After a chilly start to April, warm weather has returned to the northeastern US and it is expected to stick around for the next several days. For this, we can thank an atmospheric phenomenon known as blocking, or more specifically in this case, an omega block.

Atmospheric blocking causes the stagnation of a particular type of weather pattern. In other words, the same type of weather, be it hot, cold, wet or dry, will remain in place over a specific region for an extended period of time.

Flowing from west to east, the jet stream moves weather systems across the country and marks the boundary of cold air to its north and warm air to its south. When flowing in a zonal pattern – a fairly straight line – we generally see seasonal temperatures. But, there are times when it meanders in a more north/south pattern forming large troughs and ridges. When this type of meridional flow develops, warm air can reach farther north than normal and cold air can spill deeper into the south. It also means that weather systems get “blocked” from their typical eastward flow and therefore move more slowly.

The omega block is named after the Greek letter (Ω) that it resembles and is characterized by a high-pressure ridge sandwiched between two low-pressure troughs. Areas under the ridge experience a prolonged period of warm and dry weather, while areas under the troughs see persistent wet and cold conditions.

This current blocking event, centered over the eastern US, is letting the Northeast as well as parts of both the Midwest and West Coast to bask in unseasonable warmth. States in the central plains, on the other hand, are dealing with repetitive rounds of heavy precipitation and the risk of flooding.

Omega Blocking Pattern over US, April 2016. Credit: CBS