill Farm in West Tisbury, in a phone interview. "I wouldn't say it was a very unusual summer concerning the weather. From what I have heard, farms up north have been having a lot of trouble with the type of weather we have been experiencing. But I think we have been lucky, because there haven't been high winds associated with a lot of these storms. The frequency of these storms and aggressiveness has been a little ramped up this year, but luckily there has been very little damage."
These systems concluded in the middle of August. Since then, the Island has experience a very cool August. At press time, the average temperature for this month was 76, three degrees below the five-year average. Only eight days saw temperatures reach 80, the hottest being 84 on August 1, compared with 15 days at 80 or above last year. In 2006, August had 16 days when temperatures reached above 80, five of which were above 90.