‘A recipe for flash flooding:' Valley under flood watch Monday night

BETHLEHEM, Pa. – The National Weather Service warned Monday of the next round of severe weather taking aim at the region, and this time it will be a mostly nocturnal event.

Lehigh and Northampton counties are among the slew in the tri-state area under a flood watch from 8 p.m. Monday to 6 a.m. Tuesday.

“Have multiple ways to get warnings,” social media posts from NWS Mount Holly said, with forecasters now calling for a widespread 1 to 2 inches of rain, and more possible where thunderstorms set up and train.

• The Lehigh Valley is at risk of flash flooding and other severe weather hazards late Monday
• It will be a mostly nocturnal, or overnight event, the weather service said
• Rain should move into the area by around 8 p.m.

It comes a week after a tornado touched down in Allentown’s Midway Manor neighborhood and straight-line winds as high as 100 mph caused heavy damage in Center Valley and other areas.

The setup

After a quiet morning, things are expected to deteriorate through the mid-to- late afternoon hours, the weather service said in its latest forecast discussion.

“Current thinking is that a good portion of the day across our area should stay precipitation free under variable cloud cover,” the discussion said.

Forecasters say a “fairly potent” upper level disturbance will continue to trek across the Ohio River Valley during the day, advancing a strengthening low pressure system along with it. Ahead of that main system, a warm front will slowly lift northward, reaching the southernmost portions our region by about 5 p.m., in time for the evening rush.

“It’s going to evolve in a steady and sometimes heavy rain overnight, along with some scattered thunderstorms,” EPAWA meteorologist Bobby Martrich said in his Monday video forecast.

Timing

Forecast models indicate an approaching area of showers and storms moving eastward from western Maryland and Virginia and into the Delmarva area by early evening.

As the warm front gradually lifts northward, precipitation is expected to reach the Lehigh Valley by around 8 p.m. Monday, with heavy showers and thunderstorms continuing to develop along the front.

“Over the next several hours through the evening and into the overnight period, heavy showers and thunderstorms are expected to become more numerous,” the weather service said, noting the system will funnel “abundant moisture” into the region.

The hazards

The Lehigh Valley will face a number of severe weather hazards, with guidance continuing to show a very saturated atmosphere, forecasts warn.

“The very tricky part is there is quite a bit of uncertainty of exactly how quickly this front will move and how far north it will end up and, therefore, who will see the heaviest amount of rain and the greatest risk for both flash flooding and severe thunderstorms,” the NWS forecast discussion said.

But Martrich said he doesn’t see the prospect of greater severe weather, such as tornadoes, being a factor for most of the region.

“Generally when you have nocturnal thunderstorms and it’s associated with a warm front, you’re really not dealing with a lot of severe weather, because you don’t have the instability that you would have typically during the day,” he said, noting all the aspects of severe weather need to be met in order to have severe weather.

“If you have one key ingredient missing, you’re not going to have the severe weather aspect.”

The Storm Prediction Center has the Lehigh Valley at a marginal risk (1 out of 5) of severe weather, while the Weather Prediction Center puts the area at a slight risk (2 out of 5) of excessive rainfall.