Thursday, October 8, 2015

Weather or Not | It was sleet that got the drivers stuck

Deborah Miller (right) , of Rhode Island, speaks with Andrea Bryer of the Pennsylvania National Guard on I-78 in Bethel, Pa. Huge - perhaps record - amounts of sleet left drivers icebound.
Deborah Miller (right) , of Rhode Island, speaks with Andrea Bryer of the Pennsylvania National Guard on I-78 in Bethel, Pa. Huge - perhaps record - amounts of sleet left drivers icebound.
The most intense storm of the winter is raging in Harrisburg as lawmakers are demanding to know who didn't do what to rescue icebound motorists on I-78 on Valentine's Day.

We'll leave that overwhelming question to our esteemed elected officials and the independent investigator.

Meteorologically, we can say that what happened Feb. 13 and 14 was a wild and extraordinary event, and one that might have directed a singular fury in the vicinity of the traffic fiasco.

It was one of the most disruptive Northeast storms on record, in the view of Louis Uccellini. He is in charge of all the country's bad weather at the National Weather Service and is also recognized as a winter-storm authority.

Atop an unexpectedly stubborn layer of cold air, the Valentine's Day storm created a strange and persistent atmospheric parfait. Warm air melted snow on the way down, but then cold air refroze it before it landed. What resulted was prodigious, perhaps record, amounts of sleet.

Not that it was a total surprise. Warnings were posted well in advance, but as usual, nature had a few surprises for the computer models.

Uccellini knew early on that he was looking at something special. The models were calling for a major ice storm - with a "bull's-eye" of up to 2.5 inches of precipitation in northeastern Pennsylvania. That's the water equivalent of up to three feet of snow.

He remembers saying, "If this is all sleet, this is going to be incredible." It wasn't all sleet, a catchall term for frozen precipitation that is neither snow nor rain nor hail, but the bulk of it was.

Our best estimate is that 4 of the 7-plus snow and ice inches measured at the Allentown station - the closest to the traffic jam - consisted of sleet. That is a phenomenal amount.

Typically, it takes a forecast of a mere half-inch to trigger a "heavy sleet warning." So 4 inches would be eight times the warning criterion. The water content of what fell at Allentown was mighty close to that of the 30-inch snowstorm that hit Philly in January 1996.

It is clear that conditions deteriorated after the I-78 jam-up started, sometime after 10 a.m. Sleet fell so heavily in the early afternoon that even if that road had been treated, the material likely would have been buried under ice balls. The hourly reports indicate visibilities below a mile.

Sustained sleet is at the very least unusual. Sleet suggests an atmosphere in transition. Ordinarily it occurs when warm air aloft is entering or exiting, as rain is changing to snow or vice versa. Rarely is it a main event.

What happened?

Uccellini said a fresh batch of cold air became entrenched in the lower atmosphere on Tuesday. The models didn't show it enough respect. They figured the warmer air generated by the storm would scour out the cold layer. But fresh cold air is more stubborn than air that's been weakened by a few days of February sun.

Meanwhile, the upper atmosphere was warming in a hurry. The warming was so robust and pervasive that sleet fell as far west as central Pennsylvania.

So was the sleet worse on I-78 than anywhere else?

"There can be an optimal bad zone," said Dork Sajagian, a Lehigh University professor who is familiar with the road. That said, he is among those at a loss to explain how people could have wound up stranded on I-78 for up to 24 hours.

Even if every detail wasn't nailed, the forecasts clearly indicated trouble. The government posted a winter-storm watch on Sunday and a warning on Monday.

Neither Delaware, Maryland nor New Jersey experienced anything similar, highway officials said.

Delaware, we can see. It's flat and about the size of a Chester County backyard. But Maryland has serious mountains, as does New Jersey, which also has a stretch of I-78.

New Jersey Transportation Commissioner Kris Kolluri is sympathetic to his neighbors.

Said Kolluri: "We obviously had a challenging storm here as well."

But nothing like the storm in Harrisburg.

"Weather or Not," Anthony R. Wood's column, appears biweekly. Read his daily weather blog at

Contact Anthony R. Wood

at 610-313-8210 or

"Weather or Not" appears the second and fourth Mondays of

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