The science behind the rain-snow line in Delaware

Delawareans use the C&D Canal as a dividing line for a lot of things, but local weather experts also often point to it as the snow-rain dividing line.
That was evident Wednesday, when a mid-week nor’easter spared most areas south of the canal the heavy, slushy snow that led to business and school closures and a low-level driving restriction in New Castle County.

As Sussex County experienced a relatively normal rainy day, New Castle County may have seen one of the state’s most significant snowfalls of the season, said State Climatologist Daniel Leathers.

“It’s probably going to end up being our biggest snow of the year, and some people will hope it will be the biggest and last snow for good,” he said. “I think this will be the March storm people will remember for a while.”
Leathers said this storm, nestled between last week’s intense nor’easter and another system expected late this weekend, is not all that unusual. It is a product of normal weather patterns as the seasons shift from winter into spring.

“The pattern just shifted,” he said. “We don’t get a lot of snow here in March, but it’s not unprecedented to get March snowstorms. It’s not going to be as intense as the winds were last week, but with more moisture and cold air, that’s why in the north we’re getting the heavy snow and in the southern part of the state, you guys are missing it.”

A dip in the jet stream over the eastern part of the United States – called a trough – set up the perfect conditions for coastal storms. With temperatures close to freezing, this storm offers a stark contrast to unusually warm temperatures seen last month.

Leathers said he still needs to crunch the numbers, but it looks like February 2018 may rank in the top five warmest since record-keeping began in the late 1800s.

Back-to-back coastal storms are expected this time of year. March is the month for nor’easters and mid-latitude storms created by normal fluctuations in the jet stream, said John Callahan, a climatologist at The Delaware Geological Survey. On average, Delaware sees about 33 of these storms every year.

“This is a peak time for these kind of nor’easters, which I think we have to worry about more than tropical systems because they have a higher risk since they last longer and happen more frequently,” Callahan said. “They carry enormous potential for beach erosion and damage to infrastructure and communities for the flooding.”

The famous Storm of 1962, which wreaked as much havoc as a powerful hurricane, was actually a nor’easter, he said.

“Nor’easters are severe for us,” he said. “About 90 percent of the coastal storms we get here are non-tropical in origin, and most of them are nor’easters.”

As temperatures teetered around freezing, New Castle County saw heavy, wet snow as large flakes moved through the atmosphere and stuck together before hitting the ground. If temperatures were much cooler and below freezing, New Castle County would have a lot more snow to clean up, Leathers said.

“When snow falls, usually it has a 10-to-1 ratio of snow to water, meaning one inch of rain equals 10 inches of snow,” he said. “Because it is warm today, that ratio is a lot lower. So, there’s less snow, but it has a large amount of water in it. It’s going to stick, and you’re going to feel it when you pick up a shovel full of it.”

The First State’s geography is the reason Delaware often sees so many back-to-back coastal storms, and why weather can vary drastically from one end of the state to the other.

“I-95 is kind of the perfect distance inland where, to the south and east of it you get warm air that changes snow to rain,” Leathers said. “North of there, you’re in some higher elevation, which helps lower the temperature and are further away from the ocean and can be a little colder. That’s the dividing line.”

The longer the jet stream stays in place, the longer atmospheric conditions that produce storms can linger, Callahan said.

“Just like how we had the three hurricanes at one time last year – that was rare, but not unique,” he said. “These kind of events happen just because of normal weather patterns.”

A few hundred feet of elevation can make all the difference between rain and snow, Leathers said. Some of northern Delaware reaches 300 feet above sea level in the Piedmont region, while the southern part of the state on the Atlantic Coastal Plain is much closer to sea level.

“As you go up in elevation, you typically go down in temperature,” he said. “Just a few hundred feet in elevation can make a difference of two to three degrees. And that can make a difference of whether it falls as rain or snow.”

As the seasons transition between winter and spring and the jet stream dips over the East Coast, warm air from the ocean meets cold air to the north and west creating air flows that foster coastal storms. The bigger the difference between those two patterns, the more intense the storms, Leathers said.

For this weekend, he said Delawareans can expect a very similar storm system. Northern Delaware will be right along the rain/snow line. But it is too soon to predict exactly what will happen, he said.

“We will have to wait and see how that plays out,” he said.


Contact reporter Maddy Lauria at (302) 345-0608, or on Twitter @MaddyinMilford.

For questions and information, contact DGS at, 302-831-2833