BETHLEHEM, Pa. — Residents across the Lehigh Valley should brace for heavy rain Monday as a tropical air mass combines with an approaching cold front to deliver downpours and thunderstorms, forecasters say.
The National Weather Service has issued a flood watch for Lehigh and Northampton counties — along with much of eastern Pennsylvania and parts of New Jersey — from Monday afternoon through the evening.
Forecasters warn models are projecting localized rainfall totals in parts of the region could reach 3 to 5 inches in just an hour or two, which would be enough to quickly flood rivers, creeks, and streets if and when that rainfall occurs.
The setup
“This is going to be a solid line coming through,” EPAWA meteorologist Bobby Martrich said in his Monday video update.
With the aforementioned tropical air mass sitting over the region, any breaks in the clouds could help fuel the atmosphere further before rain arrives in the afternoon.
“One of the key ingredients for severe weather is shear," Martrich said. "That’s what keeps storms moving.”
"If the shear is lower, it’s a lower number, normally you want that 25 to 30 in terms of knots, or how fast the forward speed of the storm is.
"If it’s too slow, that means the top of the storm is going to be outracing the lower half and then you get rain-cooled updrafts.
"You might have some storms pulse up and get stronger, because there’s plenty of instability available, but then they collapse under their own rain-cooled updrafts and fizzle out."
Martrich said while the Storm Prediction Center does have the Lehigh Valley at a marginal risk (1 of 5) of severe weather, it would be very isolated activity if it occurs.
12:48am CDT #SPC Day1 Outlook Marginal Risk: in parts of the northern, eastern, and southwest states https://t.co/TgJgC6cj9Y pic.twitter.com/tjVQWDBRqS
— NWS Storm Prediction Center (@NWSSPC) July 14, 2025
Instead, the main concern is precipitable water values, a measure of how much moisture the air can hold.
Forecasters say that’s expected to reach around 2.5 inches on Monday — a sign that any thunderstorms could be extremely efficient rainmakers.
Until the storms arrive, it will remain very warm and humid, with highs in the mid to upper 80s and heat indices well into the 90s.
Forecasters say once storms move through, lingering low clouds and fog could pose additional hazards overnight as moisture remains trapped near the surface.
Wash, rinse, repeat
Unsettled weather is expected through the middle of the week, the weather service said.
Several rounds of showers and thunderstorms are likely again Tuesday, with locally heavy rainfall leading to flash flooding, the latest forecast discussion said.
Similar to Monday, precipitable water values will be about 2 inches, and training storms could result in aggressive localized rainfall totals again in just a couple hours.
“Flash flooding could result in some areas if these totals are realized,” the weather service said.
It noted it currently has no flood watches to highlight the threat for Tuesday, but one could be needed in an update.
Tropical downpours will remain a threat for Wednesday, with heat and humidity peaking from Thursday through Friday, along with continued daily chances of showers and thunderstorms.
