Watch out for a gullywasher, forecaster warns, with intense rainfall rates possible Friday

BETHLEHEM, Pa. – The National Weather Service in Mount Holly, New Jersey, has issued 22 flood warnings and 29 flash flood warnings across its forecast area this year.

Friday could bump those numbers significantly, with an atmosphere supportive of very intense rainfall rates, according to the government’s Weather Prediction Center, which puts the region at a slight risk for excessive rainfall.

• The Lehigh Valley is at risk for excessive rainfall on Friday and severe weather on Sunday
• Isolated storms Friday afternoon and evening could bring very intense rainfall rates, forecasters say
• Incidents of flash flooding are possible

“Slow cell motions and storm outflow collisions will support localized very intense rainfall rates with 2”/hr probabilities well above normal (20-40% HREF) and 2”/3hr probabilities nearing 50-60% by 21z (4 p.m. EST) across much of N PA to NW ME,” a forecast discussion said.

“Cells should be a bit more scattered to isolated across the Mid-Atlantic into the Northern Carolinas, respectively; however, the risk remains for scattered incidents of flash flooding.”

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The setup

According to the weather service, another warm and humid day is on tap. Highs will top off in the upper 80s, with dew points again in the low 70s, resulting in heat index values in the low to mid 90s.

As a cold front approaches from the west, scattered showers and storms will develop by midday, becoming likely along and west of the Interstate-95 corridor, including the Lehigh Valley.

“Although some thunderstorms may produce gusty winds of 35 to 45 mph, the main threat will be heavy rain,” the NWS forecast discussion said.

The larger issue is the lack of steering flow, according to EPAWA meteorologist Bobby Martrich.

Without steering flow, storms can form on the outflow of previous cells and can produce heavy rainfall, leading to localized flooding and flash flooding.

“A very hot, soupy environment is going to lead to high precipitable water content and that means heavy rainfall,” Martrich said in his Friday video forecast.

“It could be one of those deals where one town is getting nailed and the next town over or the next two towns over aren't really seeing too much … but if you get under one of these you’re going to get a gullywasher, basically."

Gullywasher refers to short but extreme heavy rainfall, or cloudburst.

The weekend will be wet

Storms are expected to taper off Friday evening, leading to another warm and humid night with patchy fog developing.

That sets the table for the weekend, with more rounds of showers and thunderstorms expected.

Activity is likely to be scattered on Saturday, the weather service said, but Sunday will be a different story.

“Sunday will be wet,” the forecast discussion warned, with the potential for excessive rains and severe weather for the afternoon and evening time frames.