Halloween Nor'easter: How unusual was it?
Folks in the Northeast remember the Blizzard of '78 and the April Fool's Day Blizzard, which hit in '97. They'll be talking about the limb-snapping, electricity-killing Halloween Nor'easter for a long time, too.
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If the 1991 event was a perfect-storm convergence of two storms, this latest record-breaker was a perfect-storm convergence in its own way, Uccellini suggests.Skip to next paragraph
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In this case, blame it on the jet stream, a fast moving river of air at high altitudes that steers storms across the continent from west to east. The feature also serves as a boundary between warmer air masses at mid and low latitudes and cold, polar air to the north.
Last Tuesday and Wednesday, a southward dip in the jet stream brought a storm with heavy snow to Denver and other communities along the Rocky Mountains' front range. As that dip worked its way across the continent, it in effect merged with another deep meander in the jet stream that came out of Canada. The two reinforced each other in ways that triggered the formation of a strong low-pressure system at the surface off the US East Coast.
The system's counterclockwise winds swept up moist, relatively warm air from the Atlantic. As the system moved northward along the the meander, or "trough," the moist air clashed with the cold air the trough brought out of Canada. The contrast would set up a rain-snow boundary whose location along the storm's path was initially unclear to forecasters.
Some weather-forecasting models first hinted at the storm's formation as much as six days before it arrived, Uccellini says. Three days later, the full suite of models the National Weather Service considers produced similar projections for the storm's formation.
The only question the model couldn't answer at that time involved the location of the rain-snow line, he explains. Only when the storm was a day or two away could forecasters "really start defining where that rain-snow line was going to be."
The potential for heavy snow in the areas that got it was evident three to four days out. By the time local forecast offices began issuing winter storm warnings on Friday, "it was pretty clear that he heaviest snow would stay inland, from West Virginia all the way up to Maine."
"Given that this was such a rare event, such a historic event, the fact that it was forecast, that people understood the impact with leaves on trees, was quite remarkable," he says.
IN PICTURES: October snowstorm
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