Severe storms and flash flooding threat loom for Tuesday

BETHLEHEM, Pa. — There will be no break in the unsettled weather for the Lehigh Valley.

After Monday brought rounds of thunderstorms and heavy rain, more severe weather is expected to target the region on Tuesday, forecasters say.

Beyond the Valley, saturated ground also has heightened flood concerns, prompting the National Weather Service to issue a flood watch covering the urban corridor counties northward into Berks County that runs from 2 p.m. Tuesday until 2 a.m. Wednesday.

While the flood watch does not cover Lehigh or Northampton counties, we are expecting “a prime environment for severe weather,” the NWS said in its latest forecast discussion.

Severe weather threat

The setup Tuesday includes a warm front lifting north of the region, putting all of eastern Pennsylvania into the “warm sector” — the region of warm, moist air located between a warm front and an approaching cold front.

Temperatures are forecast to reach the upper 80s to low 90s, with dew points in the mid-70s making the air feel oppressively humid and “providing fuel for storms to rapidly intensify,” the weather service said.

The combination of strong daytime heating, abundant moisture, and increased wind shear “will set the stage for severe thunderstorms to erupt during the afternoon,” the discussion said.

The Storm Prediction Center has the Lehigh Valley at a slight risk (2 out of 5) for severe storms, with damaging winds and flash flooding as the primary threats.

Flood risks

EPAWA meteorologist Bobby Martrich said the setup Monday meant some areas got “absolutely whacked” with heavy rainfall, while other areas really didn’t get too much.

He said Tuesday, however, will bring a “more uniform” line of storms. But they also could repeatedly move over the same areas — known as training — dumping heavy rain in short periods of time.

Storms will have plenty of moisture with which to work, forecasters said, noting that precipitable water values could reach 2.25 to 2.5 inches in some areas.

That setup could easily drop 1 to 3 inches of rain in just a couple of hours, the weather service said, with isolated areas seeing totals closer to 4 inches.

“Instances of flash flooding could be more numerous than what we see in many events with even the potential for some instances of significant flash flooding possible, especially if storms train over an urban area,” the weather service said.

Urban and low-lying areas will be most at risk for sudden flooding if storms stall or backbuild, which could make the evening commute hazardous.