Meteorologists say this week's crippling rush-hour snowstorm was a nightmarish mash-up of an unusually dynamic storm, some uncommonly sticky flakes and a pinch of bad luck. And it all combined to snare evening commuters who should have gone home an hour earlier.
But it was also part of a complex weather pattern that has made winter storms especially hard to forecast this year, particularly with the kind of scale and precision the public may expect.
"While it seems like we should be able to do it, even today we're not quite able to precisely indicate where the smaller-scale features in a larger storm will set up," said Pennsylvania State University meteorologist Fred Gadomski.
Storms earlier this winter intensified perhaps 100 miles farther north than Wednesday's storm. "That makes all the difference in a place like Baltimore and Washington," Gadomski said, in those cases leaving the region with only an inch or two of snow.
At least two factors have made this year's storms harder to forecast, meteorologists said.
First is high-latitude blocking. It's a pattern of high atmospheric pressure that has been parked near Greenland for two winters in a row, Gadomski said, a persistence "unprecedented" in the 60 years such records have been kept.
"It tends to be associated with discharges of arctic air into the middle latitudes and subtropics," he said. And it has contributed to both persistent cold and storminess in the Northeast and Mid-Atlantic.
The persistent blocking pattern has one other effect — it degrades the reliability of the computer models used to predict storms three to six days in advance.
"The fact that we've had unprecedented persistence of high-latitude blocking, and the fact the computer models seem to be worse than we expect them to be — those two things are related," Gadomski said.
James E. Lee, meteorologist-in-charge at the National Weather Service forecast office in Sterling, Va., offered another explanation.
Last winter was dominated by storms that swept off the tropical Pacific along the southern jet stream and across the Gulf of Mexico, following a pattern typical of El Nino winters.
Computer models handled last winter's storms pretty well, armed with data from a wealth of observing stations in the tropics and mid-latitudes.
But in this La Nina winter, it's the northern jet that's dominant, he said. Storms tend to sweep out of the arctic and across the continent.
"When storms track out of the polar regions, we have less data out there," Lee said. Short of input, the models struggle with the forecast output.
Wednesday's storm "was one of the first events this winter we've had where we've had more of an influence of the southern jet stream, so I believe the models for our area performed much better," Lee said.
Marylanders may have been lulled by recollections of the storm forecasts earlier this winter that seemed to warn of significant snows, only to see them veer away or intensify just beyond the region.
But this time it didn't. On Wednesday, the storm intensified off the Virginia Capes, and Baltimore finally found itself in the snow zone. And it was an especially dangerous storm, even compared with last winter's blizzards, Lee said.
"The snow was a lot stickier, rather than a dry, fluffy powder," he said. As cold air moved into place and changed the rain to snow, flakes melted a little on the way down and stuck together, coalescing into a heavy, sodden blanket. It became hard, icy and very slippery, coating the streets, clogging windshield wipers and burdening limbs and wires to the breaking point.
It also piled up quickly, accelerating to several inches an hour by something called meso-scale banding. The storm's intensity was signaled by claps of thunder and flashes of lightning. Forecasters saw that just before 7 p.m. and boosted their accumulation forecasts, from 3 to 6 inches to 6 to 10.
Worst of all, the snow struck in the middle of the evening rush hour. Commuters who had watched the rain for much of the day left for home just as it was changing over quickly to sleet and then snow. Very soon, the roads were impassable.
Forecasters say the warnings were out there. Just before 11 a.m., the National Weather Service issued this special weather statement:
"Rapidly deteriorating conditions for the mid- to late-afternoon and evening rush hour commute … A mix of rain and snow will quickly change to all snow during the afternoon … The heavy snow will reduce visibilities below a quarter mile, with snowfall accumulation rates at times 2 inches per hour. This will produce dangerous travel conditions during rush hour."
Whether that warning was passed along early enough, or heard widely enough, is another matter, Lee said.
Maryland weather blog: Frank Roylance on meteorology
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