Heavy rain, more forecast led to the advisory, as the worst of the storm seemed to have skipped over Northampton County.
The National Weather Service upgraded a severe thunderstorm watch issued Tuesday afternoon to a flash flood warning across parts of Warren and Hunterdon counties.
The warning was in effect through 12:15 a.m. Wednesday for southwestern Warren and north-central Hunterdon counties.
Flash flooding was forecast to follow thunderstorms moving through the warning area, producing heavy rain as of about 8:15 p.m.
"Reports of road flooding and up to 2.5 inches of rain have already been received," the warning stated. "More thunderstorms are moving into the area and additional rainfall is expected."
Locations in the warning area include Washington, Oxford, Alpha, Glen Gardner, Hampton, Bloomsbury, Brass Castle, Pleasant Valley and Phillipsburg.
"Move to higher ground now. Act quickly to protect your life," the warning stated, adding in an advisory to drivers: "Turn around ... don't drown when encountering flooded roads. Most flood deaths occur in vehicles."
Northampton County appeared to have escaped the worst of the storms, with no widespread flooding issues reported as of about 8:40 p.m., a 911 dispatch supervisor said. There was a call to pump water from the basement of First United Methodist Church, 55 N. Third St. in Bangor, the dispatcher confirmed.
The severe thunderstorm watch remained in effect through 10 p.m. Tuesday for the Lehigh Valley region.
Kurt Bresswein may be reached at kbresswein@lehighvalleylive.com. Follow him on Twitter @KurtBresswein. Find lehighvalleylive.com on Facebook.