17 November 2013
I cannot remember the last time I saw a moderate risk for severe storms in November, but it has seemed to happen more often in recent years. A negatively tilted trough will swing through the region on Sunday and severe storms will likely develop along a fast-moving cold front. The wind field will be strong as well with south winds of 20-30 mph ahead of the front, with almost all of Ohio and Indiana are at the greatest risk of seeing tornadic activity.
This may just be the beginning of some notable stormy weather because the longer range models are showing the North Atlantic Osc. to go negative in about a week. This pressure pattern (and it’s cousin the Arctic Oscillation) is a good forecast tool for a few weeks out. When the NAO goes negative the cold air in the Arctic tends to get shoved southward into North America instead of sliding east into the Atlantic. I’ve written about the NAO before here. Also here.
The best way to do long-range forecasting is to run the numerical weather model several times while very slightly changing the initial conditions. Then look at an average of the runs and compare how close together all the runs are. If there is high agreement your confidence in the model solution is much higher. The image on the right shows several of these runs and you can see the trend toward a negative AO in about ten days.