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Numerical Weather Prediction

Influence of the Great Lakes on weather patterns Besides weather radars, satellites, and actual measurements of the atmosphere, meteorologists use computer models to forecast the weather. An atmospheric computer model is a complex program of mathematical equations written by scientists to simulate air motions taking place over the earth. Based on these air motions, we can determine temperature and pressure changes and what kind of weather may take place. The National Weather Service's ability to develop and run more complex weather computer models has exploded along with our society's entering of the computer age. Faster and more powerful computer processors have allowed for more accurate calculations of air flows and more complete representation of factors that affect temperature and pressure changes on a smaller scale.

These computer models run by performing numerous calculations at specific locations called grid points. The number of grid points a model can have depends on the processing power of the computer running it. The more grid points that are in a model over a given location, the more accurate it can be in forecasting wind, pressure, and temperature. In 1975, our most advanced model had three grid points over Lake Superior. A new model introduced in 1985 had 13 grid points over Lake Superior. By 1992, National Weather Service computer models had 39 grid points over Lake Superior, and today, in 2010, our most sophisticated computer model (with a resolution of 4 km) has over 5100 grid points over Lake Superior! That's an increase of 392 times the original number in 1975!

A greater number of vertical levels in a computer model will also produce more accurate forecasts of jet stream winds and temperature motions important to developing storms. In 1975, our computer model had six vertical layers, today the National Weather Service runs computer models with up to 50 vertical layers.

If you live near the Great Lakes, you know that the lakes add heat and moisture to the air during the fall and winter, resulting in lake effect snow. The added heat and moisture can also produce deeper low pressure systems and stronger winds. Computer models in 1975 were not advanced enough to include the effects of heating and moistening from the lakes. It was like the lakes weren't even there! Today's computer models are written to approximate the heat and moisture given off by Lake Superior and all the Great Lakes, giving meteorologists better information upon which to base their forecasts.

Anyone can look at what the atmospheric computer models are showing meteorologists. Just visit the following weather models web site from the National Centers of Environmental Prediction. If you want to see more detailed modeling from the NWS Marquette, go to the mesoscale modeling page where you can see 5 km model runs every 6 hours.

The Great Lakes add heat and moisture to the air surrounding them. Adding heat and moisture results in a lowering of barometric pressure, which our computer models can now show. This figure shows the additional snowfall (shaded areas) in inches and lowered surface pressure (red lines in millibars) that occurs in the National Weather Service's computer model as a result of the Great Lakes for a 36 hour period ending 10 AM EST December 20, 1996.
(Image courtesy of Dr. Greg Mann, National Weather Service Detroit/Pontiac)