Before you put away the sunscreen, however, we offer a few caveats. This opinion isn’t unanimous. And seasonal forecasting remains one of meteorology’s most-precarious limbs; the ultra-mild winter defied almost everyone’s preseason outlooks. The next few days are going to feel like summer 2011 reruns, with tropical-style humidity and temperatures flirting with 90. Even the ocean is in the swim of things: On Thursday, the surf temperature off Cape May was near 70, more typical of late June.
However, if those commercial forecasters are right about what’s to follow, the summer of 2012 is going to be a trend-breaker.
In records dating to 1874, the last 10 summers have been the warmest of any decade in Philadelphia, averaging 76.8 degrees, a full 3 degrees Fahrenheit higher than those of the 1960s.
Perhaps global warming has been a factor, but the local trend far outpaced summer warming worldwide. Earth’s average temperatures for the last decade were only 1.1 degrees above those in the ’60s, according to the National Climate Data Center records.
Meteorologists lament that global temperature trends aren’t much help in trying to figure out what’s going to happen in a given region in a given season. Neither are the short-range computer models that have revolutionized day-to-day forecasting. So weather prognosticators have to mine the oceans, key areas of the atmosphere, recent history, and even the dirt for clues. Their quarry, however, tends to be more unpredictable than human nature and prone to make a mockery of their efforts.
Recent summers have been dominated by "blocking" in the atmosphere over the North Atlantic Ocean, said Todd Crawford, meteorologist with WSI Corp. in Massachusetts, which serves the energy industry. Stalled areas of high pressure, or heavier air, literally have blocked weather systems from moving. So once it got hot in one place — say, Philadelphia — it tended to stay hot. The summer of 2010 was Philadelphia’s warmest in nearly 140 years of data-keeping, and last year’s was No. 5.
"There are no indications that these levels of blocking will occur again this summer," Crawford said.
The blocking "may not be as strong as the last couple of years," said Accu-Weather’s Paul Pastelok.
When it does occur, he said, heat will be shunted westward into the Plains and Rockies. "Temperatures will turn out near normal for the I-95 corridor from Boston to New York City and Philadelphia to Washington, " Pastelok said.
The big cities, where mortality is a major concern among the elderly live-alone populations, should be spared prolonged heat spells, he said. The Philadelphia Health Department blamed heat for 537 deaths in the last 20 years, 35 of those last year.
During the summer of 2011, sea-surface temperatures over vast expanses of the tropical Pacific were well below normal, the condition known as La Nina. Readings were above normal in 41 of the 48 contiguous states. Over 100 daily temperature records fell in July, and at one point more than 130 million people were under government heat advisories. In Philadelphia, July was the hottest calendar month on record. at 82.4 degrees.
La Nina has faded, but its remnants could contribute to a warm start of summer, according to Commodity Weather Group in Washington. But even if that’s the case, "the intensity and especially duration of heat should not be as impressive as last year," the company said, adding that it looks for a cooling trend the second half of the summer.
Jon Gottschalk, meteorologist with the government’s Climate Prediction Center, isn’t sanguine about the prospects for a benign summer. The government’s outlooks are bleached of subjectivity and confined to statistical probabilities, but for this summer it sees the odds tilted to above-normal temperatures in about two-thirds of the nation, including Philadelphia.
The trend of warm summers is a consideration, Gottschalk said, but part of the forecast reasoning lies in the dirt. The soil is bone dry in vast areas of the Southwest and Southeast, and when the sun has no moisture to evaporate, it can go to town cooking the ground. Heat begets heat, and it can go viral.
Atmospheric and Environmental Research also factored dry soil into its forecast, said AER’s Judah Cohen, who bucked conventional wisdom and forecast a mild winter of 2011-12. He’s going his own way again, calling for a warmer-than-normal summer in much of the nation, including the Northeast corridor. "Given how warm it has been so far this year it is hard to imagine the summer being anything but warm," he said, "but just when you are most certain, the weather has a way of humbling you."
That outlook might gain supporters around here in the next couple of days. However, experience would argue against reading too much into this weekend’s weather. Recall that it snowed before Halloween around here, and hardly at all after that.
Contact Anthony R. Wood at 610-761-8423 or firstname.lastname@example.org.