First heat wave is coming; expect 90s into the weekend. Is there more to come?
The first heat wave of the year is brewing, and while it doesn't look particularly dangerous or life-threatening, air-conditioners will be getting their first serious workout. Trends and forecasts say we’d better get used to it.
While the hot spell due to get underway Wednesday doesn’t look especially dangerous, it appears that the region’s million-plus air conditioners are about to get their first serious workout of 2019.
Forecasts are calling for the first heat wave of the season to continue into the weekend, possibly lapping into Sunday. Temperatures won’t be extreme — in the 90-to-93 range in the city, just enough to meet the technical threshold of a “heat wave,” which is at least three consecutive days of 90 or better temperatures. At the Shore, highs are expected closer to 90.
“Right now, it seems marginal,” said Alex Staarmann, a meteorologist at the National Weather Service, which is unlikely to be issuing any heat advisories, although the weather service did advise that heat indices in Philadelphia and in urban areas such as Chester, Camden, and Norristown could reach 100 Friday and/or Saturday.
But it might seem a shade toastier given the generally benign temperature conditions so far this month. That’s been the case here and elsewhere in the country, likely related to the not-so-benign rains that have dampened soils, requiring the sun to divert some of its heating energy to evaporation. Virtually every part of the nation had above-normal ground moisture as of Monday.
“It’s been a slow start to the 90s,” said Paul Walker, senior meteorologist at AccuWeather Inc., adding that some cities in the Plains States still are waiting for a 90 reading.
Drier and hotter days almost certainly are coming, and if the longer-term outlooks and trends are any indicators, the 90s have only begun in the Philadelphia region.
On average in the 21st century, Philadelphia officially has experienced 28 days with temperatures of 90 or better per year, compared with 21 in the 20th century, based on weather service records. Daily highs in summer have increased by 2.1 degrees Fahrenheit this century, and nighttime warming has been even more robust, up 2.6 degrees.
That trend is a major factor in the government Climate Prediction’s Center call for a warmer-than-normal summer, including an above-normal July.
AccuWeather also is staying with its call for above-average temperatures, although that is not a done deal.
“There’s no real strong seasonal signal that we’re seeing now,” Walker said.
In the short-term, the weather is about to turn sultrier as the air becomes more swollen with water vapor.
A major spike in air-conditioning use is all but a certainty. People tend to be reluctant to turn on the compressors the first few days of a heat wave, says Elizabeth Anastasio, meteorologist with PJM Interconnection, the 13-state power-grid operator. But by the third day they are prone to yield to the inevitable.
About 1.2 million residences in the Philadelphia region have central air-conditioning, according to the U.S. Census Bureau.
Anastasio said, however, that it’s difficult to predict energy demand in summer because summer heat often is interrupted by random thunderstorms, whose downdrafts can shave 10 to 20 degrees off the temperature in a hurry.
Such showers are possible Saturday, and the first heat wave of 2019 could be over on Sunday.