Memorial Day weekend weather should be decent in the Philly region — until Monday

While the weather should be more than decent Saturday and Sunday in the Philly region, forecasters say, those heading to the Shore may have the beaches to themselves on Memorial Day if they decide to stay around — or be prepared for heavy traffic if they don’t.

On the mainland, late-May sun is due to cook temperatures well into the 80s on Saturday and Sunday, with both days rain-free.

Temperatures at the Shore are forecast to be several degrees cooler, with highs in the 70s.

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However, conditions look to deteriorate on Memorial Day. “Monday certainly looks like the day that people may be heading back from the Shore early,“ said Eric Hoeflich, a National Weather Service meteorologist in the Mount Holly office and a Shore veteran.

With the atmosphere engorged with moisture, flooding downpours and strong thunderstorms may be in play throughout the region on Monday, said Dave Dombek, senior meteorologist at AccuWeather Inc.

The government’s Storm Prediction Center has the entire region under a “marginal risk” for severe storms on Monday, defined as those with wind gusts approaching 60 mph.

Monday night, the National Weather Service issued a tornado warning for Northeastern Lancaster County in effect until 9 p.m. The warning noted “a severe thunderstorm capable of producing a tornado” was located over Rothsville at 8:24 p.m.

Not the Atlantic Ocean or the atmosphere, which own no real estate nor Boardwalk concessions at the Shore.

Even by the meteorological community’s liberal definition of the starting date of summer — that is, June 1 — Memorial Day never qualifies since it is always celebrated on the last Monday in May.

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The climate hasn’t changed that much (yet) to consistently nudge the end of May into summer.

Of the three most popular traditional “summer” holidays — Memorial Day, July Fourth, and Labor Day — “Memorial Day is the one with the highest probability of having nasty weather,” said Dombek.

Hit-and-miss thunderstorms are a staple of early July. The biggest threat in early September would be rains and winds of tropical storms or their remnants. But those are infrequent occurrences, Ida notwithstanding.

Nature, however, has an assortment of ways to spoil picnics and beach outings at the end of May, said Dombek.

For example, enough chilly air is around to create the temperature contrasts needed for the organized storms that characterize the cool seasons.

Backdoor fronts — which move northeast-to-southwest rather than west-to-east — can cause temperatures to drop in a hurry since they import chilly air off the ocean.

And the ocean is still chilly, around 60 degrees this time of year, and it takes its good, old time warming into the 70s. Even a breeze from the east would create a jacket situation for beachgoers.

The summer heat also usually takes some time to ripen. The normal high temperature at the Shore doesn’t reach 80 until June 9, and 85 until June 25.

The Memorial Day weekend outlook shines in comparison to some other May 25-27 periods in the past.

In 1991, a record heat wave that was a prelude to a blistering summer cooked Philadelphia with highs in the mid-90s, and the Shore was only a couple of degrees cooler.

On May 25, 1963, it got down to 40 degrees in the city, and to 44 in Atlantic City on May 27, 1965.

If you were around in 1918, you might remember the record 3.18 inches of rain that fell in Atlantic City on May 26.

Regarding rain, on average measurable rain falls around here about once every three days this time of year, although luckily the entire weekend was rain-free in Philly last Memorial Day weekend.

Hoeflich said he accepts what Memorial Day brings with a certain equanimity.

“It’s usually a bonus if you can get some beach time on Memorial Day,” he said.

Staff writer Ariana Perez-Castells contributed to this article.