It started with one of the most extreme El Ninos ever recorded, pumping jolts of weather-making energy across the southern United States.
Waiting on the opposite side of the continent was an area of high pressure parked over Eastern Canada, a hallmark of significant snowfalls in the Mid-Atlantic. Like water flowing around a stone in a stream, weather systems must go around it.
But the key to Baltimore's record-setting snowfall this weekend, meteorologists say, was a flow of unusually mild and wet air off the Atlantic into a storm that for hours barely moved off its position near the mouth of the Chesapeake Bay.
As ocean moisture drew into the storm from the south and east like water flowing down a drain, the system also pulled in a colder stream from the northwest — a combination that created relentless bands of heavy snow over the region. The intensity of the precipitation was reminiscent of a summer thunderstorm, said Eric Luebehusen, a government meteorologist who runs a Maryland-focused weather email list mostly for snow lovers.
"It was amazing just watching it on radar," he said. "There was a never-ending band from New York down to D.C."
Most of Baltimore's biggest snowstorms have a few factors in common with the weekend's storm, including the presences of both an El Nino and what is known as a "blocking" high-pressure system to the northeast. But none of those other snowfalls compared to the 29.2 inches measured at Baltimore/Washington International Thurgood Marshall Airport from Friday night through Saturday night.
The measurement is still preliminary, said Bryan Jackson, a meteorologist with the National Weather Service's Baltimore/Washington forecast office, and will be subject to review by the National Centers for Environmental Information for accuracy. A spokesman for that institution said no one was available to explain the review process Sunday.
The weather service measures snow on a clean and flat board, wiping it every six hours and starting fresh because snow totals can be diminished when the weight of newer snowfall presses down on the older accumulation beneath it.
Dan Pydynowski, a senior meteorologist for AccuWeather.com, said the El Nino contributed to the storm's intensity, but wasn't a direct factor. The El Nino-enhanced energy along the southern branch of the jet stream, where the storm originated, was apparent Thursday, when the storm prompted severe storms and tornado warnings in Texas and Louisiana.
Most of the moisture that was dumped on the Mid-Atlantic came not from the Pacific, where El Nino is associated with abnormally warm waters, but from the Gulf of Mexico and the Atlantic, Pydynowski said, where waters happen to also be as much as 2 or 3 degrees milder than normal.
Temperatures are still cold — in the upper 30s close to the beaches of the Delmarva Peninsula and as high as the mid-40s some 20 miles out. But they were warm enough to fuel exceptionally heavy and unusually long-lasting snowfall, Pydynowski said.
Luebehusen said measures of the vertical movement of air within the storm were "off the charts," indicating a clash of the cold, dry air on one side of the storm and the relatively mild and moist air on the other. When the warmer air was pulled into the storm, it rose rapidly and condensed, producing the heavy precipitation.
The storm's track just off the coast and its slow movement meant the area from Baltimore to the mountains of Northern Virginia and West Virginia were bombarded with that heavy snowfall for hours. It "was in the same spot for like six hours," Luebehusen said.
The result was historic. The 29.2 inches at BWI surpassed the previous record, 26.8 inches from Feb. 16-18, 2003, by more than 2 inches.
Some of the heaviest snow totals reported to the weather service across the Mid-Atlantic included 42 inches near Glengary, W.V., 38.5 inches near North Potomac in Montgomery County, 37.5 inches in Cascade in Washington County and 35.2 inches at Point of Rocks in Frederick County.
Leading the Baltimore region were an area of Harford County between Forest Hill and Street, with 33.5 inches, and Reisterstown, with 32.1 inches.
Together, the back-to-back "Snowmageddon" storms of early February 2010 dumped 16 inches more on Baltimore, for a total of nearly 45 inches. But they are counted separately for record-keeping purposes, likely making this weekend's storm — whether you call it another Snowmageddon, Jonas or Snowzilla — No. 1 in the books.