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Rarely do so many people have the same experience at the same time. Probably two thirds of the residents of North Carolina, young and old, looked out their windows on Tuesday morning, January 25, and said the same thing: "I don't believe it!" More than a foot of snow lay on the ground, and snow was still falling heavily. By the time it ended, early in the afternoon in most places, the snow was as deep as two feet.
Meteorologists at RDU Airport recorded 20.3 inches (51.6 cm). It was the largest snowfall in the history of the weather station, breaking a record set in 1902. The total January snowfall, 25.8 inches (65.5 cm) breaks the monthly record set in the legendary winter of 1898.
So what happened? Clearly, central North Carolina got caught in a historic nor'easter. But it was more than that; it was what meteorologists call a bomb cyclone. And like most bomb cyclones, the storm wasn't really predicted.
On Monday morning, North Carolinians were recovering from an ice storm of the previous day. That storm was a small disturbance along a front lying across the southeastern states. Forecasters could see a weaker disturbance coming through during the day Monday, and a third disturbance due to arrive Monday night. It is normal for such disturbances, called shortwaves, to run along a front one after another.
To predict the effects of shortwaves, meterologists look at the computerized predictions distributed by NCEP, the National Centers for Environmental Prediction, located in Washington, DC. These predictions are based on observations throughout the Northern Hemisphere and their goal is to predict the movement and development of large scale features. Local meterologists are responsible for interpreteting the results, using as much local knowledge and experience as they can muster.
Because no one has been able to create a computer program that does a good job of predicting the weather all of the time, NCEP runs several different programs, or models. The models used routinely include the Aviation Model (AVN), Eta Model (ETA), Medium Range Forecast Model (MRF), and Nested Grid Model (NGM). These models are run every 12 hours, in a cycle, so that forecasters always have a fresh model available.
Of course, the models generally disagree on the details of what is going to happen. On good days, three or four of the models will predict roughly the same thing, and then meterologists can go with that prediction with some confidence. On bad days like Monday, January 24, meteorologists have to decide which model is most likely giving the best prediction. Generally, they'll favor the model that's did the best job 12 hours ago predicting whatever's happening now.
Unfortunately, on January 24 none of the models were behaving very well, and meteorologists were left scratching their heads. When WRAL-TV meterologist Greg Fishel went on camera at 6 pm, he "was thinking this could be anything from a joke to a huge snowstorm." Like most area forecasters, Fishel went with an intermediate forecast: sleet, possibly changing to snow, maybe 2 or 3 inches total accumulation.
And then, as National Weather Service meterologist Ron Humble says, the third shortwave grew "unbelievably intense, the kind of thing you might see once in a lifetime." Instead of going mostly out to sea, snow billowed back over two thirds of North Carolina. Winds rose, the barometer crashed, and blizzard-like conditions prevailed in many places.
It was a bomb cyclone, which meteorologists define to be a non-tropical storm rapidly developing the intensity of a tropical storm, its central pressure falling at least 24 millibars (0.71 inches of mercury) in 24 hours. In many bomb cyclones, the pressure falls as much as 60 millibars (1.77 inches of mercury), reaching the low levels we usually associate with tropical storms. In this bomb cyclone, the pressure fell to about 28.00 inches of mercury, in the same range as September's hurricanes. Raleigh Mayor Paul Coble wasn't so far wrong when he called the storm a "white hurricane."
How could the computer models fail so badly? The main problem is that the models are designed to handle large scale features. They don't do well predicting what meteorologists call mesoscale features: features roughly 50-250 miles (80-400 km) wide. Although the great storm of 2000 was a large scale feature, its "bomb" core was a mesoscale feature.
Other mesoscale features include tropical storms such as hurricanes and the mesoscale convective complexes (MCC's) which generate outbreaks of tornados. These features are also predicted poorly by the general models. For tropical storms and MCC's, meterologists have had to develop special tools and computer models designed to predict the movement and development of those specific systems. Obviously, we need similar attention to the winter bomb cyclones of the Atlantic coast.
Bottom line: we just don't know everything we'd like to know about the weather. Despite our best efforts, we can still be fooled pretty badly by the behavior of the atmosphere. It's good for us to remember--and it's good for students to learn!--that a lot of good and useful science remains to be discovered.
Posted Januray 31, 2000. Features remain online as long as they remain current; they may be updated if new information becomes available.
Copyright © 2000, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
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