Why July 4's Storm Brought Chaos To Haddonfield (And Spared Almost Everyone Else)

HADDONFIELD, NJ — The Fourth of July storm was one of the most powerful weather events in Haddonfield's recent memory. It also ended within minutes and barely impacted communities outside of the borough's vicinity.

Officials have a clearer picture of Tuesday's storm that flooded Haddonfield, caused hundreds of outages and spread debris throughout the area. The borough was in the epicenter of a microburst: a powerful blow of sinking air within a thunderstorm that spreads in all directions once it hits the surface.

From about 5:30-6 p.m. Tuesday, four successive microbursts hit a stretch of South Jersey: first around Mount Laurel, then between Mount Laurel and Haddonfield, then in Haddonfield and finally around Gloucester City.

The microbursts' gusts hit about 70 mph — an estimate based on wind-damage reports, according to the National Weather Service's Mount Holly/Philadelphia office. Some areas within the heart of the storms received about 3.5 inches of rain, while less than an inch fell in communities 5 or 10 miles away.

How Do Microbursts Develop?

Microbursts begin with the development of a thunderstorm, as water droplets and hailstones get suspended within the updraft, the National Weather Service says. The updraft can get so strong that it suspends large amounts of droplets and hail in the upper portions of the storm.

Hailstones melt and raindrops evaporate, which pulls latent heat from surrounding areas and cools it. Since cooler air is denser than warmer air, the large core plummets to the ground and then spreads in all directions.

How Much Damage Can They Do?

Microburst winds can reach 100 mph or higher — equivalent to a tornado. The winds can significantly damage homes and level trees.

As a result, severe thunderstorm warnings are just as serious as tornado warnings, according to the National Weather Service, which issues the warnings.

Forecasting The Potential For Microbursts

Meteorologists can forecast microbursts within about six to 12 hours before convection is expected to develop, the National Weather Service says. Several atmospheric parameters help determine microburst potential, including instability, high precipitable water, dry air in the mid-levels and strong winds in the dry layer.

The ideal conditions usually occur during hot, humid summertime afternoons. Microbursts are most common in the Southeast during the summer.

Why Warnings May Have Short Notice

When examining radar data, forecasters look at converging air within the mid-levels of a thunderstorm, according to the National Weather Service.

"These can be very hard to detect since microbursts are so short-lived and can sometimes occur between radar scans," the federal agency says. "Therefore, unfortunately, Severe Thunderstorm Warning lead times for microbursts can be very short, or there may be no warning at all. Our understanding of microburst formation and detection continues to increase and will hopefully lead to better lead times in the future."