Lehigh Valley under excessive heat warning, with severe weather threat to follow

BETHLEHEM, Pa. — The National Weather Service on Wednesday expanded an excessive heat warning into the Lehigh Valley, calling for dangerously hot conditions with heat index values up to 105 degrees.

The warning will remain in effect for Lehigh and Northampton counties until 8 p.m., with the greatest heat index values expected to occur in and around the lower elevations and valleys.

And, for the first time, the area turned magenta — or extreme — on the NWS HeatRisk tool, an experimental color-numeric-based index that provides a forecast risk of heat-related impacts to occur over a 24-hour period.

HeatRisk takes into consideration how unusual the heat is for the time of year, daytime and nighttime temperatures, and whether those temperatures pose an elevated risk of heat-related impacts based on data from the U.S. Centers for Disease Control and Prevention.

Lehigh Valley more like San Antonio

The lingering hot spell — which already was smothering — will kick into overdrive Wednesday, with a storm threat to follow.

It comes after Tuesday marked the fifth day in a row of hitting at least 90 degrees and the 13th day of 90-plus temperatures this summer.

The area is well on its way to surpassing the average of 14.42 days in the 90s for records that date to 1912.

The nights, meanwhile, have been exceptionally toasty.

Temperatures lingered around 80 degrees early Wednesday just before sunrise, and the area already has posted several instances of low temperatures in the mid-to-upper 70s during this stretch.

It adds up to this being the third-warmest start to summer on record in the Lehigh Valley based on the astronomical summer (beginning with the summer solstice) and the fourth-warmest based on climatological summer (June through August).

Climate perspectives compare the stretch of heat in the area to cities such as San Antonio, Texas, and Salt Lake City, Utah.

Severe weather threat

Forecasters will closely watch the remnant low of what was Hurricane Beryl on Wednesday.

The center of the low still is forecast to remain well west of the region as it moves across the Great Lakes and pushes a cold front across the Mid-Atlantic and into the Lehigh Valley area.

Guidance suggests most of the region should remain dry through the day, with a small chance of a few thunderstorms developing in the late morning and early afternoon hours.

“The primary concern with any storms that develop will be heavy rain due to the very high precipitable water values and deep warm cloud layer,” the weather service said in its latest forecast discussion.

Storm motions are expected to be relatively fast, limiting the flash flood threat during the day, but forecasters aren’t ruling out some severe thunderstorm potential with a primary threat of straight-line winds.

After sunset, a cold front finally will begin to reach the area and bring another line of thunderstorms with it.

“With the loss of daytime heating, the severe threat will begin to diminish by the time these storms arrive, though an isolated severe thunderstorm wind gust cannot be ruled out,” the weather service said.

Storms are expected to reach the Poconos and Lehigh Valley by sunset, with flooding the main concern as they make their way across the region.