BETHLEHEM, Pa. — Forecasters are closely monitoring “a significant flash flood threat” for parts of the area on Thursday.
Widespread showers and storms are expected to develop in the afternoon and evening with broad intensification in the Mid-Atlantic region, the National Weather Service said in early morning updates.
The Lehigh Valley is part of a moderate risk (3 out of 4) area for flash flooding as showers and storms develop “within the vicinity of a slow-moving frontal boundary,” the weather service said.
The probability of precipitation remains near 80% to 100% for the entire area, with rain arriving in the afternoon and continuing through the evening and overnight.
“The synoptic and tropical aspects of this setup will likely lead to scattered convection continuing into the overnight period,” the latest forecast discussion said.
Understanding flash flood threat
The weather service said the environment during the event “will support considerable and potentially significant flash flooding, especially within the moderate risk area.”
The current rainfall forecast generally ranges from 1 to 3 inches across the area, but Thursday morning forecasters said locally higher amounts of 5 to 7 inches or even greater are certainly possible in the risk area.
“Rainfall rates could exceed 2” per hour, which could quickly overwhelm drainage systems,” the forecast discussion said.
But while the risk area covers a significant part of the region, the greatest flash flood threat will depend on where the frontal boundary — a boundary between two air masses — sets up into the afternoon and how quickly storms are propagating.
Propagation refers to the movement or spread of storms, or the influence of weather patterns on specific locations.
High precipitable water values and the tropical nature of the environment will not support quickly moving storms, the weather service said.
It said guidance early Thursday put the highest threat in an area from central and northern New Jersey and southwest into the Philadelphia metro areas and adjacent areas of southern New Jersey and Delaware.
Concerns for highly populated urban corridor
Excessive rainfall outlooks from the Weather Prediction Center forecast the probability that rainfall will exceed flash flood guidance within 25 miles of a point.
The moderate risk Thursday means there’s at least a 40% chance of rainfall exceeding flash flood guidance within 25 miles of a point inside the risk area.
“These probabilities serve as signals in ensemble guidance that highlight the potential for significant flash flooding throughout the country, let alone over a highly populated urban corridor,” the WPC’s excessive rainfall discussion said.
“Given the atmospheric parameters in place, it is possible that the Mid- Atlantic I-95 corridor witnesses thunderstorms that produce rainfall rates up to 3"/hr., with perhaps the most intense storms potentially generating 2" rainfall totals in as little as 30 minutes,” the discussion said.
The WPC noted “lingering uncertainty in the placement of the heaviest rainfall.” It said many individual convective models show localized totals surpassing 5 inches, with some approaching 8 inches.
Residents in the Mid-Atlantic should ensure they have reliable means of receiving warnings issued throughout the day and into Thursday night.
The state Transportation Department said anyone who drives around barriers intended to close a road can face increased penalties and fines up to $500 if emergency responders are called to rescue motorists who disregard traffic control signs.
Severe storms possible
1:02am CDT #SPC Day1 Outlook Slight Risk: for portions of the Mid Atlantic and the central High Plains https://t.co/TgJgC6cj9Y pic.twitter.com/a1F2kuqhD2
— NWS Storm Prediction Center (@NWSSPC) July 31, 2025
While flash flooding will be the main threat with Thursday’s storms, the weather service said isolated to scattered severe thunderstorms also will be possible.
It said water-loaded downdrafts ahead of the arriving cold front (south of I-78) could support locally damaging wind gusts.
The Storm Prediction Center has increased the severe weather outlook to a slight risk (2 of 5) for parts of the area, with the Lehigh Valley on the edge of that risk area.
Before storms develop, it’s expected to be another very warm and humid day, with highs in the upper 80s to low 90s across the region.
