Philly is under a tornado watch Friday, then record heat is expected this weekend

In advance of a serious preseason shot of summer heat, the National Weather Service posted a tornado watch for the entire region that was canceled shortly before 5 p.m. Friday.

A cluster of strong storms was building along the Ohio-Pennsylvania border Friday morning, and it was due to progress eastward and perhaps touch off severe storms later in the day, the weather service said.

The storms will be followed by a mini-heat wave, and the weather service has issued a “heat advisory” for the region for Saturday.

“It’s looking more likely we’re going to come close to if not breaking records this weekend,” said Amanda Lee, a meteorologist at the weather service office in Mount Holly, which was calling for a high Saturday of 97 degrees, surpassing the standing record, 95, set in 1934 in the Dust Bowl era.

Sunday’s high also could challenge the all-time high for a May 22 — 96 — set in 1941. But the forecast low of 75 — close to the average high for the date — has a better shot at setting a record, this one for the highest minimum temperature for a May 22. That distinction belongs to the 71 of 1959, and is arguably more notable than whatever happens on Saturday.

Highs of 90 or better have occurred frequently early in the warm seasons — it happened on three consecutive days in April 1990 ― but nights this warm this early would qualify as rare — or at least they used to.

Nighttime warming is one of the more well-documented symptoms of climate change, that’s likely related to urbanization and increased water vapor generated by rising temperatures.

This would mark the third year in the last five that an overnight low failed to fall below 70 degrees on a May 22 or earlier. That happened in only four years between 1951 and 2000. The earliest the daily minimum temperature never got below 75 was on May 28, 1991.

» READ MORE: Warm nights are becoming more common Philly

“It will be uncomfortable for people who don’t have air-conditioning ready to go,” said Paul Walker, a meteorologist with AccuWeather Inc., “and for people who don’t have air-conditioning, it’s going to a danger for those folks.”

The rather suddenness of it all is likely to intensify the discomfort, and heat indexes might approach 100 in the immediate Philadelphia area.

However, they are likely to remain well below levels required for a heat “warning” that trigger a variety of responses from the city.

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That’s because winds from the Southwest will be importing some dry air from the nation’s desert regions, said Walker. That’s also one reason that temperatures are going to spike so high during the day. When the air is full of water vapor, the sun has to spend some of its energy evaporating it.

However, some humidity will be present, and with some clouds and light winds, conditions in the early morning hours won’t be favorable for heat escaping into space, said the weather service’s Lee.

For those heading for the Shore, it should be more comfortable, with highs in the 80s and lows in the 60s. For the adventurous, surf temperatures are in the mid-50s, about normal for this time of year.

It should be mostly sunny both days, and the government is calling for “very high” ultraviolet index levels during the weekend. The Environmental Protection Agency advises limiting exposure during the peak sunlight hours and wearing a hat and sunglasses.

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Overexposure is considered a risk factor for cataracts and skin cancer, the agency advises, and, of course sunburn. Health experts advise against being chary with the sunscreen.

Another option would be to spend time inside, cranking up the air conditioner if it is available, and consuming something that might alleviate concerns about the energy bills.