The frequently mild and occasionally stormy weather that has been impacting our region for much of this winter has changed. As the weather pattern across the northern hemisphere shifts over the course of just a couple of days and allows much colder air from Siberia to move across our region.
This kind of rapid change in the large scale flow pattern may seem dramatic, but it's actually not that uncommon in the winter.
Ultimately, it's an outcome of the movement of the atmosphere, which is a fluid across a rotating sphere, the Earth.
Now, of course, there are many more specific factors at play that shape the timing and persistence of these wholesale changes in the atmospheric flow pattern. Some of these factors are forecastable a week or more in advance, while other features have a big effect but don't become apparent until a few days beforehand.
Putting these all together is one of the big challenges of mid range weather forecasting.