BETHLEHEM, Pa. — Excessive humidity and extremely warm temperatures have prompted a heat advisory for the Lehigh Valley on Friday.
On top of the heat, the Storm Prediction Center has the area at a slight risk (2 of 5) for severe weather, with damaging winds being the primary threat.
The heat advisory runs from 10 a.m. to 9 p.m. for heat index values up to 105 degrees expected.
The dangerous heat will lead to the potential for severe thunderstorms later in the afternoon and evening, the National Weather Service said in a briefing early Friday.
Main change with the latest forecast is an increased risk for severe storms for portions of the area this afternoon and evening. Dangerous heat and humidity as well with heat indices of 100-107 degrees today. Latest briefing: https://t.co/pVIeWfiKng #pawx #njwx #dewx #mdwx pic.twitter.com/IxntOikqtd
— NWS Mount Holly (@NWS_MountHolly) July 25, 2025
Summer stats
Friday’s expected high temperature will bring Allentown’s ninth day at or above 90 degrees this summer (though many other days have reached the upper 80s, with the heat index pushing “feels like” temperatures into the 90s).
The area’s normal number of 90 degree days through the summer is 18, but its normal number of days at or above 95 degrees is just 3.
Depending on Friday’s high, it could mark our fourth day of the summer at or above 95, according to the weather service.
Our record number of days at or above 95 degrees in one year is 15, which occurred in 1999.
Storm threat
Scattered strong to severe thunderstorms will pose a risk of damaging wind gusts across the area Friday afternoon and evening.
“Storm coverage is somewhat uncertain,” the Storm Prediction Center said in its outlook. Its said isolated to widely scattered storm development will be possible along and ahead of an arriving cold front.
“A few organized clusters and/or marginal supercells may develop,” the outlook said.
The timing looks to be late afternoon or early evening to start, the weather service said in its forecast discussion.
“We lose the daytime heating factor, so convection is expected to dwindle by late evening or early overnight.
“The showers and thunderstorms will develop northwest to southeast, which puts areas near and northwest of the I-95 corridor in the greater potential for severe weather,” the discussion said.
It also flagged a marginal (1 of 4) risk of excessive rainfall — along with a flash flooding risk — associated with the storms.
An unsettled weekend
Showers and thunderstorms are expected again Saturday night over portions of eastern Pennsylvania, the weather service said.
That will set the table for Sunday, which, forecasters say, “looks to be the more unsettled of the two weekend days.”
Showers and storms could be ongoing Sunday morning, with additional showers and storms developing Sunday afternoon.
"The main threat in terms of hazardous weather looks to be heavy rain and the potential for at least localized flash flooding.”National Weather Service briefing
“Given that there may not be much of a break between the two waves, there doesn`t look to be much sun for Sunday, which should keep temperatures and instability relatively lower,” the forecast discussion said.
“For this reason, the main threat in terms of hazardous weather looks to be heavy rain and the potential for at least localized flash flooding.”
The weather service said precipitable water values max out around 2.25 to 2.5 inches, helping add to the flood threat.
