Lehigh Valley under flood watch Tuesday, with risk for severe storms, damaging winds

BETHLEHEM, Pa. — The National Weather Service has issued a flood watch for the Lehigh Valley and much of eastern Pennsylvania from Tuesday afternoon through Tuesday evening, warning that heavy downpours could trigger flash flooding.

A cold front colliding with a surge of tropical moisture is expected to produce widespread showers and storms, the watch states.

Some of the storms could pack a punch: The Storm Prediction Center has the area at a slight risk (2 of 5) of severe weather, stating, “high water content suggests damaging downbursts are likely, especially with any organized clusters.”

Downbursts are characterized by powerful downdrafts of air within a thunderstorm that spread out rapidly upon hitting the ground, creating damaging straight-line winds.

The setup

Temperatures are expected to climb into the mid to upper 80s in the Lehigh Valley on Tuesday, helping storms shifting from the central Great Lakes into the Mid-Atlantic grow stronger and more organized.

Heat indices are expected to range between 100 and 105 degrees across the region, the weather service said in its latest forecast discussion.

Forecasters say the storms will tap into a corridor of deep moisture extending into our area, creating an environment ripe for intense rainfall rates of up to two inches per hour, potentially leading to flash flooding.

The best chance for severe weather and flash flooding will come late Tuesday afternoon into the early evening as storms move toward the coast.

Arrival of storms in the Lehigh Valley could come between 5 and 6 p.m.

“Excessive runoff may result in flooding of rivers, creeks, streams, and other flood-prone locations,” the weather service said. “Flooding may occur in poor drainage and urban areas.”

Watching the PWATs

The weather service said PWATs — or precipitable water values — will remain in the range of 1.8 to 2.3 inches.

Precipitable water is a popular metric used by meteorologists to understand how much moisture is in the atmosphere. The values give them an idea of how much rain a storm is capable of dropping.

“Global precipitable water, or the total amount of moisture in the atmosphere, has been at or near-record levels for the last three years,” meteorologist Ben Noll said on X Monday.

“How much is a global mean increase in precipitable water contributing to individual events? It would certainly vary by region,” Noll said.

He highlighted a trend that may be tilting the odds toward higher amounts of rain, on average.