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Environment & Science

Fourth of July forecast: Will there be fireworks or flash flooding for the Lehigh Valley?

Stephanie Sigafoos
Fireworks over Macungie on Sunday night.

BETHLEHEM, Pa. — Aerial fireworks create vivid flashes of color that elicit “ohhhs" and "ahhhs” from spectators.

But this Fourth of July, the fireworks in the sky might come from Mother Nature.

“We don’t mean to rain on your parades, but scattered storms are possible July 4th afternoon/evening,” the National Weather Service said on X.

“Not everyone will see storms, but those who do will get a downpour, lightning and gusty winds. If you are out on the 4th and thunder roars, go indoors!”

The setup

After a sunny and seasonable Wednesday, the big story for Independence Day will be the return of much more humid air, forecasters say.

A warm front will crawl through the region early in the day Thursday, which will bring a few isolated showers or storms in the morning for some areas.

The more widespread threat will come later in the day as most areas reach the upper 80s to low 90s, with dewpoints reaching the upper 60s to low 70s.

Whether you're looking to side with safety to celebrate Independence Day this year, want a fun event or just don't want to deal with post-celebration clean-up, plenty of spots in the Lehigh Valley are offering free and purchase-required events.

“This will result in max heat indices into the 90s for most areas along and south of I-78,” the weather service said in its latest forecast discussion.

Cloud cover is expected to increase as the day progresses, with storms to fire “in the vicinity of a weakening frontal boundary draped over PA by the afternoon hours,” the forecast discussion said.

Timing, impacts

Storms are expected to move eastward into eastern Pennsylvania and northern New Jersey later in the afternoon, potentially affecting Delmarva and southern and coastal New Jersey by the evening (for those headed down the shore, stay weather aware).

“It won’t be an all-day type rain event but unfortunately timing won’t be great as it could interfere with Independence Day festivities,” the weather service said.

Chances of precipitation are 40-50% over much of the area, and lower near the coast. But the storm threat will linger into much of the evening.

The Storm Prediction Center has maintained a marginal risk (1 in 5) for severe thunderstorms south of the Lehigh Valley, generally in the Philadelphia area, where “water-loaded downdrafts could lead to a few strong to locally damaging wind gusts.”

The bigger story may be the overall rain threat.

Excessive rainfall
Weather Prediction Center
This graphic shows the risk areas for excessive rainfall on July 4.

“Most guidance has PWats (precipitable water values) over 2.0” across the entire area, and largely within the 2.25” to 2.5” range. These values are near-record PWat values for the month of July,” the weather service said, calling the numbers “highly anomalous.”

Some of the storms that fire will be capable of producing very heavy rain, which could lead to localized flash flooding.

The Weather Prediction Center has much of the area at a marginal risk of excessive rainfall, especially from the Lehigh Valley down through the Philadelphia suburbs and metro area.