Published: 2010-12-23 16:27:00
Updated: 2010-12-23 16:51:51
Posted December 23, 2010
WRAL meteorologists – and their peers across the country – rely on four main forecasting models to draw a picture of what the weather might bring.
The relative strengths, weaknesses and variations among the models all form a part of the seven-day forecast, WRAL meteorologist Mike Maze said.
On Thursday, all four major models – the North American Mesoscale (NAM), the Global Forecasting System (GFS), WeatherScope and the so-called "European model" showed snow for Raleigh beginning Saturday night and continuing into Sunday.
"The European model generally does a better job of the day-to-day forecast, so we tend to give it a bit more credence," Maze said.
However, in the case of the coming Christmas storm, the European model, formally the European Centre for Medium-Range Weather Forecasts, is the outlier, predicting more snow on Sunday than the other models.
Each model depends on data gathered at the surface and in the atmosphere from weather balloons. The data is updated four times a day. At 1 a.m. and 1 p.m., the models update with the most recent surface data. At 7 a.m. and 7 p.m., models update with both the upper air and ground-level data, Maze said. Because they are more complete, the 7 o'clock updates are more reliable, Maze said.
The coming storm formed over the Pacific Ocean, where there are few weather balloons. "Now that the storm is over land, the models have more weather balloons to query, more data to crunch and a better idea of what the storm really is," Maze said.
As the models detect a faster-moving storm, they reduced the amount of snow forecast for central North Carolina. Where once the prediction was for a "significant" amount – more than six inches – that looked less likely Thursday, Maze said.
"It is unlikely we'd swing back into the significant category," he said.