Day 31: More Cold Air?
Above is a computer generated forecast for Tuesday morning (6 AM CST) 9 February 2010. This forecast comes from the Global Forecast System (GFS) model and is used by meteorologists around the world to help predict the weather. It is run 4 times a day (00Z, 06Z, 12Z, 18Z — 6 PM CST, 12 AM CST, 6 AM CST, 12 PM CST) and goes 384 hours into the future. Now, as the forecast goes farther into the future the less accurate it tends to be. This is why you will hardly ever see a meteorologists make a 384 hour prediction. Often times something that is seen in the later parts of a forecast run (200+ hours into the future) will change from run to run. This means that a major snow storm forecast at hour 288 might not be there on the next run.
If the forecasts are so unreliable that far into the future, then why do the models run that far into the future? Well, the short answer is that while the exact details may change, often a pattern will emerge and you can get a big picture idea of what may happen in the future. Take the above forecast image (valid 216 hours into the future). The green on the map is areas where the model is predicting precipitation and areas to the north of the blue ’0′ line are places that are forecast to be below freezing. Based on this forecast, a meteorologist might make a prediction that there is a chance of snow in northeast Oklahoma, SW Missouri, and NW Arkansas. However, being so far into the future, that is not a good idea.
What a good meteorologist should take from this image (and the entire forecast) is that the model is forecasting cold air to come southward out of Canada into the southern plains sometime next week. Furthermore, the model is predicting some precipitation mechanism to develop in the southern plains / Gulf Coast around the same time. Now, will this occur at 12Z (6 AM CST) on 9 February? It’s too far out into the future to know for sure. If several model forecasts in a row try to develop a storm on 9 February 2010, then a meteorologist would have more confidence that will happen. If the model continues to change the date and time of when this might occur, a good forecaster will refrain from making a specific prediction about where and when, but understand that ingredients are coming together for a possible winter storm.
As for what the models are currently forecasting? Well, I wouldn’t put too much stock in the exact timing of things, however, several models are forecasting cold air to move south from Canada. At the same time, the models continue to have cyclones (lows) move across northern Mexico or southern Texas into the southeast United States. The cyclones will provide a source of lift for the atmosphere and help to bring moisture from the Gulf of Mexico northward into the southern United States. Add in the cold air from Canada, and the models are hinting at the potential for more winter storms in the coming weeks. It’s far too early to tell where and when and how much. However, it isn’t too soon to begin paying attention to forecasts for next week and beyond. A lot can (and probably will) change in the coming days, so stay tuned!
