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When and where do thunderstorms occur most often in South Jersey?

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South Jersey on Sunday turns the calendar to April, a month ripe with the cracking of thunder and flashing of lightning.

How much thunder and lightning? New research shows our area experiences four to six hours of thunderstorms in a given year.

Thunderstorms are a way of life in the spring and summer, when warmer air, heated by the sun, makes its annual journey back to the area.

But first, it needs to beat out the lingering wintertime chill.

When the two air masses collide, heavy rain and lightning are born. As radar echoes go over 45 dBZ, the measure of intensity for rainfall, it is commonly accepted that a thunderstorm is occurring.

In the United States, Next-Generation Radar, or NEXRAD, tracks the skies for thunderstorms, rain, snow, hail and whatever weather phenomena come our way. The National Oceanic and Atmospheric Administration recently passed the 20-year mark of storing radar data, a critical time in the eyes of researchers.

“Once you reach 20 years, you can use data to pick out trends in how often or where thunderstorms appear,” said Frederic Fabry, director of the Radar Observatory at McGill University in Montreal. Fabry and a team of researchers from across the United States and Canada were inspired to see what could be unearthed from all the saved radar data.

They analyzed the U.S. to see how often and when thunderstorms occurred.

Where do they occur?

Most of South Jersey experiences six hours’ worth of thunderstorms per year, Fabry said. The area between Vineland and Cape May experiences four hours.

By comparison, in eastern Kansas and Oklahoma, the heart of Tornado Alley, any one spot can expect to see 13 hours of thunderstorm-intensity rainfall a year.

That might not sound like much, but consider that many times when you hear thunder and see lightning, that does not mean a thunderstorm is directly overhead. This data accounts only for when a thunderstorm is directly overhead and you are facing the brunt of the storm.

As to why extreme southern New Jersey experiences fewer thunderstorms, Fabry said, Cape May and eastern Cumberland counties are heavily influenced by the Delaware Bay. During the spring and summer, the bay is cooler than the land. If storms pass west to east, as they typically do, the bay will help stabilize the storms, weakening them to below the 45 dBZ threshold.

When do they happen most often?

Typically, the heavy rain and ferocity of a thunderstorm happens during the evening rush or while we eat dinner.

The peak thunderstorm time in New Jersey is between 5 and 7 p.m. Along the shore, from Atlantic City to Toms River, it is a bit earlier. South of Atlantic City, down to Cape May, it is later, Fabry’s research shows.

There are theories as to why.

“In the case of Atlantic City to Toms River, the sea-breeze tends to develop thunderstorms earlier because east winds meet west winds earlier, due to how the land is positioned,” Fabry said.

David Robinson, the New Jersey state climatologist, agreed the orientation of the land plays a role.

“We would have to look at more of the data. However, Atlantic City to Long Beach Island runs from south-southwest to north-northeast. South of there, the shore runs from southwest to northeast,” Robinson said.

The difference is small but matters. Robinson believes Atlantic City on north has a more effective sea breeze. This increases the airflow around the region (and cools down the shore). As a result, it could create more thunderstorms.

However, this all pertains just to spring and summer. During the fall and winter, “thunderstorms do not pay attention to the clock,” Robinson said jokingly.

That’s because fall and winter thunderstorms are driven by large-scale events, whereas the heavy rain and booming thunder of the spring and summer are driven by daytime heating, which peaks between 5 and 7 p.m.

Drier weekends?

In what is welcome news for beachgoers, businesses and shops, Fabry and his team also found that rain falls 5 percent to 10 percent less in New Jersey between Saturday and Monday.

Why is unclear.

According to Fabry, “We thought that less pollution from industry on the weekend would be the reason for this. We got excited. Everything would seem to point to that, but the data tells us that it is a fluke.”

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